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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona 3 The Science of Climate Change and Climate Variability Roger Revelle University of California, San Diego In considering the potential for climate change, we must understand what we mean by climate and what we mean by weather. Weather is the instantaneous condition of the atmosphere at any particular time and place—whether it is raining or the sun is shining, whether the wind is blowing or it is calm, whether it is warm or it is cold. Climate is the average of weather over some time period. We can talk about the climate of a particular season, a particular year, or a particular place. Or, we can talk about the average of weather over several years. Climate has often been defined as the average of weather over 30 years. We are all familiar with climate change: the biggest change occurs between the summer and the winter. This change occurs in most places in addition to any change due to greenhouse gases or the earth's orbital parameters. Between summer and winter in the United States, we may have a 10 or 15°C difference in average temperature and, of course, great differences in precipitation and atmospheric circulation. When we talk about climate change, we are not really talking about something unfamiliar, because the change that we see between summer and winter in the United States is quite large compared to any expected climate change. However, the possibility of an interannual climate change—a change from year to year as contrasted with seasonal changes—is very important for us from many perspectives, particularly from the perspective of water. There is good reason to expect that because of the increase of greenhouse gases in the atmosphere there will be a climate warming. How big that warming will be is very difficult to say. The average temperature will probably increase somewhere between 2 and 5°C at the latitudes of the United States; it will probably change more at higher latitudes and less at lower latitudes.
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona We can be certain that whatever climate change occurs will have a profound effect on some aspects of water resources. First, we can be sure (there is not much uncertainty about this) that the demand for water will increase. Higher temperatures will cause people, farmers, and industries to use more water. Second, the supply of water will certainly change seasonally. The warming climate will likely bring more rain and less snow; therefore, there will be more winter runoff and less summer runoff and probably other, similar seasonal effects caused by the change in the character of precipitation. Third, it will probably be true that the general circulation of the atmosphere will diminish, because the temperature gradients between high latitudes and low latitudes will be considerably less than they are now. Beyond these crude predictions, we are faced with a lack of certainty—really a lack of understanding—of what may happen. We know that atmospheric carbon dioxide concentrations will increase and will continue to increase as long as we use fossil fuels and as long as we persist in cutting down our tropical forests. At least in principle, we know what we can do about carbon dioxide: we could cut down on the use of fossil fuels and we could grow trees instead of cutting them down. But there are other gases that contribute to the greenhouse effect and are harder to manage than carbon dioxide. A second greenhouse gas is methane (natural gas), which is not now but may in the future be as important as carbon dioxide. Methane has a variety of sources: it is partly flared off in oil fields (though a good deal of gas comes out of the ground without being burned when oil is produced); it is produced in forest fires; it is produced by the belching of cattle and the burping of termites; it is produced in swamps and in rice paddies. As far as I can determine, none of these sources of methane is controllable, except possibly the methane released in oil fields. So, we expect the atmospheric methane concentration to continue to increase; it is doubling now about every 10 years, and it probably will continue to increase to about 8 parts per million during the next 50 to 75 years. A third greenhouse gas, nitrous oxide, comes primarily from agricultural use of fertilizers. Its concentration is increasing, though much more slowly than the carbon dioxide concentration, and we can probably do something about it. The fourth greenhouse gas, tropospheric ozone, is really a product of air pollution. If we cut air pollution, in the process we will be cutting tropospheric ozone. We are quite uncertain about the quantity of carbon dioxide that we will release in the atmosphere in the future. If we keep on
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Managing Water Resources in the West Under Conditions of Climate Uncertainty: Proceedings of a Colloquium November 14–16, 1990 Scottsdale, Arizona at the present rate, carbon dioxide emissions to the air will probably increase from 6 billion tons, which is what they are at present, to 15 billion tons as the less-developed countries develop their economies. We cannot expect, and we probably should not even ask, that these nations not use their fossil fuels. The fossil fuel that both China and India have in abundance is coal, not oil or gas, and coal is a messy, dirty, poisonous substance. I hope we can at least reduce its use to a considerable extent in the United States. But until the economies of India and China develop much more than they are now, we cannot expect much reduction in their use of coal. Many ways have been suggested for reducing carbon dioxide emissions. The principal ones, from my viewpoint, are the use of nuclear power as a substitute for fossil fuels and the use of hydrogen (produced by the electrolysis of water, with the primary energy coming from nuclear reactors) as a fuel for transportation. In any case, the concentration of carbon dioxide in the atmosphere will not increase indefinitely, because we will ultimately exhaust the world's reserves of fossil fuels. A quadrupling of atmospheric carbon dioxide compared to mid-nineteenth century levels would be about the highest atmospheric concentration that could be expected in the future and could possibly occur in the twenty-second century.
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