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'''Methane''' (CH<sub>4</sub>) is a potent greenhouse gas with a high global warming potential 72 times that of carbon dioxide (averaged over 20 years) or 25 times that of carbon dioxide (averaged over 100 years), according to the [[Intergovernmental Panel on Climate Change]] (IPCC)'s Third Assessment Report. Methane in the atmosphere is eventually oxidized, producing [[carbon dioxide]] and water. This breakdown accounts for the decline in the global warming potential of methane over longer periods of time.<ref>Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html "Climate Change 2007: Working Group I: The Physical Science Basis: 2.10.2 Direct Global Warming Potentials"], ''IPCC Fourth Assessment Report: Climate Change 2007'', Cambridge University Press, 2007.</ref>
The [[global warming]] potential of methane was estimated at [http://ghginstitute.org/2010/06/28/what-is-a-global-warming-potential/ 21 times ] that of carbon dioxide, averaged over 100 years, in the IPCC Second Assessment Report(1995), and the 21 figure is currently used for regulatory purposes in the United StatesU.S.,<ref>[http://epa.gov/methane/scientific.html "Methane,"] U.S. Environmental Protection Agency information page, accessed July 2010</ref> although in 2013 the EPA [https://s3.amazonaws.com/public-inspection.federalregister.gov/2013-06093.pdf proposed] increasing the number to 25, in line with more recent scientific estimates.<ref>Jason Mark, [http://www.earthisland.org/journal/index.php/elist/eListRead/methanes_contribution_to_global_warming_is_worse_than_you_thought/ "Methane’s Contribution to Global Warming is Worse than You Thought,"] Earth Island Journal, August 20, 2012.</ref> The 2013 IPCC report, [http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter08.pdf Chapter 8,] notes that if carbon feedbacks from methane emissions are taken into account, then the GWP for methane increases to 86 over 20 years and 34 over 100 years (Table 8.7 on page 8-58 ).
Methane is both a [[greenhouse gas]] and a contributor to background levels of [[ozone]]. Tropospheric ozone is a significant greenhouse gas and the primary constituent of photochemical [[smog]].<ref>[http://www.oar.noaa.gov/spotlite/2006/spot_methane.html "Linking climate and air pollution: Methane emission controls yield a double dividend"] NOAA, accessed November 2010.</ref>
