A '''"greenhouse gas"''' is [[gas]] traps heat in the atmosphere by absorbing radiated heat, in the form of [[atmosphereinfrared]] that . Any gas which contributes to the greenhouse effect is called a greenhouse gas, although those produced by human activity have gotten more attention in the past few decades. The most abundant greenhouse gas is [[water vapor]];<ref>Water vapor has the largest greenhouse effect, though since the quantity of water vapor in the atmosphere is determined primarily by the temperature it is not regarded as a climate forcing agent. [http://earthobservatory.nasa.gov/Library/glossary.php3?mode=all (NASA)]</ref> see [[cloud cover]]. Also important are [[carbon dioxide]], [[methane]], [[nitrous oxide]], and [[ozone]].<ref>http://yosemite.epa.gov/oar/globalwarming.nsf/content/emissions. html</ref>
The most important nature of a greenhouse gas is to pass [[water vaporvisible light]], i. Also important e., incoming energy from the sun, but to be opaque to the [[infrared]], i.e., heat energy being re-radiated outward from the earth's surface. Greenhouse gases, by definition, are good absorbers at these wavelengths. This functions not at all like the [[carbon dioxideglass]] on a [[man-made]] [[greenhouse]], and serves to trap warmth at the surface and in the lower [[methaneatmosphere]]. Some extent of greenhouse gases are necessary to maintain life at a livable temperature on Earth. These greenhouse gas molecules (principally [[water vapor]], [[nitrous oxidecarbon dioxide]], and [[ozonemethane]].) absorb and re-emit long-wave [http://yosemite.epa.gov/oar/globalwarming.nsf/content/emissions.html[infrared rays]] emitted from the earth's surface into the atmosphere (see [[radiational cooling]]).
The nature of a greenhouse gas is to pass ==Solar radiation==[[visible lightImage:158224main earth.jpg|thumb|right|300px|A diagram illustrating the so-called "greenhouse effect."]], iSunlight falling on the surface of a planet is primarily in the visible part of the spectrum.e., incoming energy from The surface absorbs some of the sunincident sunlight, but to be opaque to and some is reflected, depending on the reflectivity of the surface (see [[infraredalbedo]], i).eThe reflected light mostly passes through the atmosphere back to space., heat energy being re-radiated outward from The absorbed light warms the earth's surface. This functions much like the Anything with a temperature radiates according to [[glassPlanck's law]] on ,<ref>Thermodynamics says that any body with a temperature above absolute zero will radiate its energy away. [[man-madehttp://www.upscale.utoronto.ca/GeneralInterest/Harrison/BlackHoleThermo/BlackHoleThermo.html]] [[greenhouse]], </ref> and serves the Earth is no exception. Due to trap warmth at its relatively low temperature, the surface and emission from the ground is in the lower [[atmosphere]]infrared (IR) part of the electromagnetic spectrum, emitted from the Earth's surface.
== See also =Infrared radiation===::''See also : [[Infrared radiation]]''The gas absorbs the IR radiation, and emits IR radiation, both up and down. Because these gases are above the surface, they are at a lower temperature than the surface, so they emit at a different frequency. The net effect is that energy is retained by the Earth system, warming it.<ref>D. Hartmann, Global Physical Climatology</ref> This is the natural greenhouse effect, which allows the Earth to be habitable. When the atmospheric composition is changed by human activity resulting in increases in greenhouse gases, the warming associated with this theory is called the [[Anthropogenic global warming theory|anthropogenic greenhouse effect]].
[[Richard Lindzen]] wrote (in 1992):{{cquote|The main absorbers of infrared in the atmosphere are water vapor and clouds. Even if all other greenhouse gases (such as carbon dioxide and methane) were to disappear, we would still be left with over 98 percent of the current greenhouse effect.<ref>[http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html Richard Lindzen quote]</ref>}} However, this figure has been disputed. The textbook Chemistry of Atmospheres puts the contribution of water vapor and clouds at less than 88 percent of the greenhouse effect, while carbon dioxide is at least 12 percent.<ref>[http://www.radix.net/~bobg/climate/halpern.trap.html Chemistry of Atmospheres]</ref> This insulating effect has been compared to a [[blanket]] or to the glass walls and roof of a [[greenhouse]], but the process is not the same. ==Gas types==The greenhouse effect is caused primarily by the following*Water vapor: 95.00% of effect<ref>Multiple references:<br/>*S.M. Freidenreich and V. Ramaswamy, “Solar Radiation Absorption by Carbon Dioxide, Overlap with Water, and a Parameterization for General Circulation Models,” Journal of Geophysical Research 98 (1993):7255-7264*http://www.geocraft.com/WVFossils/Reference_Docs/PMichaels_Jun98.pdf*http://www.eia.doe.gov/cneaf/alternate/page/environment/appd_d.html*http://www.tcsdaily.com/article.aspx?id=010405M*http://www.ecoenquirer.com/EPA-water-vapor.htm*http://pubs.acs.org/subscribe/journals/ci/31/special/may01_viewpoint.html*http://www.geocraft.com/WVFossils/Reference_Docs/sci_and_techn-glacial_expansion_03-04.pdf</ref>*[[Carbon dioxide]]: 3.61% of effect*[[Methane]]: 0.36% of effect*Nitrous Oxide: 0.07% of effectThe following is a breakdown of the sources for these greenhouse gasses.{| class="wikitable"|-! Gas !! % of effect !! % natural !! % man-made|-| Water Vapor || 95.000% || 94.999% || 0.001% |-| Carbon Dioxide || 3.618% || 3.502% || 0.117% |-| Methane || 0.360% || 0.294% || 0.066%|-| Nitrous Oxide || 0.950% || 0.903% || 0.047% |-| Other gases ( CFCs, etc.) || 0.072% || 0.025% || 0.047%|-| Total || 100.00% || 99.72 || 0.28%|}<ref>http://www.geocraft.com/WVFossils/greenhouse_data.html</ref> ==Greenhouse effect==The greenhouse effect refers to the retention of heat by a planet's [[atmosphere]]. Although "greenhouse" is a handy analogy to a botanical [[greenhouse]], which insulates an indoor space by regulating convection, the atmosphere retains heat in a different way. It happens because gas molecules absorb some of the heat radiating from the ground (see [[infrared radiation]]), so that it all doesn't escape into [[outer space]]. It was discovered in 1824 by [[Joseph Fourier]]. The effect has been compared to that of the glass roof and walls of a [[greenhouse]], although the process is different. A gardener's greenhouse simply traps warm air inside the building. ==Runaway greenhouse effect==A runaway greenhouse effect has been claimed by some climate scientists, many of whom are involved in [[Climategate]], and have argued that slight increases in trace gases in the Earth's atmosphere would lead to higher air temperatures. These higher temperatures would in turn cause "greenhouse gases" such as [[carbon dioxide]] to have higher concentrations, which in turn would accelerate the [[warming trend]]. Alarming increases of 3 to 15 degrees Fahrenheit have been predicted, as modeled in [[Al Gore|Al Gore's]] [[hockey stick graph]]. Outside of the political sphere, few academics give any credence to this theory, and many scientists have been [[Climategate#Critique of the Hockey Stick Reconstruction|critical of the hockey stick reconstruction]]. MIT professor [[Richard Lindzen]] wrote:{{cquote|It is still of interest to ask what we would expect a doubling of carbon dioxide to do. A large number of calculations show that if this is all that happened, we might expect a warming of from .5 to 1.2 degrees centigrade. The general consensus is that such warming would present few, if any, problems. [...] current models depend heavily on undemonstrated positive feedback factors to predict high levels of warming.<ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>}} Another professor wrote:{{cquote|[[Computer model]]s suggest that a doubling of the concentration of the main greenhouse gas, carbon dioxide, may raise the [[average global temperature]] between 1 and 3° Celsius. However, the numeric equations of computer models do not accurately simulate the effects of a number of possible [[negative feedback]]s. For example, many of the models cannot properly simulate the negative effects that increased [[cloud cover]] would have on the [[radiation balance]] of a warmer Earth. Increasing the Earth's temperature would cause the [[ocean]]s to evaporate greater amounts of water, causing the atmosphere to become cloudier. These extra clouds would then reflect a greater proportion of the Sun's energy back to space reducing the amount of [[solar radiation]] absorbed by the atmosphere and the Earth's surface. With less solar energy being absorbed at the surface, the effects of an enhanced greenhouse effect may be counteracted.<ref>http://www.physicalgeography.net/fundamentals/7h.html</ref>}} Some scientists have noted a [[Global warming#Natural Climate Change on Other Planets|natural climate change on other planets]]; others have pointed to [[Venus]] as an example of a planet where a runaway greenhouse effect has already occurred. However, Venus has many differences with Earth, including a lack of plate tectonics which make the comparison faulty. ==References==<references/> ==See also==*[[Sea surface temperature]] (SST) - anomalies throughout the global climate system [[Category:Gases]]
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