Agriculture Reference
In-Depth Information
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greenhouse effect is good, not bad. The average temperature of the surface
of the earth is about 15°C, and in the absence of any greenhouse gases in
the atmosphere it would be reduced by about 33°C (i.e., to -18°C). An
average temperature of -18°C would make the earth a rather inhospitable
place, and any semblance of modern agriculture would only be possible
in a few locations near to the equator. A change of only a few degrees
in average temperature leads either to ice ages or to melting of the polar
ice, and either of these would destroy much of our present civilization.
Smaller changes lead to productive agricultural land being turned into a
desert. A rise or fall of a few tenths of a degree in the mean temperature
may correspond to quite a large change in environmental conditions. The
difference in mean temperature between now and one of the recent ice ages
is only about 3.5°C. In earlier ice ages the drop in average temperature has
been less than 10°C (not 33°C ).
What people are concerned with, and what is behind the enormous ef-
fort currently being devoted to running climate models on large comput-
ers, is the worry about the possibility of substantial climate change (i.e.,
global warming and changes in precipitation) as a result of changes in the
greenhouse effect induced by human activities. It would be most useful if
it were possible to identify the human activities that affect the climate and
to make reliable predictions of the direct effects of these human activities
on the future climate. However, this cannot be done directly because of
the natural changes that occur all the time. Unfortunately, the system is
so complicated that our present knowledge is somewhat uneven, and our
historical knowledge is very sparse indeed. However, in spite of all the
difficulties, a great deal of effort has gone into climate modeling in recent
years, and some very useful results have been obtained.
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limate Prediction
There are three categories of events that may affect the climate: (1) events
that occur outside the earth, (2) natural events on the surface of or within
the earth, and (3) human activities.
The source of virtually all the energy that drives the atmosphere is the
sun. Thus, any changes in the intensity of the solar radiation arriving at
the earth will clearly have a significant effect on the weather and on the
climate. The intensity of the radiation arriving at the top of the atmosphere
will depend on variations in the intensity of the radiation emitted by the
sun, changes in the transmission properties of space between the sun and
the earth, and changes in the sun-earth distance. Further details can be
found in Cracknell (1994a).
The natural events on the surface of the earth that have an effect on the
climate include plate tectonics, variations in the polar ice caps, volcanic
eruptions, and ocean circulation. These factors have various time scales
associated with them (for further details see, e.g., Cracknell, 1994a).
Human activities that may affect the climate include: (1) increase in the
concentration of carbon dioxide in the atmosphere, largely from the con-
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