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A '''"greenhouse gas"''' traps heat in the atmosphere by absorbing radiated heat, in the form of [[infrared]]. 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 level of carbon dioxide in the atmosphere has increased by about 37 percent since the start of industrialization. It is now at its highest level in at least 650,000 years.<ref>[http://www.nature.com/nature/journal/v453/n7193/abs/nature06949.html (Nature)]</ref>
The nature of a greenhouse gas is to pass [[visible light]], i.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 [[glass]] on a [[man-made]] [[greenhouse]], and serves to trap warmth at the surface and in the lower [[atmosphere]]. Some extent of greenhouse gases are necessary to maintain life at a livable temperature on Earth. These greenhouse gas molecules (principally [[water vapor]], [[carbon dioxide]], and [[methane]]) absorb and re-emit long-wave [[infrared rays]] emitted from the earth's surface into the atmosphere (see [[radiational cooling]]).
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 effect
The 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==
