| Summary: | The chapter describes the mechanisms by which methane is produced in anaerobic environments. Various methane sources and sinks, both natural (e.g. wetlands) and anthropogenic (e.g. landfills, agriculture, fires), are described. The decomposition of organic material in the soil is described as a continuum within the soil matrix, rather than a separation into labile and stable pools. The different pathways of methane production under anaerobic conditions—the acetate pathway and the hydrogen pathway—are described. The roles of wetlands, water bodies, permafrost and clathrate in storing and emitting methane are elucidated. At the geological scale, the chapter discusses the role of methane as a greenhouse gas in providing a habitable climate under a fainter sun (the faint sun paradox), in glacial–interglacial transitions and in the current anthropogenic perturbation. Future methane emissions, global warming potential and the sensitivity of the important methane stores to climate change are also discussed.
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