Methane is the most abundant reactive trace gas in the atmosphere and arises from both natural and anthropogenic sources. It is a valuable gas and is usable at a wide range of concentrations, down to 5%. In concentrated form it is flammable, representing an explosion risk in confined conditions. The global atmospheric burden of methane (in 1998) was 4850Mt(CH4), equivalent to an average concentration of 1745 parts per billion (ppb). The global methane budget can be modelled by simply considering emissions as increasing the atmospheric burden of methane, with sinks removing methane from the atmosphere – the methane cycle. The concentration of methane in the atmosphere is thought to be increasing at a rate of 22 Mt/yr, due to the imbalance between estimated annual global emissions of 598 Mt and removals of 576 Mt (Figure 4).1 It is therefore important to reduce global emissions to such a level that they are outweighed by methane sinks, so that the concentration of methane in the atmosphere decreases and its subsequent warming effect is reduced. A reduction of global emissions by just 22 Mt per year would result in stabilisation of methane concentrations in the atmosphere. Such a reduction represents just 3.6% of total methane emissions, or 6.1% of anthropogenic emissions. Such small reductions should be attainable. Obtaining a reduction in atmospheric methane concentrations would provide an encouraging example in the fight against global warming.

Methane is emitted from a range of natural and anthropogenic (relating to human activity) sources as a result of the anaerobic decomposition of organic matter, land use changes and fossil fuel related emissions.

The role of bacteria: There are two classes of bacteria actively involved in the methane cycle. Methanogenic bacteria generate methane by breaking down organic matter in the absence of oxygen (anaerobically), releasing carbon dioxide and methane. Conversely, methanotrophic bacteria oxidise methane to carbon dioxide. Methanotrophic bacteria are of two sorts; low affinity oxidation, where methanotrophs oxidise high concentrations of methane at the source of production (usually a population of methanogenic bacteria), and high affinity oxidation, which can oxidise methane present at atmospheric concentrations.

Natural sources: The main natural sources of methane are wetlands, termites and oceans. Wetlands are by far the largest source, accounting for 30% of total emissions (Figure 5), with methane being produced from the anaerobic decomposition of organic matter covered by water. Because this process involves the action of bacteria, the rate of methane production is strongly temperature dependent. Maximum methane production is experienced at temperatures between 37℃ and 45℃ and so future increases in global temperature may enhance methane production from wetlands, thereby reinforcing the greenhouse effect.

Methane is also produced by the digestive processes of termites resulting in the generation of around 20 Mt per year was approximately 5% of world methane emissions. This value is unlikely to change as termite populations are not expanding despite greater availability of biomass due to deforestation.22 Methane emissions from termites should be treated as a significant, but background, source that is likely to remain constant.

Anthropogenic sources: Approximately 60% of emissions are related to human activities. The key anthropogenic sources of methane include fossil fuels, agriculture, landfill and the burning of biomass. Methane emissions arising from the fossil fuel industry form the largest anthropogenic source of methane, estimated to be between 80 and 100 Mt per year. The main sources of fossil fuel related methane emissions are the release of natural gas from coal mining and leakage from gas processing and distribution pipes.