Earth Sciences / 45climate, or ‘forces’ it into a (warmer) state than it would be otherwise. The actual average surface
temperature is about 288 K (+15°C), a difference of 38°C. The forcing that achieves this difference
is called the ‘greenhouse effect’ and, clearly, without it life on Earth would be very uncomfortable,
if it were possible at all with surface water, including the oceans to considerable depth, frozen solid
and the liquid water beneath the ice markedly more saline because it would contain the salt removed
from solution as water crystallized to form ice.
Nitrogen and oxygen are almost completely transparent to electromagnetic radiation at wavelengths
greater than 0.29 μm, but some of the incoming solar radiation is absorbed by other constituents of
the atmosphere. Of the total, about 4 per cent is absorbed by stratospheric ozone, 20 per cent in the
infra-red band by carbon dioxide and 13 per cent by water vapour (in three narrow infra-red wavebands
at about 1.5 μm, 2.0 μm, and 2.5–4.5 μm), and 6 per cent by water droplets and dust. At wavelengths
greater than about 4.0 μm, however, several atmospheric gases absorb radiation, each in particular
wavebands related to the size of its molecules. The significance of this arises from the fact that the
Earth, warmed by the Sun, behaves like a black body at a temperature of 288 K and emits
electromagnetic radiation at 4–100 μm, with a peak of intensity around 10 μm. More than 90 per cent
of this outgoing long-wave radiation is absorbed in the atmosphere (BARRY AND CHORLEY,
1982, pp. 33–35). The remainder, about 6 per cent, with wavelengths between those at which it can
be absorbed, escapes into space. These ‘gaps’ in the absorption bands, at about 8.5 μm and 13.0 μm,
are called the ‘atmospheric window’.
Molecules which absorb radiation reradiate it in all directions (see Figure 2.12). Some returns to the
surface, some is absorbed by other atmospheric molecules and some is radiated upwards, out into
space. Of course, the surface is warmed by the Sun only during daytime, but its heat is radiated away
by night as well as by day. Eventually, as much energy leaves the Earth as reaches it from the Sun. It
must do, because otherwise the atmospheric and surface temperatures would either rise or fall steadily
over time; the overall energy budget must balance, and it does, although the transfer of energy is
complicated.
The ‘greenhouse’ metaphor is colourful but a little misleading. It is true that the glass of a
greenhouse is transparent to short-wave radiation and partly opaque to infra-red radiation,
so its action is similar to that of the absorbing gases in the atmosphere, but the temperature
difference inside and outside a greenhouse is due mainly to the fact that air inside is prevented
from being cooled by mixing with air outside. With this minor qualification, however, the
Figure 2.12 The greenhouse effect