is not sharp; it depends on how rapidly an environmental change spreads in space.

The most prominent global changes, as noted in Chapter 1, are increases in atmospheric greenhouse gases, depletion of the stratospheric ozone layer, and loss of biological diversity. There are others, however, such as pollution of the oceans (a systemic change) and possibly degradation of soil quality (a cumulative change)— and yet others are probably still unknown. People also debate whether the acidification of lakes and forests caused by the long-range transport of airborne pollutants is a global or only a regional change. Nevertheless, the appropriate strategy for scientific analysis of global change seems straightforward. One may conceive of the earth as a complex system composed of a number of differentiable but interacting spheres or subsystems. Some of these, including the atmosphere, the biosphere, the geosphere, and the hydrosphere, can be thought of as environmental systems, in that from the human perspective they constitute the environment. Others, sometimes called the noosphere or the anthroposphere and further subdivided—for example, into economic, political, cultural, and sociotechnical systems—can be distinguished as human systems (the terms environmental systems and human systems are taken from Clark, 1988). Approached in this way, the study of global change centers on efforts to understand how environmental systems at the global level affect or are affected by changes in any one of these spheres or subsystems. Key to this study is understanding the feedback mechanisms between subsystems that either amplify or dampen the initial impacts.

Much public concern with global change comes from the sense that amplifying (positive) feedback mechanisms involving environmental systems may be impossible to control once they get started. Accumulation of greenhouse gases, for example, may raise temperatures sufficiently to increase rainfall in the high north latitudes, threatening the capacity of plants and animals to survive in a rapidly changing environment (ARCS Workshop Steering Committee, 1990). These changes in turn may contribute to an acceleration of climate change by lowering the earth's albedo through reduction in snow cover and sea ice. Warming will also affect the global hydrological cycle by changing precipitation and evaporation patterns, leading to shifts in vegetative cover; these changes in turn could amplify the warming in areas where desertification is taking place. Another possibility is that some global changes may trigger dampening (negative) feedbacks that offset or even dominate the forces unleashed by positive feedback processes.

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