Atmospheric methane comes from both manmade and natural sources. The main sources of methane emissions from human activity are agriculture (especially cattle and rice paddies) and fossil fuels (extraction, transport, and use). Fossil methane is emitted from coal mines, fracking, gas leaks and venting of oil wells. Together, fossil fuels and agriculture account for about 60% of methane emissions. Natural sources of atmospheric methane account for the remaining 40%.
They include vegetated wetland and inland water systems (lakes, small ponds, rivers), thawing terrestrial and marine permafrost, ocean water, and sediments on the ocean floor. Natural, or biogenic, methane is produced and emitted wherever organic material builds up and breaks down in the absence of air – for example, underwater or in landfills – a process called anaerobic fermentation. It is also produced by bacteria in the digestive tracts of animals (for example, cows and termites are significant methane emitters).
Burning biomass, whether naturally or from human activity, also contributes moderately to methane emissions.
A small amount of methane is consumed by soil bacteria. Methane emitted into the atmosphere breaks down naturally through oxidation. Methane’s chemical formula is CH4. When it reacts with oxygen in the atmosphere, it forms water (H2O) and carbon dioxide (CO2). The carbon dioxide produced in methane oxidation is a greenhouse gas, but it has only one-fiftieth (2%) of the global warming impact of unoxidized methane. So despite the carbon dioxide produced, methane oxidation is a huge net gain for the climate.
Increases in atmospheric methane levels are caused by a combination of changes in emissions and in the rate of atmospheric oxidation. When methane emissions are stable, atmospheric methane levels are usually stable. But today, neither is stable. Atmospheric methane concentrations are rising fast – faster than carbon dioxide or other GHGs – and are now at record levels. Since the pre-industrial era, atmospheric methane has risen almost threefold while atmospheric carbon dioxide has increased by less than 50%. The increase in methane is predominantly due to agriculture and fossil fuel use.