Agriculture Reference
In-Depth Information
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to predict the effect of future climatic conditions, it is necessary to consider
the local spatial variations.
To quantify the effects of past human activities on the climate is difficult.
The climate is a temporal average taken over a long time scale, whereas re-
liable detailed observations of the weather (from conventional records or
from satellite data) have only been made over a very short and recent pe-
riod of time. Archaeological or geological evidence that we have for longer
periods does exist but is of a much less detailed nature. Consequently it
is difficult to determine the extent to which human activities over the last
century or two have already affected the climate significantly. For the fu-
ture, it is extremely important to try to construct models that make use
of the environmental parameters (such as the increase in carbon dioxide
concentration) that can be quantified and use these models to predict the
effects on the climate. Constructing such models is possible, but serious
difficulties begin to arise when one tries to use them. This is because the
atmosphere/earth system is very complicated and one is trying to extrapo-
late its behavior over a long period of time, when we cannot even predict
the weather more than a few days in advance with any degree of reliabil-
ity. It is, perhaps, not surprising that different groups of people working
with different models often produce quite different predictions of climate
change; nevertheless, there is enough agreement that a number of general
conclusions have been obtained.
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Long
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limate Models
In constructing a climate model one needs to identify the processes that
affect the atmosphere, including the interactions between the atmosphere
and the surface of the earth (land, oceans, ice masses) (Cracknell, 1994a,
1994b, 2001). The mathematical set of simultaneous integro-differential
equations that denote these processes then needs to be constructed. These
equations will include various atmospheric parameters, for which values
have to be assigned before the equations can be solved. The process of
solving these equations has to be programmed for a (large and fast) com-
puter. Such a computer model is described as a General Circulation Model
(GCM).
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reenhouse Effect
An important factor is the extent to which solar radiation that reaches the
earth is trapped in the atmosphere and the extent to which it is reflected or
reradiated to outer space. Various atmospheric constituents, most notably
water vapor and carbon dioxide, act in much the same way as the glass
in a greenhouse and cause solar energy to be trapped; this is commonly
referred to as the greenhouse effect. The greenhouse effect thus causes the
atmosphere to be considerably warmer than it would be if these materials
were not present in the atmosphere.
It is important to realize that from the point of view of life on earth, the
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