Introduction

Methane is a powerful greenhouse gas produced both naturally and through human activities. It was first discovered by Alessandro Volta in 1776 who noticed bubbles rising from a pond and found that these bubbles could be ignited with a naked flame. Methane has a relatively short lifetime in the atmosphere, most molecules having been destroyed within 10 years of their release. However, the concentration of methane actually affects its own atmospheric lifetime. The primary mechanism for its destruction is by reaction with hydroxyl radicals (OH-), the greater the concentration of methane the more the reductive power of the atmosphere (the supply of hydroxyl radicals) is reduced. If this feedback is included, then the true atmospheric lifetime of methane extends to about 12 years.

Methane has a global warming potential (GWP) of 23. This means that every kilogram of methane emitted to the atmosphere has the equivalent forcing effect on the Earth's climate of 23kg of carbon dioxide over a 100 year period.

Concentrations of methane in the atmosphere have more than doubled since the pre-industrial period, rising from around 750 parts per billion (ppb) in 1800 to the current level of around 1750 ppb. In the last decade the rate of increase has slowed, but recent analyses indicate that this has been due to transient reductions in emissions from natural sources brought about by unusually low rainfall. If rainfall in these major source areas returns to normal, methane concentrations can be expected to increase further.

Sources

Natural

Natural sources of methane are dominated by wetlands. Where soils are waterlogged and oxygen concentrations are low or zero, a group of microorganisms called methanogens may produce large amounts of methane as they respire carbon dioxide to derive energy. Wetland methane emissions are thought to comprise around 80 percent of the total natural methane source, with methane release from termites, methane hydrates (forzen deposits of methane), and the emission from the oceans also being important. Total annual methane emissions from natural sources are estimated to be around 250 million tonnes. A paper by Frank Keppler in 2006 suggested that vegetation may also be an important natural source of methane. His study indicated that up to a third of natural methane emissions may actually arise from this source. If true, then the current estimate for wetland methane emissions is likely to be an overestimate, but the true magnitude of the newly-discovered vegetation source is still uncertain.

Human-made

Methane emissions resulting from human activities are now thought to exceed those from natural sources, annual emissions being around 320 million tonnes. The main human-made sources arise from losses occurring during oil, coal and gas extraction, from ruminant livestock and waste treatment, from landfill sites, rice cultivation and biomass burning.

Sinks

The primary sink for methane is its destruction in the atmosphere by hydroxyl radicals. Significant amounts of methane are also oxidised by microorganisms (called methanotrophs) which use the methane as a source of carbon and energy. Methanotrophs are aerobic microbes and are usually found in well-drained soils or similar enevironments where they have a good supply of both oxygen and methane. The uptake of methane by these micobes constitutes only about 5 percent of the total methane sink of between 500 and 600 million tonnes per year. However, the methanotrophs are more important than this figure might indicate, as they also consume a great deal of methane before it is released to the atmosphere.

Further Reading

• Climate Change 2001: The Scientific Basis. IPCC 2001. Full text.
• Greenhouse Gas Sinks. Reay et al. (eds). CABI Publishing (in press).
• Keppler F. et al. (2006) Methane emissions from terrestrial plants under aerobic conditions. Nature 439, 187-191