From AMS Glossary
(Also called marsh gas.) Colorless, inflammable gas of formula CH4; the simplest hydrocarbon.
Methane enters the atmosphere as a result of the anaerobic decay of organic matter in, for example, swamps and rice paddies, and is also produced in large quantities by cattle and termites. It is formed along with coal and oil in fossil fuel deposits, and released to the atmosphere on mining. Methane is itself burned as a fuel, being the major constituent of natural gas. The atmospheric mixing ratio of methane is currently about 1.7 parts per million and has been rising gradually since the industrial era began. The atmospheric lifetime of methane is about eight years. As well as influencing the chemistry of the atmosphere, methane is a strong greenhouse gas and an important source of stratospheric water vapor, and it contributes to global warming.