Environmental Engineering Reference
In-Depth Information
molecule it is about twenty-one times more
effective than CO
2
, however, and its
concentration is increasing at about 0.8-1.0 per
cent per year (Blake and Rowland 1988; Shine
et al.
1990). The most important causes of this
increase are to be found in agricultural
development (see Table 7.1). Biomass burning,
to clear land for cultivation, adds CH
4
to the
atmosphere, as does the world's growing
population of domestic cattle, pigs and sheep,
which release considerable amounts of CH
4
through their digestive processes (Crutzen
et al.
1986). By far the largest source of
agriculturally-produced CH
4
, however, is rice
cultivation. Rice paddies, being flooded and
therefore providing an anaerobic environment
for at least part of the year, act much like
natural wetlands. Their total contribution to
rising CH
4
levels is difficult to measure since 60
per cent of the world's rice paddies are in India
and China—both areas from which reliable
data are generally unavailable. However,
annual rice production has doubled over the
past 50 years, and it is likely that CH
4
emissions
have increased in proportion (Watson
et al
1990), although perhaps not by as much as was
once thought (Houghton 1992).
The energy industry is another important
source of anthropogenic CH
4
. As a by-product
of the conversion of vegetable matter into coal,
it is trapped in coal-bearing strata, to be
released into the atmosphere when coal is
mined. It is also one of the main components of
natural gas, and escapes during drilling
operations or through leaks in pipelines and at
pumping stations (Cicerone and Oremland
1988). Together these sources may account for
15 per cent of global CH
4
emissions (Hengeveld
1991). The disposal of organic waste in landfill
sites, where it undergoes anaerobic decay, is
also considered to be a potentially significant
source of CH
4
. Attempts to provide accurate
estimates of emissions, however, are hampered
by the absence of appropriate data on the
nature and amounts of organic waste involved
(Bingemer and Crutzen 1987).
The lifespan of CH
4
in the atmosphere
averages 10 years. It is removed by reaction with
hydroxyl radicals (OH) which oxidize it to water
vapour and CO
2
, both of which are greenhouse
gases, but less potent than CH
4
(Watson
et al.
1990). Atmospheric OH levels are currently
declining as a result of reactions with other
anthropogenically produced gases such as carbon
monoxide (CO), causing a reduction in the rate
of removal of CH
4
(Hengeveld 1991). The total
impact of decreased concentrations is difficult to
assess, but CH
4
emissions into the atmosphere
continue to grow, and it has been estimated that
an immediate reduction in emissions of 15-20
per cent would be required to stabilize
concentrations at their current levels (Watson
et
al.
1990). The IPCC Supplementary Report noted
some evidence that the rate of growth in CH
4
concentration in the atmosphere may be already
beginning to slow down (Houghton 1992). Even
with this, however, potential feedbacks working
through such elements as soil moisture levels and
rising high latitude temperatures, could result in
significant increases in future CH
4
emissions. All
of these trends suggest that CH
4
will continue to
contribute to the enhancement of the greenhouse
effect well into the future.
The current atmospheric concentration of
N
2
O—at 310 parts per billion by volume
(ppbv)—is about a thousand times less than that
of CO
2
, and it is increasing less rapidly than either
CO
2
or CH
4
. N
2
O is released naturally into the
atmosphere through the denitrification of soils,
and is removed mainly through photochemical
decompostion in the stratosphere, in a series of
reactions which contribute to the destruction of
the ozone layer (see Chapter 6). It is thought to
owe its present growth to the increased use of
fossil fuels and the denitrification of agricultural
fertilizers. The IPCC assessment has concluded,
however, that past estimates of the contribution
of fossil fuel combustion to the increase are too
large—by perhaps as much as ten times—and
N
2
O production rates during agricultural activity
are difficult to quantify. Thus, although the total
increase of N
2
O can be calculated, the amounts
attributable to specific sources cannot be
predicted with any accuracy. It is even possible
