Editing Trace gas (section)

== Trace greenhouse gases ==
{{See also|List of greenhouse gases}}
A few examples of the major [[greenhouse gas]]es are [[water]], [[carbon dioxide]], [[methane]], [[nitrous oxide]], [[ozone]], and [[Chlorofluorocarbon|CFCs]]. These gases can absorb [[Infrared|infrared radiation]] from the Earth's surface as it passes through the atmosphere.

The most influential greenhouse gas is [[water vapor]].  It frequently occurs in high concentrations, may transition to and from an aerosol (clouds), and is thus not generally classified as a trace gas.  Regionally, water vapor can trap up to 80 percent of outgoing IR radiation.<ref name=":1">{{Cite journal | doi=10.1021/ed072p973|title = The Environmental Chemistry of Trace Atmospheric Gases| journal=Journal of Chemical Education| volume=72| issue=11| pages=973|year = 1995|last1 = Trogler|first1 = William C.| bibcode=1995JChEd..72..973T }}</ref>  Globally, water vapor is responsible for about half of Earth's total [[greenhouse effect]].<ref name="ngis">{{cite web |url=https://www.giss.nasa.gov/research/briefs/2010_schmidt_05/ |title=Taking the Measure of the Greenhouse Effect |publisher=NASA Goddard Institute for Space Studies - Science Briefs |date=2010-10-01 |author=Gavin Schmidt}}</ref>

The second most important greenhouse gas, and the most important trace gas affected by man-made sources, is carbon dioxide.<ref name=":1" /> It contributes about 20% of Earth's total greenhouse effect.<ref name="ngis" />  The reason that greenhouse gases can absorb infrared radiation is their molecular structure. For example, carbon dioxide has two basic modes of vibration that create a strong [[Transition dipole moment|dipole moment]], which causes its strong absorption of infrared radiation.<ref name=":1" />

In contrast, the most abundant gases ({{chem|N|2}},{{chem|O|2}}, and {{chem|Ar}}) in the atmosphere are not greenhouse gases.  This is because they cannot absorb infrared radiation as they do not have vibrations with a dipole moment.<ref name=":1" /> For instance, the triple bonds of atmospheric [[dinitrogen]] make for a symmetric molecule with [[Selection rule#Vibrational spectra|vibrational energy states]] that are almost totally unaffected at infrared frequencies.

Below is a table of some of the major trace greenhouse gases, their man-made sources, and an estimate of the relative contribution of those sources to the ''enhanced greenhouse effect'' that influences [[global warming]].

'''Key Greenhouse Gases and Sources'''<ref name=":1" />
{| class="wikitable"
!Gas
!Chemical formula
!Major human sources
!Contribution to Increase<br>(Year 1995 estimate)
|-
|Carbon dioxide
|CO<sub>2</sub>
|fossil fuel combustion, deforestation
|55%
|-
|Methane
|CH<sub>4</sub>
|rice fields, cattle and dairy cows, landfills, oil and gas production
|15%
|-
|Nitrous oxide
|N<sub>2</sub>O
|fertilizers, deforestation
|6%
|}