Climate change science
Methane is currently responsible for almost a fifth of the enhanced greenhouse effect, second in importance only to carbon dioxide. Methane has a warming potential more than 20 times greater than carbon dioxide on a volume basis. It is released into the atmosphere from agriculture—rice, cattle and sheep—from landfills, from biomass burning, from the mining and use of fossil fuels— coal, oil and gas—as well as from natural wetlands. Methane has an atmospheric lifetime of about ten years.
There has been a 150 per cent rise in atmospheric methane since pre-industrial times. The 1990s saw a decline in emissions of methane from human activities, resulting in the slower growth of methane in the global atmosphere in recent years. Since 1999 sources of methane from human activities have again increased, although atmospheric methane levels remain relatively stable due to drying of wetlands and consequent reductions in the amount of methane they release.
If the wetland drying trend reverses, and wetland methane emissions return to their normal levels, atmospheric methane levels may again increase. Methane may also be released in the future as tundra thaws in the northern hemisphere and perhaps as methane hydrates in ocean sediments destabilise due to oceanic warming.
Greenhouse gas emission scenarios for the 21st century indicate that changes in carbon dioxide will play the dominant role in future global warming. At present, carbon dioxide accounts for 60 per cent of the total greenhouse gas forcing (that is, the extra heat absorbed in the atmosphere as a result of atmospheric composition changes relative to pre-industrial times).
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