25human set fires (biomass burning), microbial processes in the stomachs of
ruminants,^22 as well as anaerobic conditions common in rice paddies and landfills.
As detailed in Chap. 4 , if human release of methane is to be curtailed, many aspects
of modern society will need to be addressed, including how we heat our homes,
generate our electricity, and produce our food.
Two considerable complications for the proper accounting of the human release
of CH 4 are posed by possible alteration of the wetland source due to climate-change
induced changes of the hydrologic cycle (i.e., floods and drought) as well as the
possible release of prodigious amounts of CH 4 from the Arctic as permafrost thaws,
due to global warming (Koven et al. 2011 ). For now, at least, the source of CH 4 due
to Arctic permafrost is small on a global scale (Kirschke et al. 2013 ).
We turn our attention to the scientific importance of the numerical estimates of
the CH 4 source and sink strengths shown in Fig. 1.9. The globally averaged sink for
CH 4 is 550 Tg per year. The mass of CH 4 in the atmosphere, at present,^23 is about
5326 Tg. The atmospheric lifetime of CH 4 is given by:
(^22) Ruminants are mammals such as cattle, sheep, deer, giraffes, etc. that acquire nutrients by fer-
menting plant-based foods in a specialized stomach prior to digestion.
(^23) We can approximate the mass of CH 4 in the atmosphere by multiplying the mass of the entire
atmosphere, 5.2 × 10^21 g, by the mixing ratio of CH 4 , which is 1.84 ppm or 1.84 out of every million
air molecules. We must also account for the ratio of the atomic mass of CH 4 (16) to the mean
atomic mass of air (28.8). The atmospheric mass of CH 4 therefore equals 1.84 × 10−6 × 5.2 × 10^21 g
× (16/28.8) = 5.326 × 10^15 g = 5326 Tg.
Sewage
Landfills
Biomass Burning
Rice fields
Ruminants
Fossil Fuels
Other Natural
Wetlands
Soil
Tropospheric Cl
Stratospheric Loss
Tropospheric OH
Sources
Sinks
Human
Natural
Sources Sinks
Met
h
ane F
lux
(T
g CH
4
yr
-^1
)600
500
400
300
200
100
0
HumanNaturalGlobal Methane Budget, 2000 to 2009Fig. 1.9 Global methane budget. Source and sinks of atmospheric methane, over the decade
2000–2009, expressed as flux either into or out of the atmosphere. After Conrad ( 2009 ) and
Kirschke et al. ( 2013 ). Human and natural sources, as well as components of all terms, are indi-
cated. See Methods for further information
1.2 The Anthropocene