Figure 7.12b illustrates how this emitted radiation is absorbed by various atmos-
pheric gases. As Figure 7.12 shows, much of the UV radiation impinging on the
atmosphere is absorbed by O 3 molecules in the stratosphere, and this explains the
concern that human-induced decreases in stratospheric O 3 may lead to larger
amounts of harmful UV radiation reaching the Earth’s surface (see Section 3.10).
Much of the remainder of the solar energy passes through the atmosphere
without major absorption.
Turning now to the Earth’s emission spectrum, it is the CO 2 absorption band
centred around 15mm which is particularly important here. This, together with
other absorption bands due to water molecules, means that the atmosphere is
considerably warmer (mean temperature about 15°C) than the effective emission
temperature of the Earth (–23°C; Fig. 7.12a). The combined effect of the atmos-
phere’s transparency to most of the incoming solar radiation, and the absorption
of much of the Earth’s emitted radiation by water and CO 2 molecules in the
atmosphere, is often referred to as the ‘greenhouse effect’—by analogy to the role
played by the glass of a garden greenhouse.
From the above discussion, it is easy to see why elevated concentrations of
CO 2 in the atmosphere resulting from fossil fuel burning are likely to lead to a
warmer climate. However, close inspection of Fig. 7.12 indicates that there is suf-
ficient CO 2 in the pre-industrial atmosphere for the 15mm band to be absorbing
almost 100% of the energy in that wavelength range coming from the Earth.
Although the CO 2 absorption band will broaden as CO 2 concentrations rise, a
major effect is for more of the absorption to occur lower in the atmosphere with
less at higher altitudes. The result is that the lower layers warm, whereas higher
up there is cooling.
Highly sophisticated mathematical models are used to predict the details of
the temperature changes to be expected from rising levels of atmospheric CO 2.
The results of one rather straightforward model are shown in Fig. 7.13. The
model confirms the simple prediction made above; the lower atmosphere warms
by about 3°C for a doubling of atmospheric CO 2 (although the distribution of
the increase varies considerably with latitude), with a concomitant decrease in
temperatures aloft. Figure 7.11 shows that, for many fossil fuel consumption
scenarios, such a doubling might occur some time in the second half of this
century.
Although CO 2 is the most important of the anthropogenic greenhouse gases,
it is not the only one of significance. Figure 7.14 shows, for the period 1980–90,
the relative contributions of various gases to the change in the total greenhouse
gas forcing over that decade. Just over half the effect was due to CO 2 but other
gases, including methane (CH 4 ), nitrous oxide (N 2 O) and chlorofluorocarbons
(CFCs) also contributed substantially to the total effect. In the case of these other
gases, although the absolute amounts entering the atmosphere were small com-
pared with CO 2 , their contributions to the greenhouse effect were proportion-
ately large due to their absorption of energy being in parts of the Earth’s emission
spectrum (Fig. 7.12) which are not saturated. To illustrate this we should note
that on a molecule-for-molecule basis methane is about 21 times more effective
at absorbing energy than CO 2 , and CFC-11 is 12 000 times more effective. The258 Chapter Seven