Methane has been the second most important greenhouse gas, after CO2, since the Industrial Revolution. Current atmospheric levels are 1,772 parts per billion by volume (ppbv) - almost 1,000ppbv higher than at any other point in the past 400,000 years.
But measurements by University of California scientists indicate the rate of growth has been falling since the 1980s and reached a virtual standstill in 1998.1 It now seems unlikely the UN’s Intergovernmental Panel on Climate Change’s 2001 prediction of methane levels reaching 1900ppbv by 2020 will be realised.
"If one really tightens emissions, the amount of methane in the atmosphere 10 years from now could be less than it is today," says Professor Sherwood Rowland, one of the paper’s researchers. This would compensate for rising CO2 emissions and cut production of atmospheric ozone, another greenhouse gas, of which methane is a source.
The study suggests that measures to reduce leaks from oil and gas pipelines and storage facilities may have led to the stabilisation. It could also be caused by decreasing natural gas production, or a fall in emissions from coal mining or paddy fields.
The main sources of methane derived from human activity are rice farming, biomass burning, energy use, landfill emissions and the farming of ruminants such as cattle. The gas lasts in the atmosphere for only eight or nine years, but is more than 20 times as potent a greenhouse gas as CO2.
Professor Rowland’s team found considerable fluctuations in methane levels, particularly during the 1990s when there were three significant peaks. They compared these patterns with levels of two other gases that break down in the atmosphere through the same route as methane: ethane, released by biomass burning and the use of natural gas; and the industrial solvent tetrachloroethene.
Their findings suggest the methane spikes were caused by fires in Indonesia and Russia. There was also a correlation with El Niño activity which could be the result of more forest fires during these periods.
The researchers warn that the lull in methane emissions may be temporary. The former Soviet Union’s economic recovery is likely to increase energy-related emissions and various feedback mechanisms could kick in as CO2 emissions rise.
Recent studies suggest the potential for release of methane from thawing permafrost around the Arctic Circle could be disastrous. Hundreds of billions of tonnes of carbon are stored in the frozen peat bogs of Alaska and Siberia and the fossilised soils below them. The rate of melting seems to be faster than originally supposed (ENDS Report 381, pp 34-36 ).
Indeed, a study published by Swedish scientists in November shows peatlands in the country’s north now emit 22% more methane than they did 30 years ago.2 This counterbalances the effect a longer growing season has on carbon sequestration by vegetation and has led to a net rise in climate forcing of about 47%.
A recent report from the UN’s Food and Agriculture Organization reveals that 37% of human-induced methane emissions come from livestock production.3 Once the sector’s nitrous oxide and CO2 emissions are also taken into account, it is responsible for 18% of all human-induced greenhouse gas emissions. This is more than transport, the report says.
Changing dietary habits and growing population and income levels are likely to double meat and milk production by 2050. Knock-on effects will include increased deforestation, desertification and water pollution, the FAO predicts.