nature of so much human activity, and considering the great uncertainties about the future course of global change, the level of concern about global environmental change now expressed in a variety of public forums is extraordinary. In the interest of providing a common vantage point from which to examine the human dimensions of global change, we present a brief synopsis of current expectations in the scientific community regarding trends in ozone depletion, climate change, and the extinction of species.

There is general agreement that the yearly reductions in ozone over Antarctica are a reliable pattern (seasonal depletions of up to 50 percent were measured in 1987 and again in 1989 and 1990), that emissions of CFCs into the atmosphere are a principal cause of this phenomenon, and that CFCs already in the stratosphere will constitute a growing source of ozone depletion for several decades, regardless of efforts to reduce or eliminate additional emissions. The effects of the resultant increase in ultraviolet radiation (more specifically, UV-B) reaching the earth's surface, moreover, are widely believed to include damage to human health (in such forms as skin cancers, cataracts, and suppression of the human immune response system) and to plants and aquatic organisms (including crops of considerable importance to humans). Beyond this, our knowledge of ozone depletion is less clear-cut. There is some evidence of Arctic depletions that are significant, though less severe, than those recorded in Antarctica; decreases of a few percent are observed. Predictions of future trends in ozone depletion are sensitive to changes in a number of variables, including human responses to the threat of severe ozone depletion. And much remains to be learned about the consequences of increased ultraviolet radiation (Solomon, 1990).

Global climate change is undoubtedly more complex than ozone depletion, more difficult to project, and more important in terms of its potential impacts on human welfare. Projections of temperature trends over the next century are based largely on scenarios of increasing concentrations of greenhouse gases in the earth's atmosphere (chiefly carbon dioxide, methane, CFCs, and nitrous oxides). Projections of the equilibrium temperature response, expressed as a global mean temperature, are shown in Figure 2-1 for four scenarios, one assuming current growth rates and the others assuming progressively increasing controls of greenhouse gas emissions to the atmosphere. The best available analytical tools project that, assuming current growth rates for emissions, we can expect a significant rise in worldwide equilibrium temperature (perhaps 1-5 degrees Celsius by the middle of the

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