Warming may increase low-level sea clouds leading to cooling due to the radiative effect of increased planetary albedo. Increases in the level of carbon dioxide in the atmosphere, for example, may stimulate the growth of plants through leaf changes that increase photosynthetic activity and CO2 uptake, thereby moving the planetary climate system back toward its initial condition.

There is nothing new about global change. The earth has always been a highly dynamic system whose atmospheric, biological, and geological properties have changed, sometimes dramatically, over the course of time. And there is nothing new about global change forcing humans to make drastic changes in their ways of life. Between about 12,000 and 25,000 years ago (and later, in Canada)—a recent time in the two-million-year existence of the human species—thick sheets of ice covered most of northern Europe and Canada. The sites of present-day New York and Paris had arctic climates, and sea levels were about 100 meters lower than they are today.

But the global environmental changes occurring now differ from those of the past in at least two ways that have profound consequences for our thinking about this subject. For one thing, the pace of global change has picked up dramatically. Methane concentration in the atmosphere has doubled in the past century; chlorofluorocarbons (CFCs), which accounted for about one-quarter of the anthropogenic contribution of greenhouse gases in the 1980s, were not present in the atmosphere before the 1930s (Houghton et al., 1990). Such changes require analysis on the time scale of centuries or even decades for understanding ozone depletion or global climate change. Equally important is the fact that the global changes we are concerned with today are largely anthropogenic in origin. Humans are no longer simply innocent victims compelled to adapt, in some cases rapidly, to large-scale changes in environmental systems resulting from forces beyond their control. Instead, it is human behavior itself that must be controlled if we are to succeed in ameliorating or redirecting global change.

TRENDS IN GLOBAL CHANGE

A remarkable feature of the current concern about global change is that it is largely anticipatory of any effect on humanity. None of the environmental changes in question has moved beyond the early stages of its projected trajectory, and several of the global changes of greatest concern have yet to manifest themselves in any unambiguous and convincing fashion. Given the short-term



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