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Global warming

2008-01-03T17:37:18Z

Arkark:

[[Image:RobinsonWhopisgohifdohiwfEOHEFHatWarmsTheEarth.gif|right|frame]]

'''Global Warming''' is any increase in average air temperature at the [[Earth]]'s surface over a period of decades or more. Since 1989, advocates have clashed on the issue of whether human beings are more responsible than nature for modern periods of global warming; see "[[greenhouse warming]]" and [[global warming theory]].<ref>Some people predicted Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
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* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

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2. ^ Hegerl, Gabriele C.; et al. (2007-05-07). Understanding and Attributing Climate Change (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 690. Intergovernmental Panel on Climate Change. Retrieved on 2007-05-20. “Recent estimates (Figure 9.9) indicate a relatively small combined effect of natural forcings on the global mean temperature evolution of the seconds half of the 20th century, with a small net cooling from the combined effects of solar and volcanic forcings”
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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

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85. ^ "EU agrees on carbon dioxide cuts", BCC, 2007-03-09. Retrieved on 2007-05-04.
86. ^ Begley, Sharon. "The Truth About Denial", Newsweek, 2007-08-13. Retrieved on 2007-08-13.
87. ^ Adams, David (2006-09-20). Royal Society tells Exxon: stop funding climate change denial. The Guardian. Retrieved on 2007-08-09.
88. ^ "Exxon cuts ties to global warming skeptics", MSNBC, 2007-01-12. Retrieved on 2007-05-02.
89. ^ Sandell, Clayton. "Report: Big Money Confusing Public on Global Warming", ABC, 2007-01-03. Retrieved on 2007-04-27.
90. ^ "Greenpeace: Exxon still funding climate skeptics", USA TODAY, 2007-05-18. Retrieved on 2007-07-09.
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92. ^ "Group: China tops world in CO2 emissions", Associated Press, 2007-06-20. Retrieved on 2007-10-16. ; "Group: China surpassed US in carbon emissions in 2006: Dutch report", Reuters, 2007-06-20. Retrieved on 2007-10-16.
93. ^ China: US should take lead on climate, by Michael Casey, Associated Press, 12/7/07.
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99. ^ Raven, John A.; et al. (2005-06-30). "Ocean acidification due to increasing atmospheric carbon dioxide" (ASP). Royal Society. Retrieved on 2007-05-04.

"global warming," which has come to mean extreme [[greenhouse warming]] of the atmosphere leading to catastrophic environmental consequences. [http://www.junkscience.com/news/robinson.htm]</ref>

As an issue, it is a political battle over the science of climate change and is mainly used to justify support for the [[Kyoto Protocol]], which seeks to restrict [[carbon dioxide]] production on the grounds that it accelerates the "[[greenhouse effect]]".

Most non-scientific support for the global warming theory rests on popular accounts such as Al Gore's ''[[An Inconvenient Truth]]'' or on reports issued by the UN's [[IPCC]]. However, Keston Green of Monash University and Scott Armstrong of the Wharton School wrote that:

*Because the forecasting processes examined in Chapter 8 overlook scientific evidence on forecasting, the IPCC forecasts of climate change are not scientific. <ref>http://www.forecastingprinciples.com/Public_Policy/WarmAudit31.pdf</ref>

Politician [[Al Gore]] calls it a [[moral issue]], while scientist [[Richard Lindzen]] says, "It is probably the most immoral thing you could do to restrain energy so that the billions in the earth who don’t have access to electricity won't conveniently get it." <ref>http://www.americaabroadmedia.org/media/On%20line%20extra%20materials/Climate%20change%20-%20lindzen%20interview.doc</ref>
However, it is to be noted that many proponents of the theory propose energy conservation only until 'green' energy generation becomes predominant.{{Fact}}

In the general, the term ''global warming'' can be used for any upward trend in the [[global mean temperature]], but it is mostly used to refer to the 1° F warming since 1850.

The two sides are divided on whether this warming is a natural recovery from the [[Little Ice Age]] or a man-made environmental phenomenon. There are also debates regarding the potential positive or negative effects of a warming.

Republicans and Democrats in the U.S. are evenly split on the issue. ''Boston Globe'' writer [[Ellen Goodman]] wrote:
* 23 percent of college-educated Republicans believe [[anthropogenic global warming theory|the warming is due to humans]], while 75 percent of college-educated Democrats believe it. <ref> [http://www.boston.com/news/globe/editorial_opinion/oped/articles/2007/02/09/no_change_in_political_climate/] </ref>

As we learn more about climate, the "settled" conclusions of global warming [[alarmist]]s, appear more and more ridiculous.
"Clinging to alarmist scenarios for which there is no evidence, is simply exploitation of public gullibility." <ref>http://64.233.169.104/search?q=cache:A0llnpZ71lMJ:www.physics.harvard.edu/~motl/lindzen-nature-of-arguments.pdf+warming&hl=en&ct=clnk&cd=1&gl=us</ref>

==The Modern Warm Period==
[[Image:Artic 1979.jpg‎|left|thumb|275px|A computer projection comparing the Polar Ice Mass in 1979 (left) and 2000 (right).]]
Average [[Earth]] surface air temperature has risen about 1° F since 1850.
<ref>Hansen's group at the Goddard Institute wrote, "Global warming is now 0.6 °C [1.0 °F] in the past three decades and 0.8 °C [1.4 °F] in the past century." http://data.giss.nasa.gov/gistemp/2005/</ref> How much of this is due to contamination of the temperature record from the [[urban heat island]] effect is hard to assess. Efforts are currently underway to detect and remove this bias. Spurious warming trends that might be considered as global warming can occur almost anywhere. <ref>http://climatesci.colorado.edu/2006/01/29/do-urban-areas-have-larger-long-term-temperature-trends-than-other-locations/feed/</ref>
Nonetheless, controversy has persisted over the influence of urban warming on reported large-scale surface-air temperature trends. <ref>http://climatesci.colorado.edu/2006/01/29/do-urban-areas-have-larger-long-term-temperature-trends-than-other-locations/feed/</ref>

According to temperature reconstruction made within an [[Old Earth]] paradigm, there have been many cycles of naturally-caused global warming and cooling over many millions of years (see [[climate cycles]]). Some scientists, including [[Richard Lindzen]] of [[MIT]], [[Sallie Baliunas]] of [[Harvard]] and [[Fred Singer]] (independent), say that the recent warming could be part of another natural cycle or random fluctuations in the atmosphere. However, many scientists also think that human activities were most likely the cause of the the planet's recent warming.

A [[United Nations]] panel ([[IPCC]]) asserts that most of the warming since around 1850 is human-caused. Global warming alarmists call this panel "authoritative". Others question its objectivity, and a handful of scientists have resigned from the panel in protest over unauthorized changes in scientific reports.

In February, 2007, the [[IPCC|Intergovernmental Panel on Climate Change]], whose members were appointed by their governments, released their fourth assessment report on climate change. That report concluded that most of recent global warming was "very likely" caused by human activity<ref>http://news.bbc.co.uk/2/hi/science/nature/6321351.stm</ref>. It says that increases in atmospheric concentrations of [[greenhouse gas]]es cause warming. Several members of this panel later complained their contributions were misrepresented in the final report.

===IPCC controversy===
A reporter for the ''Chicago Tribune'' called the IPCC "the top body of climate change scientists in the world". <ref>http://www.chicagotribune.com/news/nationworld/chi-0704280477apr29,1,4275902.story?coll=chi-newsnationworld-hed</ref>

Fred Singer and other scientists dispute this view. 


==False Claims of "Consensus"==

[[Richard Lindzen]] wrote in 1992,

:Indeed, a recent Gallup poll of climate scientists in the American Meteorological Society and in the American Geophysical Union shows that a vast majority doubts that there has been any idenGlobal warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

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tifiable man-caused warming to date (49 percent asserted no, 33 percent did not know, 18 percent thought some has occurred; however, among those actively involved in research and publishing frequently in peer-reviewed research journals, none believes that any man-caused global warming has been identified so far). <ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>

Oddly enough, even though 82% of US climate scientists refused to support the global warming theory then, liberal activists were already claiming a scientific consensus for [[anthropogenic global warming]]. (It's hard to understand how 18 percent credence in ''any'' global warming translates into "consensus" support for ''human-caused'' global warming.)

The campaign to convince the public (and their elected representatives) that the "science is settled" began in 1988 or 1989, shortly after the communist nations lost the Cold War.


==The Hockey Stick Temperature Reconstruction==
[[Image:Hockey stick chart ipcc.jpg|right|thumb|300px|The flawed Hockey Stick graph in a 2001 IPCC report shows blue proxy values along the "shaft" with red instrumental values superimposed as the "blade".]]
One of the main pieces of evidence for anthropogenic global warming presented during the 3rd IPCC report was a temperature reconstruction done by climatologist [[Michael Mann]]. The resulting graph of the Northern Hemisphere temperature is shaped like a hockey stick, and it is used as evidence for the unusualness of the earth's temperature in the past few decades.

While dramatic, the graph encountered serious questions from the beginning. It failed to reproduce two Northern Hemisphere climate episodes that were well documented in the literature, the Medieval Warm Period and The Little Ice Age (it should be noted that these were not global phenomenons, but were limited to Europe and Greenland; the Mann diagram concerned global average temperatures). Concerned about these discrepancies, [[Stephen McIntyre]] and [[Canada|Canadian]] economist Ross McKitrick attempted to reproduce the result and discovered a serious flaw in the statistical methods used to construct the hockey stick graph. Because of this, the 4th Assessment Report of the IPCC included several different temperature reconstruction methods instead of favoring just one hypothesis.

==Natural Variability of the Climate System==
Evolutionary scientists who believe in global warming think that the Earth has experienced numerous [[Ice Age|ice age]]s over two million years, during which global temperatures dropped approximately 11° F and then returned to normal. The frigid temperatures allowed ice sheets to expand southward, covering much of [[Asia]], [[Europe]], and [[North America]]. The cooling associated with ice ages is gradual, while the terminations are relatively rapid. However, even the rapid terminations of ice ages take centuries to millennia.

==Natural Climate Change on Other Planets==

Since the [[Viking spacecraft]] reached [[Mars]] in the 1970s until recent readings were taken, the average temperature on Mars has risen 1.1° F just as the average temperature on the earth has risen. Since human industrialization is clearly not to blame for the change on Mars, other causes are being considered. One possibility is that dust storms are changing the albedo of the planet, allowing it to warm, while another possibility is that solar variations from the sun are causing the warming.<ref>http://www.space.com/scienceastronomy/070404_gw_mars.html</ref><ref>http://www.heartland.org/Article.cfm?artId=17977</ref>

Recently, it has also been found that similar to the Earth and Mars, [[Neptune]] is also undergoing global warming. Measurements taken at the Lowell observatory in [[Arizona]] have shown an increase in Neptune's brightness and temperature since 1980 following the same pattern seen on Earth and Mars. The researchers who discovered this warming suggest there may be a correlation between the warming and solar variations.<ref>http://www.agu.org/pubs/crossref/2007.../2006GL028764.shtml</ref>

[[Pluto]] has also been found to be undergoing global warming. The overall temperature increase on Pluto has been greater than that on the earth.<ref>http://www.space.com/scienceastronomy/pluto_warming_021009.html</ref>

On the other hand, [[Uranus]] has been found to be undergoing rapid cooling, for reasons not entirely understood.<ref>http://www.boulder.swri.edu/~layoung/eprint/ur149/Young2001Uranus.pdf</ref>

==Politics of Global Warming==

'Global Warming Now World's Most Boring Topic’ <ref>http://newsbusters.org/node/14167</ref>

The need to fight "global warming" has become part of the dogma of the liberal conscience. <ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>

Clearly, "global warming" is a tempting issue for many very important groups to exploit.
... dealing with the threat of warming fits in with a great variety of preexisting agendas [like] dissatisfaction with industrial society (neopastoralism), ... governmental desires for enhanced revenues (carbon taxes), and bureaucratic desires for enhanced power. <ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>

The global average surface temperature warmed about 0.9 °F over the second half of the 20th Century.

Assessments of climate science by the [[United Nations]] (see [[IPCC]] - ''Intergovernmental Panel on Climate Change'')
have claimed that scientists are 90% sure that over 50% of the observed global warming in recent decades is human-caused, and that continued global warming should be expected over at least the next century. ''Science'' published a literature search by Naomi Oreskes concluding that "scientific consensus" supports the IPCC reports. <ref>http://www.sciencemag.org/cgi/content/full/306/5702/1686]</ref>

Several prominent scientists have pointed out the [[politicized science]] of the UN's assessment methods. The scientific reports are submitted to a panel of representatives appointed by each country in the IPCC. Several scientists whose research demonstrates that climate change is taking place have complained about their work being misrepresented by the U.N.

*In addition, a number of the participants have testified to the pressures placed on them to emphasize results supportive of the current scenario and to suppress other results. That pressure has frequently been effective, and a survey of participants reveals substantial disagreement with the final report. <ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>

Richard Lindzen wrote:
:Perhaps more important are the pressures being brought to bear on scientists to get the "right" results. Such pressures are inevitable, given how far out on a limb much of the scientific community has gone. The situation is compounded by the fact that some of the strongest proponents of "global warming" in Congress are also among the major supporters of science (Sen. Gore is notable among those). <ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>

==Al Gore's Assault on Science==

*Gore claims that Himalayan glaciers are shrinking and global warming is to blame. Yet the September 2006 issue of the American Meteorological Society's Journal of Climate reported, "Glaciers are growing in the Himalayan Mountains, confounding global warming alarmists who recently claimed the glaciers were shrinking and that global warming was to blame." <ref>http://www.suntimes.com/news/otherviews/450392,CST-EDT-REF30b.article</ref>


*Gore claims the snowcap atop Africa's Mt. Kilimanjaro is shrinking and that global warming is to blame. Yet according to the November 23, 2003, issue of Nature magazine, "Although it's tempting to blame the ice loss on global warming, researchers think that deforestation of the mountain's foothills is the more likely culprit. Without the forests' humidity, previously moisture-laden winds blew dry. No longer replenished with water, the ice is evaporating in the strong equatorial sunshine." <ref>http://www.suntimes.com/news/otherviews/450392,CST-EDT-REF30b.article</ref>

*Gore claims global warming is causing more tornadoes. Yet the United Nations Intergovernmental Panel on Climate Change stated in February that there has been no scientific link established between global warming and tornadoes. <ref>http://www.suntimes.com/news/otherviews/450392,CST-EDT-REF30b.article</ref>

== Scientists ==

[[Image:Rogerrevelle.jpg|thumb|right|200px|Roger Revelle]]

{|
| valign="top" width="35%"|
* [[Roger Revelle]]
* [[Achim Steiner]]
* [[Fred Singer]]
* [[Richard Lindzen]]
* [[Sallie Baliunas]]
* [[Lonnie Thompson]]
| valign="top" width="33%"|
* [[James E. Hansen]]
* [[Michael Mann]]
* [[Charles Keeling]]
* [[Edwin A. Hernández-Delgado]]
|}

==See Also==
*[[Anthropogenic global warming theory]]
*[[Global Warming Controversy]]
*[[Politics of global warming]]
*[[Liberal_Hysteria#Global_Warming_Derangement_Syndrome|Global Warming Derangement Syndrome]]
*[[Socialist Environmental Disasters]]
*[[An Inconvenient Truth]]
*[[Previous Breaking News/Global Warming|Articles about '''Global Warming''' from previous "Breaking News"]]
*[[More News/Global Warming|Articles about '''Global Warming''' from "More News"]]

==External Links==
<small>
*[http://global-warming.accuweather.com/2007/12/attempting_to_stop_global_warm_1.html Attempting to Stop Global Warming is Futile and a Mistake, says letter to the UN]
*[http://www.nationalpost.com/news/story.html?id=164004 Signatories of an open letter on the UN climate-conference]
*[http://schwinger.harvard.edu/~motl/global-temperature-not-exist.pdf Does a global temperature exist?], ''Journal of Non-Equilibrium Thermodynamics'', June 2007.
*[http://environment.guardian.co.uk/climatechange/story/0,,2032821,00.html The appliance of science] by Mike Hulme.
*[http://www.climatecrisis.net/ ''An Inconvenient Truth''], Film by [[Al Gore]]
*[http://video.google.com/videoplay?docid=2332531355859226455&q=great+global+warming+swindle ''The Great Global Warming Swindle'' - Documentary Film]
*[http://www.physics.harvard.edu/%7Emotl/iris-effect.pdf Climate Sensitivity and Observed Negative Feedbacks], lecture by Richard Lindzen and Roberto Rondanelli.
*[http://www.physics.harvard.edu/%7Emotl/lindzen-nature-of-arguments.pdf Nature of Arguments for Anthropogenic Global Warming], by Richard Lindzen
*[http://www.independent.org/store/book_detail.asp?bookID=42 ''Hot Talk, Cold Science: Global Warmings's Unfinished Debate'', by S. Fred Singer]
*[http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html Global Warming: The Origin and Nature of the Alleged Scientific Consensus] - [[Richard S. Lindzen]], Alfred P. Sloan Professor of Meteorology at the [[Massachusetts Institute of Technology]]
*[http://www.independent.org/publications/tir/article.asp?issueID=47&articleID=604 "Should We Have Acted Thirty Years Ago to Prevent Climate Change?", by Randall G. Holcombe].
*[http://www.independent.org/publications/tir/article.asp?issueID=25&articleID=296 "After Kyoto: A Global Scramble for Advantage," by Bruce Yandle].
*[http://www.ncpa.org/sub/dpd/index.php?Article_ID=2319 The physical evidence of earth's unstoppable 1,500-year climate cycle]
*[http://www.independent.org/publications/article.asp?id=1714 "Is There a Basis for Global Warming Alarm?", by Richard Lindzen]
*[http://www.independent.org/publications/policy_reports/detail.asp?type=full&id=5 "New Perspectives in Climate Change: What the EPA Isn’t Telling Us"]
*[http://www.heartland.org/Article.cfm?artId=17181 Survey Shows Climatologists Are Split on Global Warming]
*[http://www.magma.ca/~hurleyp/FightingTheHoax.htm Fighting the Hoax]
*[http://www.climateaudit.org Climate Audit], Steve McIntyre's blog
*[http://www.realclimate.org Real Climate], blog by a group of climatologists including Michael Mann</small>

==Critique of the Hockey Stick Reconstruction==
<small>*[http://www.climateaudit.org/pdf/mcintyre.mckitrick.2003.pdf Corrections To The Mann et. al. (1998) Proxy Data Base And Northern Hemispheric AveraGlobal warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

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85. ^ "EU agrees on carbon dioxide cuts", BCC, 2007-03-09. Retrieved on 2007-05-04.
86. ^ Begley, Sharon. "The Truth About Denial", Newsweek, 2007-08-13. Retrieved on 2007-08-13.
87. ^ Adams, David (2006-09-20). Royal Society tells Exxon: stop funding climate change denial. The Guardian. Retrieved on 2007-08-09.
88. ^ "Exxon cuts ties to global warming skeptics", MSNBC, 2007-01-12. Retrieved on 2007-05-02.
89. ^ Sandell, Clayton. "Report: Big Money Confusing Public on Global Warming", ABC, 2007-01-03. Retrieved on 2007-04-27.
90. ^ "Greenpeace: Exxon still funding climate skeptics", USA TODAY, 2007-05-18. Retrieved on 2007-07-09.
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92. ^ "Group: China tops world in CO2 emissions", Associated Press, 2007-06-20. Retrieved on 2007-10-16. ; "Group: China surpassed US in carbon emissions in 2006: Dutch report", Reuters, 2007-06-20. Retrieved on 2007-10-16.
93. ^ China: US should take lead on climate, by Michael Casey, Associated Press, 12/7/07.
94. ^ India's glaciers give grim message on warming, by Somni Sengupta, 7/17/07, New York Times via oregonlive.com.
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98. ^ Caldeira, Ken; Wickett, Michael E. (2005-09-21). "Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean". Journal of Geophysical Research 110 (C09S04): 1–12. Retrieved on 2006-02-14.
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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

1. ^ a b c d e f g h i Summary for Policymakers (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2007-02-05). Retrieved on 2007-02-02. “The updated 100-year linear trend (1906 to 2005) of 0.74°C [0.56°C to 0.92°C] is therefore larger than the corresponding trend for 1901 to 2000 given in the TAR of 0.6°C [0.4°C to 0.8°C].”
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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

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Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000.[1] The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"[1] via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.[2][3] These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions.[4]

Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[1] The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans.[1]

Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.
Contents
[hide]

* 1 Terminology
* 2 Causes
o 2.1 Greenhouse gases in the atmosphere
o 2.2 Feedbacks
o 2.3 Solar variation
* 3 Temperature changes
o 3.1 Recent
o 3.2 Pre-human climate variations
* 4 Climate models
* 5 Attributed and expected effects
o 5.1 Economic
o 5.2 Security
* 6 Adaptation and mitigation
* 7 Social and political debate
* 8 Related climatic issues
* 9 See also
* 10 References
* 11 Further reading
* 12 External links

Terminology

The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence.[5] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[6] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.

Causes
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.
Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

Main articles: Attribution of recent climate change and scientific opinion on climate change

The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[7][8][9] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[10][11] identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity.[12][13][14]

None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[15]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.[16][17] Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[18][19] Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[20] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.[21]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[22] Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[23] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[24]

Feedbacks

Main article: Effects of global warming#Further global warming (positive feedback)

The effects of forcing agents on the climate are complicated by various feedback processes.

One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[25] This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.[25]

Another important feedback process is ice-albedo feedback.[26] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.

Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming.[27] A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[28]

Solar variation
Solar variation over the last 30 years.
Solar variation over the last 30 years.

Main article: Solar variation

A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000.[29] A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[31] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[32]

One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming.[33] Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming.[34][35] A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[36] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.[37] Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.[38]

Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

Main article: Temperature record

Recent

Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.[39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land.[41] Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[42] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[43][44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[45] though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.[46]

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[47] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[48]

Pre-human climate variations
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.
Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph.

Further information: Paleoclimatology
See also: Snowball Earth

Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures.[49]

A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years.[50][51]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago).

Climate models
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions.
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[52] However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity.

Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[1] Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[53] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[54][55][56]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[57] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Main article: Effects of global warming

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes,[58] reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[59] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[60]

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.[61] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[59] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[62] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[63] However, few mechanistic studies have documented extinctions due to recent climate change[64] and one study suggests that projected rates of extinction are uncertain.[65]

Economic
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.
The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

Main articles: Economics of global warming and Low-carbon economy

Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[61] One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%.[66] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,[67] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[68][69]

In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk.[70]

Security

In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change.[71] These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts.

Adaptation and mitigation

Main articles: Adaptation to global warming, Mitigation of global warming, and Kyoto Protocol

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[72][73]

There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.[74] Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[75]

Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.[76] Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005.[77]

China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[78]

The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product.[79]

Social and political debate

Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
See also: Climate change denial

Per capita greenhouse gas emissions in 2000
Per capita greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000
Per country greenhouse gas emissions in 2000

Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[80] Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world.[81] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S.[82] In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States.[83][84]

The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions.[85] Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls.[86][87][88][89] Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years,[90] or called for policies to reduce global warming.[91]

Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report.[92] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[93] India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.[94] However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same.[95]

Related climatic issues

Main articles: Ocean acidification, global dimming, and ozone depletion

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[96] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[97] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][98] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[99]

Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]

Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong.

See also

* Glossary of climate change

References

1. ^ a b c d e f g h i Summary for Policymakers (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2007-02-05). Retrieved on 2007-02-02. “The updated 100-year linear trend (1906 to 2005) of 0.74°C [0.56°C to 0.92°C] is therefore larger than the corresponding trend for 1901 to 2000 given in the TAR of 0.6°C [0.4°C to 0.8°C].”
2. ^ Hegerl, Gabriele C.; et al. (2007-05-07). Understanding and Attributing Climate Change (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 690. Intergovernmental Panel on Climate Change. Retrieved on 2007-05-20. “Recent estimates (Figure 9.9) indicate a relatively small combined effect of natural forcings on the global mean temperature evolution of the seconds half of the 20th century, with a small net cooling from the combined effects of solar and volcanic forcings”
3. ^ Ammann, Caspar; et al. (2007-04-06). "Solar influence on climate during the past millennium: Results from ransient simulations with the NCAR Climate Simulation Model" (PDF). Proceedings of the National Academy of Sciences of the United States of America 104 (10): 3713–3718. “However, because of a lack of interactive ozone, the model cannot fully simulate features discussed in (44)." "While the NH temperatures of the high-scaled experiment are often colder than the lower bound from proxy data, the modeled decadal-scale NH surface temperature for the medium-scaled case falls within the uncertainty range of the available temperature reconstructions. The medium-scaled simulation also broadly reproduces the main features seen in the proxy records." "Without anthropogenic forcing, the 20th century warming is small. The simulations with only natural forcing components included yield an early 20th century peak warming of ≈0.2 °C (≈1950 AD), which is reduced to about half by the end of the century because of increased volcanism.”
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ge Temperature Series] by Stephen McIntyre and Ross McKitrick, ''Energy & Enviornment'' volume 14, 2003.
*[http://blogs.nature.com/climatefeedback/2007/05/the_decay_of_the_hockey_stick.html The Decay of the Hockey Stick by Von Storch] published on the journal ''Nature'''s blog, May 3, 2007.
*[http://www.technologyreview.com/Energy/13830/ A Global Warming Bombshell] by Richard A. Muller, ''Technology Review'' , Oct. 2004; calls into question famous graph by Michael Mann.
*[http://www.sciencemag.org/cgi/content/abstract/306/5696/679 Reconstructing Past Climate from Noisy Data] by Hans von Storch, Eduardo Zorita, Julie M. Jones, Yegor Dimitriev, Fidel González-Rouco, Simon F. B. Tett, ''Science'' magazine, 22 October 2004.
</small>
Now this article has facts in it. S out.
==References==
{{reflist|1}}

[[category:earth science]]
[[Category:Environmentalism]]
[[Category:Liberal Bias]]

George W. Bush

2008-01-03T17:32:08Z

Arkark:

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'''George "I need a new diaper" Walker Bush''' (born New Haven, [[Connecticut]] 1946) was the Governor of [[Texas]] (1996-2001) and has served as the [[Republican]] [[President of the United States of America]] since 2001. Anybody reading this, EDIT OTHER CONSERVAPEDIA ARTICLES BY PUTTING IN FACTS NOW, NOW, NOW! Campaigning on the notion that the United States should not be in the business of nation-building,but proved himself a liar when he engaged in nation building by putting us in a Vietnam-esque war in Iraq.<ref>http://www.cnn.com/ELECTION/2000/debates/transcripts/u221003.html</ref> he won the office by a narrow margin in the decisive State of [[Florida]]. Legal challenges to the certified vote count went all the way to the [[U.S. Supreme Court]] in 2000. Democratic I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate!I've just figured out a WAY more annoying way to do my job. I got the idea trying to edit from an iphone. Anyway, onto the facts: Bush was in the national guard, but thanks to daddy's money, he was able to bail out right before drug blood tests. Fishy. Also, recent reports in Iraq show that Syria and Iran are rounding in their terrorists. Man, this regime is stupid! Don't they know that most of these terrorists, including Bin Laden, are FROM Saudi Arabia, our trusted ally? Also, Bush's tax policies are insane. He cuts taxes for the rich and raises them for the poor. I feel that if Bush got a nickname, it would be Prince John. Gore would be Robin Hood. More facts to come. Now, to duplicate! contender [[Al Gore]] initially conceded defeat on the night of the election, but then contested the outcome for weeks until the Supreme Court case [[Bush v. Gore]].

In 2004 Bush won reelection, helped in part by a 300,000 vote victory (5%) in the State of Florida, where the outcome had been so close in 2000 <ref>[http://election.dos.state.fl.us/elections/resultsarchive/SummaryRpt.asp?ElectionDate=11/2/2004&Race=PRE&DATAMODE= Florida Election Records], ''[[Florida Dept. of State]]''</ref>. Democratic candidate [[John Kerry]] quickly conceded defeat the day after the election.

== Foreign policy ==
George W. Bush has spent most of his Presidency focusing on foreign policy issues, namely the war on terror in response to the September 11, 2001 terrorist attacks against the Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!Anyway, more facts! Bush's administration was warned about the threat of Bin Laden many times by agents Clinton had working on him. Eventually, Rumsfeld told one to quit bugging him because he'd "covered his ass". Wow, so the Bush negligence helped cause 9/11! Also, the intelligence Bush received on Iraq's nukes: fake. There was word that it was fake from Britain too, but Bush, according to former employees, had been planning the invasion from the beginning, and he wasn't going to let some dumb fake intel ruin him! However, according to you, Clinton's lies are worse. I disagree, because although both lies are wrong, Clinton's lie didn't KILL THOUSANDS OF IRAQI CIVILIANS AND AMERICAN SOLDIERS!USA. As part of the fight against terrorism, he received Congressional approval to invade the countries of [[Afghanistan]] and [[Iraq]], and the military has successfully effected regime changes in those countries. He is now working to end the Iraqi insurgency and stabilize the region.

Egyptian President [[Hosni Mubarek]] and Libyan [[Socialist]] General Secretary [[Muammar Gaddafi]] in negotiations with [[Saddam Hussein]] relayed a message through Italian Prime Minister [[Silvio Berlusconi]] and Spanish President [[Jose Maria Aznar]] to UK Prime Minister [[Tony Blair]] and US President [[George W. Bush]] that Saddam would be willing to avoid removal by force and voluntarily go into exile, provided Saddam was allowed to take $1 billion dollars and "all the information he wants about [[weapons of mass destruction]]." <ref>[http://www.weeklystandard.com/weblogs/TWSFP/2007/09/saddam_risked_his_life_for_wmd_1.asp Saddam Risked His Life for WMD Secrets], ''WeeklyStandard.com. Spetember 29, 2007.</ref><ref>[http://www.mediainfo.com/eandp/news/article_display.jsp?vnu_content_id=1003646639 Scoop for Spanish Daily: Transcript of Private 2003 Bush Talk Promising Iraq Invasion], ''Editor & Publisher'', September 26, 2007.</ref><ref>[http://www.washingtonpost.com/wp-dyn/content/article/2007/09/26/AR2007092602414.html Report Says Hussein Was Open To Exile Before 2003 Invasion], He Is Said to Have Sought $1 Billion and Information on Arms, By Karen DeYoung and Michael Abramowitz, ''[[Washington Post]], September 27, 2007; Page A17.</ref><ref>[http://www.elpais.com/articulo/espana/Llego/momento/deshacerse/Sadam/elpepunac/20070926elpepinac_1/Tes Llegó el momento de deshacerse de Sadam], ''El Pais'', 26/09/2007. (In Spanish).</ref>

In a show down with Congressional Democrats after taking control of the U.S. House of Representatives and Senate in the 2006 Congressional Midterm elections, Democrats retreated on their pledge to end the [[Iraq War]] early and bring the troops home. Democrats had threatened to withhold funding for the troops unless a date certain for withdrawal was set. After the final vote, 280-142 in the House and 80-14 in the Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. 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Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Actually, the reason democrats aren't getting much done in congress is because the Republicans keep fillibustering everything with a pulse. Since you guys don't want to admit this, I'll just repeat it until you do.Senate, the anti-War movement was defeated. The deal cut with Democratic leaders in exchange for their acquiescing to fund the troops calls for the President to sign legislation raising the [[minimum wage]]. <ref>[http://washingtontimes.com/national/20070525-122215-4854r.htm Congress OKs war bill sans time-line,] By S.A. Miller, The Washington Times'', May 25, 2007. </ref> One commentator remarked, "Despite all the talk of standing up to George W. Bush, despite all the bravado about taking control of Congress, despite the so-called mandate to change direction, Democrats caved....They claim that the majority of Americans are with them on the Iraq issue, but...President Bush, at the weakest moment of his presidency, still bested his Democratic rivals." <ref>[http://www.newmediajournal.us/staff/phyrillas/05292007.htm Democrats Show True Colors], Tony Phyrillas, ''New Media journal'', May 29, 2007, </ref>

== Economic issues ==
Though the liberal media continues to disparage Bush's handling of the economy, they neglect to report all aspects of the economy are at historically high levels. This is partially due to changes in the stock market that lead to a record high in 2007. Corporations show profits growing by double digits growth.<ref>http://www.usatoday.com/money/markets/us/2006-05-08-mart-usat_x.htm</ref> Even the working class is benefiting from the Bush economy, as unemployment hits an all time low in March 2007.<ref>http://news.yahoo.com/s/ap/economy</ref> Bush worked with Democrats to raise the minimum wage to a more livable level.<ref>http://www.washingtonpost.com/wp-dyn/content/article/2006/12/20/AR2006122001784.html</ref> Tax policies have been favorable to reducing the Capital Gains Tax, with a subsequent surge in investment.

== Family ==
[[Image:GWBush.jpg|thumb|300px|right|President Bush admires pictures of his family in the Oval Office.]]
George W. Bush is the son of [[George H. W. Bush]], who served as vice-president from 1981 to 1989 and as president from 1989 to 1993, and [[Barbara Bush]]. President Bush is married to Laura Welch Bush, a former teacher and librarian, and they have twin daughters, Barbara and Jenna. John Ellis "Jeb" Bush, Neil Bush and Marvin Bush are his brothers. Dorothy Bush Koch is his only surviving sister, as Pauline Robinson Bush died at age four of leukemia.

==Faith==
George W. Bush is a member of the [[United Methodist Church]], and most people feel that George W. Bush's faith is sincere and profound. ''The Faith of George W. Bush'', a non-political book by author Stephen Strang, made the [[New York Times]] best-sellers list. <ref>http://www.christianlifemissions.org/ministries/georgewbush.htm</ref>
When asked where he would be without the influence of family friend Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again!Remember, in Iraq we fight theocracies. A theocracy is a religion controlled country. Republicans seem desperate to make us a religion controlled country, going so far as to say that we are "a Christan nation". Therefore, REPUBLICANS ARE HYPOCRITES. Also, the Democrats have stressed accepting other religions, whereas one of Guliani's employees urges genocide of Muslims. Wow, it's hard to tell which party is bad. Again! [[Billy Graham]], George W. Bush said "I wouldn't be president." "It was just a conversation," said the younger Bush about a talk with Graham in the mid-'80s that changed his life. During the pivotal conversation Bush recalled saying, "'You know, Billy, I'm longing for something.' And I know that he sent me a Bible I still have. All I can tell you is that as a result of being inspired by Billy Graham, I started reading the Bible and shortly after, I quit drinking." <ref>Billy Graham and the White House [http://religion.beloblog.com/archives/2007/08/billy_graham_and_the_white_hou_1.html]</ref>

==Threats==
No threats, except for joking remarks. Many wish Bush had been threatened, because the Bushies might resign out of fear, thus saving America. S out.

==See Also==
*[[Previous Breaking News/George W. Bush|Articles about '''George W. Bush''' from previous "Breaking News"]]

==References==
{{reflist|2}}

[[Category:Presidents of the United States]]
[[Category:Political people]]

{{DEFAULTSORT:Bush, George W.}}
{{USPresidents}}