Methane Cycle

Methane has many sources, both natural and human-related, but just two major sinks: its destruction in the atmosphere, and its use by certain soil bacteria. Methane is a powerful greenhouse gas, each kg. emitted to the atmosphere having a global warming potential (GWP) 23 times greater than that of a kg. of carbon dioxide over a 100-year time horizon. Since the pre-industrial era, the concentration of methane in the atmosphere has more than doubled, rising from around 715 parts per billion (ppb) in 1800, to more than 1,770 ppb today—higher than at any time in the preceding 650,000 years. Methane emissions resulting from human activities now exceed those from natural sources, comprising around 60 percent of the 600 million metric tons of methane emitted globally to the atmosphere each year.

Methane (CH4) is the main component of natural gas, first discovered in 1776, by Alessandro Volta. It has a relatively short lifetime in the atmosphere, most molecules having been destroyed within 10 years of release. Despite this limited lifespan, it is a powerful greenhouse gas, having a global warming potential (GWP) of 23. This means that every kg. of methane emitted to the atmosphere has the equivalent forcing effect on the Earth’s climate of 23 kg. of carbon dioxide over a 100-year period.

The global methane cycle is comprised of a wide range of sources and sinks. Major natural sources include wetlands, termites, the oceans, and release from hydrates (lattice-like structures of ice and methane, also known as clathrates, occurring in polar regions and in oceanic sediments). Recently, living vegetation has also been suggested as an important natural source of methane. Of known sources, wetlands dominate natural emissions, with between 100 and 200 million tons of methane emitted from these waterlogged soils each year. Wetland methane emissions arise from methane-producing bacteria known as methanogens, which are active in the anaerobic, carbon-rich environments common to wetland soils. The rate of wetland methane production increases rapidly with increasing temperatures, raising concerns that human-induced climatic warming, particularly at high latitudes, may increase global methane emissions and amplify warming.

While the methanogens in the anaerobic soils of wetlands are major producers of methane, soils also represent significant sinks for methane. Methane-utilizing bacteria, called methanotrophs, oxidize much of the methane produced in wetland soils before it can escape to the atmosphere. In well aerated soils, such as those common to forested areas, the methanotrophs are also able to remove methane from the atmosphere. Globally, soil methanotrophs are estimated to remove around 30 million tons of methane from the atmosphere each year. By far the largest sink for atmospheric methane is that of destruction in the troposphere by the hydroxyl (OH) radical; this process removes around 500 million tons of methane each year. Additional destruction of methane OH radicals in the stratosphere is thought to remove 40 million tons of methane per year.

At around 320 million tons per year, methane emissions related to human activities are estimated to exceed those from all natural sources. The bulk of these emissions come from losses occurring during fossil fuel extraction and transport, from ruminant livestock and waste treatment, from landfill sites, from rice cultivation, and from biomass burning. Mitigation policies designed to reduce human-induced methane emissions include the capture of methane produced during fossil fuel extraction and by landfill sites, for flaring or use as an energy source; the use of feed additives to reduce methane production by ruminants; and changes in rice cultivation methods to reduce the length of time that rice paddy soils are in a waterlogged, methaneproducing state.

Concentrations of methane in the atmosphere have more than doubled since the pre-industrial period, rising from around 715 parts per billion (ppb) in 1800, to the current level of around 1,770 ppb. This concentration is higher than at any time in the last 650,000 years, and has arisen because of an imbalance between methane sources and sinks. Since the early 1990s, the rate of increase has slowed markedly, with atmospheric methane concentrations remaining relatively constant since 1999. Analyses indicate that this recent stabilization may be the result of transient reductions in emissions from wetland areas from unusually low rainfall. If rainfall in these areas returns to normal, atmospheric methane concentrations may increase still further, unless emissions from human activities are reduced.