Environmental Engineering Reference
In-Depth Information
atmosphere. In summary, the model is started off with a known climatology, usually
resembling that of the present Earth. The data are provided for a grid network with
horizontal separation of several hundred kilometres (usually 3° latitude by 3° longitude)
and information for several heights into the atmosphere for the vertical resolution; more
information is obtained about the lower levels of the atmosphere than about the higher
levels. The solar input and radiational output are readily known and the main problem is
to model the relationship between the surface and the atmosphere. To do so, a number of
assumptions about and simplifications of the interactions have to be incorporated into the
model. An example is the role of clouds, which was discussed in Chapter 4. A major
complication is how to link the rapid atmospheric movement with the much slower
circulation of the oceans and even slower responses in ice sheets. Early models used
fixed sea surface temperatures based on present-day values which were allowed to vary
seasonally or incorporated the meridional energy transport of the oceans. The latest
models can allow for vertical and horizontal exchanges in the oceans to give more
realistic results. Close interaction between the two subsystems of air and ocean is
impossible because the ocean layer needs a much longer time to reach equilibrium from
any given change. That is why sea surface temperature anomalies are more persistent
than those of the atmosphere.
General circulation models simulate the behaviour of the real atmosphere and
reproduce the main circulation features outlined in this chapter. Even individual weather
systems are generated by the computer model. The models can either be used for short-
period weather prediction extending to about ten days ahead, or they can be modified for
climate prediction. In that case the model is run to simulate several decades, to ensure
that it reproduces the real atmosphere adequately. Once it is in equilibrium, a variable
may be changed. We could alter the concentration of carbon dioxide or the nature of the
ground surface to simulate Amazonian deforestation. The model is then run repeatedly
with increasing levels of carbon dioxide or reduced areas of forest to see what the effect
on the circulation would be. A novel use of GCMs is to attempt to reproduce former
circulations. With improved observational techniques world-wide climatic records are
being obtained from soils, lake and ocean sediments and ice strata. This wealth of
knowledge can be used to infer the nature of the atmosphere and ground surface
conditions in the past. It is now possible to allow for changes in Earth's orbit around the
sun, to simulate the effect of increased areas of ice at the surface during the last Ice Age,
and even to change the location of the continents to determine what their impact might
be.
Although GCMs have a number of limitations they are at present the best way of
estimating possible climate change. Developments are taking place in two ways. First,
improvements in computer power will allow us to incorporate more information and
make calculations even more quickly. In that way the horizontal and vertical resolution of
a model can be increased so that the initial state of the systems can be portrayed more
precisely. Second, the modelling of the interaction of air, land, ice and water needs to be
improved, perhaps with the incorporation of chemical interactions such as the changes in
stratospheric ozone.
Numerical modelling of the global climate system has led to a better understanding of
how it works. As human activities may be altering the climate, it is of vital importance to
be reasonably certain what the implications are
It will require major resources in
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