Climate Models

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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) computer. Such a computer model is described as a General Circulation Model (GCM).

Greenhouse 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 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 effort currently being devoted to running climate models on large computers, 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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