Environmental Engineering Reference
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anthropogenic aerosols is not readily available,
but the aerosols ejected by Mount Pinatubo
raised stratospheric temperatures by between
3.5 and 7°C (McCormick 1992; Gobbi et al.
1992). Some of the energy trapped in this way
will be reradiated back towards the earth's
surface, but most will be lost into space,
particularly if the aerosols have been injected to
high levels in the stratosphere (Lacis et al.
1992). As the particles begin to sink, however,
the zone of warming will be brought closer to
the surface, and an increased proportion of the
energy radiated from it will reach the
troposphere, helping to offset the cooling. The
warming of the stratosphere will also intensify
the stratospheric temperature inversion,
creating greater stability and reducing the
vigour of the atmospheric circulation.
Experiments with GCMs suggest that an
increase in particulate matter in the stratosphere
would dampen the Hadley circulation (see
Chapter 2), slowing down the easterlies in the
tropics and the westerlies in the sub-tropics
(Bolle et al. 1986). However, from their studies
of the Mount Pinatubo aerosols, Brasseur and
Granier (1992) have estimated that the
stratospheric warming following the eruption
strengthened the mean meridional circulation
by approximately 10 per cent.
Records from which anthropogenically
generated aerosol trends can be determined are
sparse, and estimates of their impact have been
developed mainly through theoretical study
rather than by direct observation in the
atmosphere. In reality, it is not yet possible to
prove that human activities have or have not
induced climatic change through the release of
aerosols, nor is it possible to make realistic future
projections. The SMIC Report (1971) recognized
the ability of particulate matter in the atmosphere
to cause warming, but suggested that it was
insufficient to compensate for the cooling caused
by the attenuation of solar radiation. In contrast,
some estimates suggest that it is possible that the
net effect of elevated atmospheric aerosol levels
could be a slight warming, rather than a cooling
(Bach 1979).
Atmospheric turbidity has received less
attention from academics and the media in recent
years. Perhaps the success of local air pollution
control measures has helped to reduce the general
level of anxiety. In the early 1970s, it seemed
possible that anthropogenic aerosols would
increase turbidity sufficiently to cause global
cooling, and possibly contribute to the
development of a new Ice Age (Calder 1974).
Concern for cooling has been replaced by concern
over global warming, mainly as a consequence
of the intensification of the greenhouse effect (see
Chapter 7). It has been argued that the presence
of particulate matter in the atmosphere has
tempered the impact of the greenhouse effect
(Bryson and Dittberner 1976), but it now appears
possible that under some conditions aerosols
actually add to the warming (Bach 1979). Kellogg
(1980) has suggested that, on a regional scale,
the warming effect of aerosols is more important
than the effect of increased carbon dioxide,
although, on a global scale, the situation is
reversed. He also points out that efforts to control
air pollution in industrial areas will ensure that
aerosol effects will decline while the impact of
the greenhouse effect will continue to grow.
Similar issues are considered by Charlson et al.
(1992) in their examination of climate forcing
by anthropogenic aerosols in the troposphere.
They claim that current levels of anthropogenic
sulphate particles are sufficiently high to offset
substantially the global warming produced by
greenhouse gas forcing, but acknowledge the
many uncertainties which remain to be resolved.
The lack of solid data means that many
questions involving the impact of atmospheric
turbidity on climate remain inadequately
answered. The extent of the human contribution
to atmospheric turbidity is still a matter of
speculation. Air pollution monitoring at the local
and regional level provides data on changing
concentrations of particulate matter over cities
and industrial areas, but there is as yet insufficient
information to project these results to the global
scale. In studying the impact of turbidity on
climate, most of the work has dealt with
temperature change, but it is also possible that
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