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