Geoscience Reference
In-Depth Information
Figure 25.2. Absorption of solar radiation by certain atmospheric gases. (Adapted
from Anon.,
1989
.)
radiation, and it therefore acts as a form of thermal blanket, keeping the earth warmer
than it would otherwise be. If the earth did not have an atmosphere, the mean tem-
perature at the surface of the earth would be
C.
The 'greenhouse effect' is a metaphor for this phenomenon. Our present atmosphere
consists of 79 per cent nitrogen, 20 per cent oxygen and 1 per cent other gases. It is
certain of these atmospheric gases that are so important in regulating earth temper-
ature. They include water vapour (H
2
O), carbon dioxide (CO
2
), oxygen (O
2
), ozone
(O
3
), nitrous oxide (N
2
O) and methane (CH
4
), as well as the entirely synthetic green-
house gases: the chlorofluorohydrocarbons (CFCs), hydrofluorocarbons (HFCs) and
halocarbons. Each of these gases has the ability to absorb incoming solar radiation
and outgoing terrestrial radiation at various wavelengths of the electromagnetic spec-
trum (
Figure 25.2
). Ozone is especially important in absorbing potentially harmful
ultraviolet radiation in the upper atmosphere.
Figure 25.2
shows very clearly that if the atmospheric concentration of any or all
of these gases changes, so too will the amount of solar radiation absorbed. If there is
−
18
°
C, rather than the present
+
14
°
