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1.2.2. Sources and Sinks of Methane
Methane (CH4) is the most important greenhouse gas in the atmosphere after
water vapor and CO2. CH4 concentrations have increased from about 700 ppbv in
pre-industrial times to about 1700 ppbv today (Etheridge et al., 1992;
Prather et al., 1995). About 550 Mt CH4 yr-1 is emitted into the atmosphere
from a variety of sources; chemical reaction with OH radicals and (to a smaller
extent) uptake by soils remove approximately the same amount (Prather et
al., 1995). The small imbalance between global production and destruction
of CH4 resulted in an increase in the atmospheric concentration at a rate of
13 ppbv yr-1 during the early 1980s. This rate diminished, however, to 8 ppbv
yr-1 in 1990 and dropped further to 4 ppbv yr-1 in 1996 (Steele et al.,
1992; Dlugokencky et al., 1998). The lifetime of CH4 with respect to
the OH sink is about 9 years (Prinn, 1994), so the characteristic adjustment
time of the atmospheric concentration to a perturbation in emissions is much
shorter than for CO2.
During pre-industrial times, wetlands (bogs at high northern latitudes and
swamps in the tropics), termites, and wild animals (Chappelaz et al.,
1993) controlled atmospheric CH4. Anthropogenic CH4 sources are associated with
rice cultivation, cattle breeding, biomass burning, waste treatment (landfills,
sewage, and animal waste), and the use of fossil fuels, including natural gas
and coal extraction as well as petroleum industry activities in general (Prather
et al., 1995) (Table 1-3). At present, anthropogenic
sources represent about 70 percent of total CH4 emissions. Although the global
CH4 source is relatively well known, the magnitude of individual sources is
still uncertain (Fung et al., 1991; Prather et al., 1995). Table
1-3 lists estimated land use-related emissions of CH4 on both a CH4- and
a CO2-equivalent basis.1 The likely changes
in these CH4 sources and sinks associated with changes in land use and other
modifications of terrestrial ecosystems are uncertain. There may also be indirect
changes resulting from human activities in accordance with Articles 3.3 and
3.4 of the Kyoto Protocol.
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Table 1-3: Global estimates (Prather et al., 1995) of recent sources
of CH4 and N2O that are
influenced by land-use activities.
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|
| CH4 Sources |
Mt CH4 yr-1
|
Gt C-eq yr-1 a b
|
|
| Livestock (enteric fermentation and animal waste) |
110 (85-130)
|
0.6 (0.5-0.7)
|
| Rice paddies |
60 (20-100)
|
0.3 (0.1-0.6)
|
| Biomass burning |
40 (20-80)
|
0.2 (0.1-0.5)
|
| Natural wetlands |
115 (55-150)
|
0.7 (0.3-0.9)
|
|
| N2O Sources |
Mt N yr-1
|
Gt C-eq yr-1 a c
|
|
| Cultivated soils |
3.5 (1.8-5.3)
|
0.9 (0.5-1.4)
|
| Biomass burning |
0.5 (0.2-1)
|
0.1 (0.05-0.3)
|
| Livestock (cattle and feed lots) |
0.4 (0.2-0.5)
|
0.1 (0.05-0.13)
|
| Natural tropical soils-wet forests |
3 (2.2-3.7)
|
0.8 (0.6-1)
|
| Natural tropical soils-dry savannas |
1 (0.5-2)
|
0.3 (0.1-0.5)
|
| Natural temperate soils-forests |
1 (0.1-2)
|
0.3 (0.03-0.5)
|
| Natural temperate soils-grasslands |
1 (0.5-2)
|
0.3 (0.1-0.5)
|
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a 12 Gt C-equivalent = 44 Gt CO2-equivalent.
b Carbon-equivalent emissions based on CH4 GWP of 21.
c Carbon-equivalent emissions based on N2O GWP of 310.
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