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.
Methane has 25 times the capacity of carbon dioxide to trap heat in the atmosphere. Its pre-industrial level was 0.75 parts per million and the 1986 level was 1.65 parts per million. The current annual increase is 1%.