Gennady V Menzhulin Sergey P Savvateyev Arthur P Cracknell And Vijendra K Boken

The climate of a region is a representation of long-term weather conditions that prevail there. Over the millions of years of the existence of the atmosphere on the earth, the climate has changed all the time; ice ages have come and gone, and this has been the result of natural causes. Recently (on geological time scales) the human population has expanded—from half a billion in 1600, to 1 billion in 1800, to almost 3 billion in 1940, and it now stands at about 6 billion. The climate may well now be influenced not only as before by natural events but also by human activities. For example, we are producing vast amounts of carbon dioxide by burning fossil fuels, and this is causing the temperature of the earth to rise significantly. If we argue that we should control our activities to preserve this planet as a habitable environment for future generations, we need to have some scientific knowledge of the effects of our present activities on climate.

In recent years the evidence has been accumulating that on the time scale of decades there is global warming (i.e., the global annual mean surface temperature is increasing). There is also evidence accumulating that part of this increase is a consequence of human activities. The evidence is largely statistical. Within this trend there are bound to be temporal fluctuations and spatial variations. Moreover, in addition to the increase in temperature, it is reasonable to assume that there is, overall, an increase in evaporation of water from the surface of the earth and that there will be a consequent increase in precipitation. But within this overall scenario there are bound to be local variations; some areas may experience more precipitation, but some areas may experience less precipitation. The effect of climate change on the proneness to drought is therefore not uniform but can be expected to vary from place to place. Therefore, whether one is concerned with considering the relation between climate and proneness to drought from the historical evidence or whether one is trying to use models 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 reliable detailed observations of the weather (from conventional records or from satellite data) have only been made over a very short and recent period 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 future, 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 extrapolate 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 reliability. 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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