Atmospheric methane, anthropogenic

Concerns over atmospheric methane as a greenhouse gas and the large contribution of biomethanogenesis as a source of this gas make it important to determine the relative significance of various components of this activity. A recent paper (8) summarized estimates (28-30) of source fluxes of atmospheric methane based on several carbon isotopic studies and presented new data on natural sources and biomass burning. These data (Table III) show that of a total flux of 594 million tons (Tg) per year, 83% is produced via biomethanogenesis from a combination of natural (42%) and anthropogenic (41%) sources. 

A tritium content of approximately 2 x 10 TU in atmospheric methane was measured between 1960 and 1963. The tritiated methane, CH3T, in the atmosphere could be accounted for by a leakage equivalent to less than 1% of the maximum amount of tritium estimated to have been released anthropogenically. 

Etheridge D. M., Steele L. P., Francey R. J., and Langenfelds R. L. (1998) Atmospheric methane between 1000 AD and present evidence of anthropogenic emissions and climatic variability. J. Geophys. Res.-Atmos. 103, 15979-15993. 

The total annual input of methane from all sources to the atmosphere shown in Table 6.4 is 540 Mt, while the estimated output from atmosphere to sinks is 500 Mt. The potential inaccuracies in flux data can be seen by comparing the observed carbon isotopic signature of atmospheric methane of —47%o with that calculated from the data in Table 6.4 of c— 54%o (the latter is actually equivalent to —58%o upon correcting for the kinetic isotope effect (see Box 1.3) that operates during the hydroxyl abstraction reaction). There are clearly major gaps in our understanding of the pathways of methane into and out of the atmosphere and the fluxes involved, as there are for many anthropogenic substances (see Chapter 7). 

Etheridge, D.M., L.P. Steele, R.J. Francey, and R.L. Langenfelds, Atmospheric methane between 1000 AD and present Evidence of anthropogenic emissions and climate variability. J Geophys Res 103, 15 979, 1998. 

Wetlands, including tropical and subtropical irrigated rice, have soil conditions suitable for both methane and nitrous oxide formation and, as a result, are major anthropogenic sources of atmospheric methane and nitrous oxide. Aerobic and anaerobic enviromnents existing in wetland soil-plant systems provide conditions for both production and consumption of methane and nitrous... 

Atmospheric methane is produced from both natural sources (e. g. wetlands) and human activities. The total sources of methane released into the atmosphere for 1990 were (the IPCC 1995 estimates are in parenthesis) in the range of 200-520 (410-660) Tg CH4 per year, of which 70-140 (110-210) Tg CH4 yr derived from natural sources and 130-380 (300-450) Tg CH4 yr" from anthropogenic sources (IPCC 2007). The anthropogenic sources are further broken down into 50-70 (70-120) Tg CH4 yr related to fossil fuels and 80-310 (200-350) Tg CH4 yr from biospheric sources (Table 2.69). All estimates are considerably smaller than those provided by IPCC (1995). 

Also, human activity 8,000 and 5,000 years ago may have added. .. L. Mitchell et al. Constraints on the late holocene anthropogenic contribution to the atmospheric methane budget. 20i3. Science 342(6161), p. 964. DOl 10.1126/science. 1238920. 

With the addition of CO caused by photochemical oxidation of methane, a significant flux enters the atmosphere annually, but the principal global contributions are terrestrial, anthropogenic and from atmospheric photolysis of methane. 

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