Other greenhousegases 43Table 3.2Estimated sources and sinks of methane in millions of tonnes
per year.aThe first column of data shows the best estimate from each
source; the second column illustrates the uncertainty in the estimates
by giving a range of values
Source Best estimate Uncertainty
Natural
Wetlands 150 (90–240)
Termites 20 (10–50)
Ocean 15 (5–50)
Other (including hydrates) 15 (10–40)
Human-generated
Coal mining, natural gas, petroleum industry 100 (75–110)
Rice paddies 60 (30–90)
Enteric fermentation 90 (70–115)
Waste treatment 25 (15–70)
Landfills 40 (30–70)
Biomass burning 40 (20–60)
Sinks
Atmospheric removal 545 (450–550)
Removal by soils 30 (15–45)
Atmospheric increase 22 (35–40)
aFrom Prather, M., Ehhalt, D.et al. 2001. Atmospheric chemistry and greenhouse
gases. Chapter 4 in Houghtonet al.,Climate Change 2001. See also Prather,
M.et al. 1995. Other trace gases and atmospheric chemistry. InClimate Change
1994. Cambridge: Cambridge University Press. The figure for atmospheric in-
crease is an average for the 1990s.
The main natural source of methane is from wetlands. A variety
of other sources resultdirectly or indirectly from human activities, for
instance from leakage from natural gas pipelines and from oil wells, from
generation in rice paddy fields, from enteric fermentation (belching)
from cattle and other livestock, from the decay of rubbish in landfill
sites and from wood and peat burning. Details of the best estimates of
the sizes of these sources are shown in Table 3.2. Attached to many of
the numbers is a wide range of uncertainty. It is, for instance, difficult to
estimate the amount produced in paddy fields averaged on a worldwide
basis. The amount varies enormously during the rice growing season
and also very widely from region to region. Similar problems arise when
trying to estimate the amount produced by animals. Measurements of
the proportions of the different isotopes of carbon (see Box on page 37)
in atmospheric methane assist considerably in helping to tie down the