activity, glacial surges, and other natural external and internal forcing mechanisms. It is impossible to predict the future direction of natural climatic variations, although, on the basis of the temperature record, the natural rate of change of mean surface temperature has been predicted to be about -0.15°C for the next decade.^ Natural trends in the temperature are not likely to exceed _+0.25°C per decade.Assuming a doubling of CO2 levels by the year 2040 and a resulting temperature perturbation of +1.5° to 3.0°C, the rate of change of temperature due to this impact would be of the order of +0.3° to +0.6°C per decade. It is apparent that man's influence on climate can exceed the natural fluctuations of climate; however, the latter at present are still greater than man-induced fluctuations. The occurrence of natural climatic fluctuations necessitates the development of climate models to predict climatic change, because the record of measured temperature change may not reveal the forcing parameters at work. 2.4.3 Other Forcing Parameters In addition to the impacts of waste heat, particles, CO2, and natural climatic fluctuations, numerous other anthropogenic emissions can influence climatic change. Of particular importance may be the pollutants that affect the stratospheric ozone layer. Chlorofloromethanes (FCs) have been recently introduced into the atmosphere through uses such as propellants in spray cans. The migration of FCs and their reactive by-products to the stratosphere can result in a depletion of stratospheric ozone.$0 A similar effect can be produced by nitrous oxide (N2O) and its by-products. N2O is produced by biological decay, as well as by conversion processes, and may be increasing in the atmosphere because of the increased use of nitrate fertilizers.52 An increase in CO2 could have a significant impact on stratospheric ozone levels due to feedback effects; however, the direction of the temperature change is not altogether certain. There is no simple, straightforward way of relating perturbations of stratospheric ozone (O3) to climatic impacts. The vertical distribution of O3 in the atmosphere is as important as the total amount. Above-average ozone concentrations in the northern hemisphere have been associated with below- average temperaturesbut this does not establish a cause-effect relation-
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