Environmental Engineering Reference
In-Depth Information
result of anaerobic decomposition of organic matter associated with rice paddy
cultivation and the digestive processes of ruminants such as cattle. As both rice area and
ruminant numbers have increased with the rising human population over the last two
centuries, the present annual increase is about 1 per cent per year compared with 0·48 per
cent for carbon dioxide.
Conversely the emission of sulphur by-products from biomass and fossil fuel burning
into the atmosphere leads to the formation of sulphate aerosols which have a strong
regional effect on climate. The suspended particles increase scattering and reflection of
insolation at a greater rate than they absorb outgoing long-wave radiation, so leading to
cooling. Incorporation of sulphate aerosols into climate models does improve the
temperature predictions of the models in comparison with observed temperature changes
of the instrumental period. The enhanced greenhouse effect may be modified to some
extent but the sulphate aerosols will sustain the acidity of precipitation.
The increase in CFCs has had two disturbing impacts which were never foreseen.
CFCs are extremely stable molecules which gradually disperse throughout the
atmosphere. They are destroyed by the action of ultra-violet light in the stratosphere,
yielding free chlorine atoms. The highly reactive chlorine reacts with ozone to produce
chlorine monoxide and oxygen. Chlorine monoxide is unstable, reacting with free oxygen
atoms to form a further oxygen molecule and releasing another free chlorine atom which
can react and destroy more ozone. Although the details are still not fully understood, the
levels of ozone in the stratosphere have been declining, especially over Antarctica in
spring, when very cold temperatures prevail. Concentrations have fallen by over 50 per
cent within the last decade (see box, p. 41). As well as affecting ultra-violet levels at the
surface, and the implications for skin cancer, the degree of heating in the stratosphere will
decrease, changing its temperature structure and perhaps its circulation. CFCs are very
effective absorbers of long-wave radiation, so any increase in their concentration will
lead to an increase in the natural greenhouse effect.
Direct warming of the atmosphere by waste heat also affects atmospheric
temperatures. Estimates of global energy production have indicated that 8 × 10 6 MW are
generated annually, most of it in densely populated urban and industrial areas. Long-
period temperature records at city-centre sites usually show an increase of temperature
through time because of this effect coupled with heat storage by buildings.
FEEDBACK EFFECTS
Whilst external and internal forcing systems may give rise to changes of climate, the
results are not always as straightforward as we might expect because of the complex
interactions which take place within the Earth-atmosphere system. Our climatic system
consists of several subsystems, such as the atmosphere, the oceans, the ice sheets and the
land surfaces. They are all closely related as a system, and changes in one may affect the
others. Moreover, changes within one of the components may act as positive or negative
feedback, ultimately influencing inputs of solar radiation to the ground surface. These
feedback mechanisms are likely to have been responsible for many of the more rapid
fluctuations in climate that have occurred throughout Earth's history.
Positive feedback leads to more dramatic and farreaching changes. The initial effect is
magnified, so that quite small changes in the environment produce major
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