Environmental Engineering Reference
In-Depth Information
Methane
Since the Industrial Revolution, the global atmospheric concentration of
methane has increased by 137 percent. About 645 million tons of
methane were emitted by the United States in 2007, nearly the same
amount as in 2000. Its concentration in the atmosphere is only 1.8 ppm,
but its overall impact on climate is relatively large because methane is
about twenty times more potent as a heat absorber than carbon dioxide
is. It is removed by chemical processes in the atmosphere (oxidation)
much faster than carbon dioxide; it has a lifetime of only about twelve
years.
The abundance of methane in the atmosphere rose signifi cantly
between 2007 and 2008, probably because of the increasing and unstop-
pable thawing of permafrost and consequent production of wetlands, the
increasing number of cattle being produced in the developing world, and
the increasing cultivation of rice among the exploding populations of
southern and southeastern Asia (
table 9.4). Cows, sheep, and goats pro-
duced one-third of the methane released in the world in 2007—90
percent by silent burping and 10 percent by fl atulence.
The Environmental Protection Agency estimates that 24 percent of
methane emissions in the United States can be attributed directly to
livestock. Natural outgassing of methane hydrates is estimated to
account for 1 or 2 percent of atmospheric methane. Methane emissions
will certainly increase if methane hydrate reservoirs are exploited for
natural gas (chapter 6), but a more critical concern is a possible cata-
strophic release of methane to the atmosphere or ocean from shallow
Table 9.4
Global sources of anthropogenic methane, 2005
Source
Percentage
Domestic livestock
34
Oil and natural gas
19
Landfi lls
12
Rice cultivation
10
Waste water
9
Other agriculture
7
Coal
6
Biomass burning
3
Source:
Environmental Protection Agency (2008).
Note:
Note the importance of cattle and also that methane from thawing per-
mafrost and from wetlands are not included in this list.
