B

H—I—I—I—i I I I I -I—i—I—i—i—I—i—I—I—)■■

H—I—I—I—i I I I I -I—i—I—i—i—I—i—I—I—)■■

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 84 02 03

Year

Fig. 11.1. Changes in global methane (CH4) surface concentrations over the last 20 years. (a) Global average atmospheric CH4 mixing ratios (wavy line) determined using measurements from the NOAA/CMDL cooperative air sampling network. The smooth line represents the long-term trend. (b) Global average growth rate for CH4. (From GLOBALVIEW, 2005.)

are summarized in Table 11.1. It is apparent that anthropogenic sources dominate, being more than double the total from natural sources. The three main anthropogenic sources are evenly spread between fossil fuel-related (27%), waste management (24%) and enteric fermentation of cattle (23%), with biomass burning (11%) and rice paddies (15%) making a non-negligible contribution to the total. Natural sources are dominated by wetland emissions, particularly in the tropics. CH4 can be generated by the decay of organic matter under oxygen-deficient conditions, and the microbes that initiate this decomposition are most active where temperatures are highest.

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