Geoscience Reference
In-Depth Information
to the enhanced greenhouse effect even if they are less damaging to the ozone layer. It is hoped that by the end of
the twenty-first century stratospheric ozone levels should have returned to their earlier values.
The implications of increased ultra-violet light in significant quantities in the southern hemisphere spring are not fully
known. Children at school in Australia are encouraged to wear protective hats and to avoid bright sunlight, as large
quantities of ultra-violet light are believed to cause skin cancer. Australia does have the highest per capita rate of
skin cancer in the world. There are suspicions that marine plankton may be affected, with unknown effects on the
food chain and ecosystems. Measurements of planktonic production have shown that the reduction of photosynthesis
induced by ultra-violet radiation increases linearly with the dosage of radiation. The productivity was reduced by a
minimum of between 6-12 per cent. Even amphibian eggs appear to be affected by ultra-violet radiation, perhaps
leading to the current decline in the number of amphibians. Land-plant growth can be affected by higher doses of
ultra-violet light, and materials such as plastics can deteriorate with exposure.
There is still much we do not know about the consequences of having used CFCs in the past. It provides a good
example of how human activities can unwittingly have a major impact on a system we do not fully understand.
is emitted by the surface but little is received from the
atmosphere and therefore the temperature falls rapidly. If
the sky is cloudy, the clouds will absorb much of the
radiation from the surface and, because they are also
emitters, more of the radiation will be returned to the
ground as counter-radiation than if the sky were clear. It
is absorbed by the ground, compensating for the emission
of long-wave radiation and so reducing the rate of cooling
at the ground.
Figure 3.5
compares temperatures on clear
and cloudy nights to demonstrate this effect.
Some of the radiation given off by the surface is lost
to space but the majority gets caught up in the two-way
exchange between the surface and the atmosphere and is
the basis of the natural greenhouse effect.
Figure 3.2
shows
the emission and absorption of long-wave radiation as a
proportion of incoming energy. Radiation from the
atmosphere is emitted spacewards as well as downwards.
As there is less water vapour at higher levels, absorption
by the atmosphere is less and proportionally more is lost
to space.
Global radiation balance
Taking Earth as a whole, we know that no part is getting
warmer (apart from the possible 'enhanced' greenhouse
effect) or cooler and so there must be an overall balance.
More short-wave radiation appears to be absorbed by
Earth than leaves it by a mixture of short and long-wave
radiation. The surface seems to be gaining heat (
Figure
If radiation were the only process operating, Earth's
surface should be getting warmer and the atmosphere
cooler. They do not do so because, in addition to radia-
tion, there are thermal energy transfers in the form of
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Figure 3.5
Contrasting diurnal temperature variations on cloudy and clear days at Sheffield, South Yorkshire. Cloudy weather
temperatures increased but night-time temperatures became lower. There is some indication of a slight progressive warming of
both day and night temperatures as a result of the storage of solar energy.
