Environmental Engineering Reference
In-Depth Information
from ultra-violet radiation, scientists and politicians agreed that something had to be
done. The first aim was to reduce the production of CFCs, which appeared to be the main
source of chlorine. An agreement was reached at Montreal in 1987 to phase out the
production and use of CFCs as soon as possible; 'as soon as possible' meant a 50 per cent
reduction by 1999. Recognizing that this was too slow, a further agreement in London in
1990 recommended the elimination of the use of CFCs by industrialized countries by
2000. Despite this, ozone levels have continued to decline, though the rate of decrease
does appear to have slowed down. Ironically the replacement gases will still contribute 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. 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 and 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.
There is still much we do not know about the consequences of using CFCs in aerosol
cans and refrigerators. It provides a good example of how human activities can
unwittingly have a major impact on a system we do not fully understand.
scattered (in the atmosphere) or reflected (by clouds and at the surface). When reflected,
the radiation is returned to space in the short-wave form and becomes part of the outflow
of energy from Earth. Similarly, some of the scattered radiation is returned to space to
give a short-wave albedo for our planet of 28 per cent. The modifications of the solar
beam by the atmosphere are shown diagrammatically on the left-hand side of Figure 3.3.
LONG-WAVE RADIATION
All substances emit long-wave radiation in proportion to their absolute temperature.
Earth's surface absorbs most short-wave radiation and therefore normally has a higher
temperature than the atmosphere. It follows that more long-wave emission will be from
the ground surface. The atmosphere is much more absorbent of long-wave radiation than
of short-wave radiation. Carbon dioxide and water vapour are very effective absorbers of
much of the longer part of the spectrum except between 8 µm and 12 µm. As water
vapour is concentrated in the lowest layers of the atmosphere, that is where most
absorption will take place. Clouds are also very effective at absorbing longwave radiation
and hence their temperature will be higher than otherwise. This cloud effect is most
noticeable at night. With clear skies and dry air, long-wave radiation 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
