Methane is released into the atmosphere by a variety of processes, the most prominent being enteric fermentation of plant material in the stomachs of ruminants, release from both natural wetlands and rice fields, biomass burning, coal mining operations and leakage of natural gas during transmission. Methane concentrations in the atmosphere are more than twice as high as they have been for most of the past 160,000 years. Methane traps heat very efficiently, making it one of the greenhouse gases of most concern.
The atmospheric concentration of methane increased by 10 ppbv in 1992, rising to approximately 1740 ppbv, or more than 130 percent greater than pre-industrial levels. The modest 1992 increase continues a trend towards slower accumulation of atmospheric methane that has been evident since at least 1984, when annual increases were approximately 14 ppbv per year. The declining growth rate appears to be linked to decreasing anthropogenic sources of methane in the mid to high latitudes of the northern hemisphere, where the decreases in annual increments in concentration have also been the most dramatic.
About 70 percent of the estimated 510 Megatonnes (Mt) of methane released into the atmosphere each year comes from natural and human-induced surface biological processes, about 20 percent from the escape of natural gas from fossil fuel sources, and 10 percent from biomass burning. Global wetlands appear to contribute about 75 percent of natural methane emissions, relatively larger in peat-rich wetlands. Past drainage of temperate swamps may already have reduced global emissions from these sources, and additional drainage of swamps or development of drier conditions due to global warming could cause further reductions and even add new sinks. This may be at least partially offset by increased emissions associated with higher ecosystem productivity under warmer climates. The methane content of frozen hydrate in polar regions is very large (over 1,000 million Gt) and could become a major source of emissions if hydrates begin to thaw. Hydrate in the Beaufort-Mackenzie region alone cover an area of 50,000 sq km.
Total anthropogenic sources of methane are estimated at about 275 Mt per year. However, as with natural sources, these estimates have large uncertainties. Measurements of fluxes into the atmosphere from this source vary from a small sink to a source of more than 50 mg/sq m/day. Net global emissions from rice paddies are now estimate at 60 to 100 Mt/year, with China and India collectively contributing between 15 and 30 percent of this amount. Research into landfill methane suggests daily emissions as large as 250 tonnes/sq km of landfill. Methane emissions from energy use are dominated by methane leakage and venting during production, transportation and /or distribution of natural gas and coal. Certain types of hydro power projects may also be significant sources of methane during the first few years after biomass flooding, although recent assessments indicated that most hydro reservoirs are deep enough to allow methane generated from bottom sediments to oxidize before being released into the atmosphere, and hence contribute little to methane emissions. Finally, increased deforestation and wood fuel use may be releasing about 34 Mt of methane each year.