Environmental Engineering Reference
In-Depth Information
example, the nature and rate of industrial
development, the extent to which the earth's
forests continue to be destroyed and the success
of programmes aimed at reducing the output of
CO
2
. Most studies have based their predictions
on several scenarios, one of which is commonly
a direct projection of the
status quo
—the IPCC
'business-as-usual' scenario, for example—with
others based on either increase or decreases in
CO
2
and combinations of other gases (Bolin
et
al.
1986, Houghton
et al.
1990). The net result
is that future greenhouse gas levels are usually
presented as ranges of possibilities rather than
discrete values (see Figure 7.7).
Since CO
2
is a radiative forcing agent known
to warm the atmosphere, the rising CO
2
values
can be translated into temperature increases. It
has been estimated, for example, that a 0.3-
0.6°C increase in the earth's surface
temperature has taken place since 1900, at a
rate broadly consistent with that expected from
the rising levels of greenhouse gases (Houghton
et al.
1990). Schneider (1987) claims that the
earth is 0.5° warmer in the 1980s than it was in
the 1880s. The change has not been even,
however. The main increase took place between
1910 and 1940, and again after 1975 (Gadd
1992). Between 1940 and 1975, despite rising
greenhouse levels, global temperatures
declined, particularly in the northern
hemisphere. In addition, analysis of the records
suggests that the relatively rapid warming prior
to 1940 was probably of natural origin (Folland
et al.
1990). Such changes are well within the
range of normal natural variations in global
temperatures (Crane and Liss 1985), but
Hansen and Lebedeff (1988) have calculated
that the warming between the 1960s and 1980s
was more rapid than that between the 1880s
and 1940s, which suggests that the greenhouse
warming may be beginning to emerge from the
general background 'noise'. Hansen, of the
Goddard Institute of Space Studies, claimed
subsequently that the global greenhouse signal
is sufficiently strong that a cause-and-effect
relationship between the CO
2
increase and
global warming can be inferred (Climate
Institute 1988a). The controversy continues,
however. Kheshgi and White (1993) concluded
that it will not be possible to separate a
greenhouse warming signal from the overall
noise until more is known about the dimensions
and causes of natural climate variability. Wigley
and Barnett (1990), in their contribution to the
IPCC Scientific Assessment, took the middle
ground. They noted that there is as yet no
evidence of an enhanced greenhouse effect in
the observational record, but cautioned that
this may be in part a function of the
uncertainties and inadequacies in current
investigative techniques. In short, although
greenhouse-gas-induced warming may not have
been detected, it does not follow that it does not
exist.
Estimates of global warming are commonly
obtained by employing atmospheric modelling
techniques based on computerized General
Circulation Models (GCMs) (see Table 2.4). To
examine the impact of an enhanced greenhouse
effect on temperature, for example, the CO
2
component in the model is increased to a
specific level. The computer program is allowed
to run until equilibrium is established among
the various climatic elements included in the
model and the new temperatures have been
reached (see Figure 7.8). This approach had
produced a general consensus by the mid-
1980s, that a doubling of CO
2
levels would
cause an average warming of 1.3-4°C (Manabe
and Wetherald 1975; Cess and Potter 1984;
Dickinson 1986; Bolin
et al.
1986). The IPCC
assessment produced values of 1.5-4.5°C, with
a best estimate of 2.5°C. These results compare
with the estimate of 4-6°C made by Arrhenius
at the beginning of the century (Kellogg 1987).
Smaller increases have been calculated by
Newell and Dopplick (1979) who estimated
that the temperatures above the tropical oceans
would increase by only 0.03°C on average and
by Idso (1980) who estimated a global increase
of 0.26°C. These lower values are generally
considered to be unrepresentative by most
scientists investigating the problem, however
(Cess and Potter 1984; Webster 1984).
