The global methane budget

The global CH4 budget is composed of a wide range of sources (see Table 1.1 and also Figure 4.5 in Chapter 4) balanced by a much smaller number of sinks, any imbalance in these sources and sinks resulting in a change in the atmospheric concentration. There are three main sinks for CH4 emitted into the atmosphere, with the destruction of CH4 by hydroxyl (OH) radicals in the troposphere being the dominant one. This process also contributes to the production of peroxy radicals, and it is this that can subsequently lead to the formation of ozone and so induce a further indirect climate-forcing effect of CH4 in the atmosphere. In addition this reaction with OH radicals reduces the overall oxidizing capacity of the atmosphere - extending the atmospheric lifetime of other CH4 molecules - and produces CO2 and water vapour. Each year an estimated 429-507Tg (teragram; 1Tg = 1 million tonnes) of CH4 are removed from the atmosphere in this way.

The other sinks are much smaller, with ~40Tg CH4 removed each year by reaction with OH radicals in the stratosphere, and ~30Tg CH4 removed by CH4-oxidizing bacteria (methanotrophs) in soils that use the CH4 as a source of carbon and energy. A relatively small amount of CH4 is also removed from the atmosphere through chemical oxidation by chlorine in the air and in the surface waters of our seas. Though several chapters within this book refer to the global CH4 sinks in terms of their impact on net CH4 emissions - in particular the soil CH4 sink - the focus of this book is on the sources of

Table 1.1 Global estimates of methane sources and sinks

Natural sources

Methane flux (Tg CH4yr')a

Rangeb

Wetlands

174

100-231

Termites

22

20-29

Oceans

10

4-15

Hydrates

5

4-5

Geological

9

4-14

Wild animals

15

15

Wild fires

3

2-5

Total (natural)

238

149-319

Anthropogenic sources

Coal mining

36

30-46

Gas, oil, industry

61

52-68

Landfills and waste

54

35-69

Ruminants

84

76-92

Rice agriculture

54

31-83

Biomass burning

47

14-88

Total, anthropogenic

336

238-446

Total, all sources (AR4)c

574 (582)

387-765

Sinks

Soils

-30

26-34

Tropospheric OH

-467

428-507

Stratospheric loss

-39

30-45

Total sinks (AR4)

-536 (581)

484-586

Imbalance (AR4)

38 (1)

-199-281

Note:' Values represent the mean of those provided in Denman et al (2007, Table 7.6) rounded to the nearest whole number. They draw on eight separate studies, with base years spanning the period 1983-2001. b Range is derived from values given in Denman et al (2007, Table 7.6). Values from Chen and Prinn (2006) for anthropogenic sources are not included due to overlaps between source sectors. c Values in parentheses denote those provided in the IPCC Fourth Assessment Report (AR4) as the 'best estimates' for the period 2000-2004. Source: Values derived from Denman et al (2007)

Note:' Values represent the mean of those provided in Denman et al (2007, Table 7.6) rounded to the nearest whole number. They draw on eight separate studies, with base years spanning the period 1983-2001. b Range is derived from values given in Denman et al (2007, Table 7.6). Values from Chen and Prinn (2006) for anthropogenic sources are not included due to overlaps between source sectors. c Values in parentheses denote those provided in the IPCC Fourth Assessment Report (AR4) as the 'best estimates' for the period 2000-2004. Source: Values derived from Denman et al (2007)

methane, their determinants and their mitigation. Detailed reviews of the key CH4 sinks and their role globally can be found in Cicerone and Oremland (1988), Crutzen (1991) and Reay et al (2007).

Of the many significant sources of CH4 on a global scale, both natural and anthropogenic, the bulk have a common basis - that of microbial methanogenesis. Though CH4 from biomass burning, vegetation and geological or fossil fuel sources may be largely non-microbial in nature, understanding the processes that underpin microbially mediated CH4 fluxes is central to quantifying and, potentially, reducing emissions from all other major sources. In Chapter 2 'The Microbiology of Methanogenesis', Fons Stams and Caroline Plugge review our current understanding of microbial methano-genesis and the interactions between different microbial communities that result in the bulk of CH4 emissions to the global atmosphere.

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