Agriculture Reference
In-Depth Information
8 Trace Gases
This chapter considers the extent, mechanisms and possibilities for control of
emissions of trace gases from submerged soils. The focus is on ricefields because
this is where research has been most intense and because ricefields are the focus
of the greatest scrutiny for possibilities to reduce emissions.
8.1 METHANE
8.1.1 GLOBAL BUDGET
Table 8.1 shows recent estimates of the global methane budget made by the
Intergovernmental Panel on Climate Change (IPCC) (Prather et al ., 2001). There
are substantial emissions from natural sources, particularly wetlands. But anthro-
pogenic sources account for 60% of the total emission and the abundance of CH 4
in the atmosphere is now more than double its pre-industrial value-1745 ppb
(molar mixing ratio in the troposphere) compared with 700 ppb in 1750. The rate
of increase has been near exponential over the last 300 years. However the annual
rate of increase has been highly variable and in the last 20 years it has declined
for reasons that are not fully understood. The current percentage rate of increase
is comparable to that of CO 2 -about 0.4% year 1 . The radiative forcing effect of
CH 4 is about 21 times that of CO 2 per mole of gas. Currently, increases in CO 2
account for about 50% of global warming and increases in CH 4 about 20%.
There is therefore political pressure to decrease man-made emissions of CH 4
by whatever means possible. Since CH 4 has a short lifetime in the atmosphere
(8 years compared with 50-200 years for CO 2 ), modest decreases in emissions
can quickly have a large effect on atmospheric abundance. Adjusting the global
CO 2 balance requires larger percentage as well as absolute changes. Further,
it is argued that the fossil fuels responsible for most of the atmospheric CO 2
increase also produce aerosols that have negative radiative forcing effects in
the troposphere (Hansen et al ., 2000). These include aerosols of non-absorbing
sulfates, which both directly reflect radiation and increase reflection by clouds.
They therefore to some extent mitigate the effects of increased CO 2 , and this
complicates calculations of the relative importance of CO 2 emissions versus other
greenhouse gases.
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