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

Human activities are to blame for around 60 percent i methane emissions into the atmosphere each year.

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 metha-notrophs 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 tones 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; changes in rice cultivation methods to reduce the length of time that rice paddy soils are in a waterlogged, methane-producing 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 due to 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.

sEE ALsO: Carbon Cycle; Greenhouse Effect; Greenhouse Gases; Greenland Cores; Radiation, Absorption.

bibliography. Philippe Bousquet, et al., "Contribution of Anthropogenic and Natural Sources to Atmospheric Methane Variability," Nature (v.443, 2006); Dave Reay, et al., eds., Greenhouse Gas Sinks (CABI Publishing, 2007); Susan Solomon, et al., eds., Climate Change 2007: The Physical Science Basis (Cambridge University Press, 2007).

Dave S. Reay

University of Edinburgh, United Kingdom

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