Environmental Engineering Reference
In-Depth Information
CFCs (see Chapter 6) are aimed at preventing
further damage to the ozone layer, but they will
also have some impact on the greenhouse effect.
However, CFCs have a long residence time in
the atmosphere—up to 400 years in the case of
CFC-13 and CFC-115—and even as emission
rates fall, they will continue to contribute to
global warming for some time to come.
The presence of these other greenhouse gases
introduces a number of uncertainties into the
predictions of future greenhouse levels. None of
them is individually as important as CO
2.
It has
been suggested, however, that their combined
influence on the greenhouse effect is already
equivalent to half that of CO
2
alone (Bolle
et al.
1986), and by early next century their
contribution to global warming could be equal
to that of CO
2
(Ramanathan
et al.
1985). Their
impact would become increasingly important in
the low CO
2
emission scenarios envisaged by
some investigators. The involvement of the CFCs
and N
2
O in the depletion of the ozone layer adds
a further complication. Attempts to mitigate the
effects of these gases on the ozone layer would
also impact on the greenhouse effect. Thus,
although such gases as CH
4
, N
2
O and the CFCs
have received much less attention than CO
2
in
the past, it is clear that plans developed to deal
with global warming must include consideration
of all the greenhouse gases, not just CO
2
(see
Figure 7.10).
which provide a general indication of what the
consequences might be in certain key sectors.
The impact of elevated CO
2
levels on natural
and cultivated vegetation has received
considerable attention. Two elements are
involved since elevated CO
2
has an effect on both
photosynthesis and on temperature. Through its
participation in photosynthesis, CO
2
provides the
carbon necessary for proper plant growth, and
in laboratory experiments under controlled
conditions it has been shown that increased
concentrations of the gas enhance growth in most
plants. The bulk of the earth's vegetation can be
classified into two general groups—C
3
and C
4
—
which differ in the biochemistry of their
photosynthetic processes. The C
3
group makes
up about 95 per cent of the earth's biomass, and
includes important grain crops such as wheat,
rice, and soybeans, while the smaller C
4
group is
represented by maize, sorghum and millet. Most
trees belong to the C
3
group. Both C
3
and C
4
plants react positively to increased levels of CO
2
,
with the former being most responsive (Melillo
et al.
1990). A doubling of CO
2
has increased
yields of maize, sorghum, millet and sugar cane
by 10 per cent in controlled experiments, and
increases of as much as 50 per cent have been
achieved with some C
3
plants (Environment
Canada 1986). The response of natural
vegetation or field crops might be less, because
of a variety of non-climatic variables, but some
trees do seem capable of responding quite
dramatically. For example, Sveinbjornsson
(1984) has estimated that a doubling of CO
2
would double the rate of photosynthesis in the
Alaskan paper birch. Chinese experiments also
indicate that higher concentrations of CO
2
—up
to 400-700 ppmv—would bring about increases
in the trunk weight, diameter and height of
nursery stock, as long as appropriate soil and
moisture conditions could be maintained
(NCGCC 1990). Some investigators consider the
present levels of CO
2
to be suboptimal for
photosynthesis and primary productivity in the
majority of terrestrial plants. They suggest,
therefore, that the biological effects of enhanced
CO
2
would likely be beneficial for most plants
ENVIRONMENTAL AND SOCIO -
ECONOMIC IMPACTS OF INCREASING
GREENHOUSE GASES
Given the wide range of possibilities presented
in the estimates of future greenhouse gas levels
and the associated global warming, it is difficult
to predict the environmental and socio-economic
effects of such developments. However, using a
combination of investigative techniques—
ranging from laboratory experiments with plants
to the creation of computer generated models of
the atmosphere and the analysis of past climate
anomalies—researchers have produced results
