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2 Definitions and Implications Of Global Change Global change is a term widely used to describe the effects of human activi- ties on the Earth. Although the term sometimes refers only or primarily to global climate change, the Ecosystems Pane! takes it to mean the interactions between natural changes in the Earth's physical and biological structure and the broader effects of human activity. Thus, global change includes changes in many aspects of the globe's environmental systems, including climate. Global change defined in this way has natural and anthropogenic compo- nents; the latter is largely due to the increasing human population and its activities. The global environment has changed constantly over the history of the Earth end would still be changing in the absence of humans. In this sense, global change is not entirely a byproduct of human actions. Even so, there is ample evidence that humans have vastly accelerated the pace of many other- wise natural kinds of change and have introduced numerous kinds of change previously absent from Earth. These two features of human-induced global change accelerated charges end new kinds of changes act together to alter, impair, or eliminate many of the environmental amenities and services on which human societies are based. The motivation of global change research is to understand the kinds and magnitudes of global change caused by humans, to project the course of change and, where possible, prescribe intervention that would moderate its harmful effects or sustain beneficial effects. Thus the focus is on the anthropogenic rather than the natural component of global change. Even so, prior change is frequently relevant as a scale against which to measure human 8

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DEFINITIONS AND IMPLICATIONS OF GLOBAL CHANGE 9 influences on the magnitude and scope of global changes at present and for the future. Global change connected with human activities first came to broad public attention through forecasts of global climate change occurring primar- ily as a result of human-induced enrichment of the atmosphere with green- house gases. The analysis ofthis problem has proven difficult, in part because of its complexity and in part because the scientific community was initially unaccustomed to environmental work of global scope. Many aspects of the global climate change problem now have been explored through the use of models, experiments, and observations on global scales. The breadth and sophistication of climate change analysis have increased over the past two decades. Scientific momentum has developed around studies of the global carbon cycle, which will ultimately be central to an understanding of the mechanism of human-induced climate change. Global change either in its natural or human-induced forms extends well beyond climate change. Although much is yet to be done in the analysis of climate change, other kinds of change are as immediate and of equally press- ing concern from the viewpoint of human welfare and must be added to the research priorities ofthe USGCRP, as well as international research programs of global scope. Global change occurs at all scales, but this report focuses only on changes that, when aggregated, are significant at a global scale. We are especially interested in processes whose scale is global and in changes that have cumulative global effects, even if the scale of operation is primarily regional or even local. (By regional the panel means areas such as southern Africa, western Europe, or eastern North America; by local it means below the size of a medium-sized U.S. state (e.g., Maine) or country (e.g., Austria, Ghana).) What constitutes significance, of course, is a judgment. As an example, the panel judges the conversion of natural landscapes to provide transportation infrastructure- mainly roads, but also railways, harbors, and airports to be globally significant. Even more significant are the greenhouse gases emitted by vehicles using that infrastructure; the conversion of natural landscapes, including coastal zones, to agricultural and urban landscapes; intensification of various biogeochemical cycles, including nitrogen and carbon; the growing concentration of human populations along coasts and in large urban areas, producing a global change in the distribution of human populations and a global change in the structure and functioning of coastal ecosystems; changes in the structure and functioning of natural ecosystems, including species extinctions and species introductions; changes in the fre- quency and severity of infectious human diseases; changes in the Earth's climate, both natural ones and those caused by human-caused changes in the

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10 GLOBAL CHANGE ECOSYSTEMS RESEARCH atmosphere's Face gases (commonly referred to as global warming); global dispersion of pollutants, such as persistent organic pesticides; and changes in the hydrologic cycle caused by various human uses of water, structural alterations to rivers and their surrounding watersheds, and by climate changes. The implications of these and other changes are the appropriate topics of a global change research program. We need to know whether a change is merely Troubling to those who remember what the world used to be like (but see Pauly 1995 for a description of the "shifting baseline syndrome") or whether it poses significant threats to the health, welfare, and well-being of humans and the other members of the biota. We need to understand the socioeconomic and biophysical forces affecting global changes and the socioeconomic and biophysical consequences of global changes. These are research questions. The results of such research must be understandable and relevant to policy makers.