Methane (CH4) is second only to carbon dioxide (CO2) in its contribution to the greenhouse effect and accounts for about 15% of anthropogenic warming.  Moreover, sudden release of CH4 from the melting of seabed methane hydrate is implicated in such cataclysmic events as the Great Dying and the Late Paleocene Thermal Maximum. Once in the atmosphere, CH4 oxidizes to CO2 in about a dozen years.
Contrary to popular belief, the majority of humans do not release CH4 when they pass flatus gas  — this provides yet another reason why attempts to ignite these emissions are seldom worthwhile. Other human activities, however, do emit large amounts of CH4. Over 75% of the world’s rice is grown in flooded paddies. Soils during flooding soon become anaerobic, and soil microbes generate CH4, or “marsh gas,” through anaerobic respiration. Clearing of agricultural land through burning also produces CH4 because of incomplete combustion. Animals, particularly ruminants such as cows, sheep, and goats, have bacteria in their digestive tracts that generate CH4. A primary component of natural gas is CH4, and losses of CH4 occur during natural gas extraction, processing, storage, transmission, and distribution. Coal deposits also contain trapped CH4 that is released during normal mining operations in both underground and surface mines.
All together, the human activities of agriculture, biomass burning, and energy production release somewhere around 320 gigatons (Gt) of CH4 per year.  These sources, together with CH4 produced by natural wetlands and non-domestic animals such as wild ruminants and termites, emit a total of between 410 and 660 Gt CH4 per year.  Various scenarios about future demographic, social, economic, technological, and environmental conditions predict that global CH4 emissions will be between 500 and 1150 Gt CH4 y-1 by the end of the century and that atmospheric concentrations will reach between 1.4 and 3.6 parts per million (ppm).
 Hansen, J. E. and M. Sato (2001) Trends of measured climate forcing agents. Proceedings of the National Academy of Sciences of the United States of America 98:14778-14783.
 Suarez, F., J. Furne, J. Springfield, and M. Levitt (1997) Insights into human colonic physiology obtained from the study of flatus composition. American Journal of Physiology-Gastrointestinal and Liver Physiology 35:G1028-G1033.
 Chen, Y. H. and R. G. Prinn (2006) Estimation of atmospheric methane emissions between 1996 and 2001 using a three-dimensional global chemical transport model. Journal of Geophysical Research-Atmospheres 111: doi:D10307.
 Wuebbles, D. J. and K. Hayhoe (2002) Atmospheric methane and global change. Earth-Science Reviews 57:177-210.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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