Environmental Engineering Reference
In-Depth Information
going infrared radiation and only about 38% is lost to space. The remaining energy is re-
cycled back to warm Earth and its atmosphere, giving us our more familiar average global
surface temperature of about +14°C. This is known as the
greenhouse effect
and the re-
sponsible atmospheric gases are known as
greenhouse gases
(GHGs). The most important
are:
·
Carbon dioxide (CO
2
).
·
Methane (CH
4
).
·
Nitrous oxide (N
2
O).
·
Sulphur hexafluoride(SF
6
).
·
Hydrofluorocarbons (HFCs).
Perfluorocarbons.
4
·
In addition to concentrations of a GHG in the atmosphere, two other factors are also im-
portant: their relative atmospheric lifetimes and their efficiency as GHGs. Together, these
define their global warming potential (GWP) relative to CO
2
. The most common GHG in
the atmosphere is water vapour. However, it is not thought of as a forcing gas in the context
of global warming (and therefore not included in the list) because it has an atmospheric
lifetime of only about 10 days and, furthermore, its atmospheric concentration is a direct
function of temperature. Nevertheless, it is a very important player in the warming mech-
anism.
By far, the most important GHG in terms of the combination of atmospheric concen-
tration and GWP is carbon dioxide (CO
2
). Of course, our atmosphere has contained CO
2
for billions of years without any anthropogenic input. The question, therefore, is what is
unusual about events at the present time? The answer is that humankind is upsetting the
way in which our planet maintains carbon in a number of linked “reservoirs”. Very briefly,
these reservoirs include the atmosphere, the oceans, the biosphere and the lithosphere (or
outerandsolidlayerofEarth'scrust).Carbonmovesbetweentheminacyclicfashion,such
that CO
2
can dissolve into the oceans and be incorporated into organic matter, sediments
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