component of epidemiology. With the rise of new virulent viruses, the importance of a geographic perspective on infectious diseases is more critical than ever. Studies of the diffusion of AIDS (e.g., Gould, 1993; see Sidebar 5.9) offer the promise of enhancing our understanding of not only the behavior of the human immunodeficiency virus (HIV) but also the social and political conditions that have been most conducive to the spread of the virus.

Global Climate Change

During the summer of 1993, record rainfall brought devastating floods to the American Midwest. Plagued by drought only a few years earlier, California experienced damaging floods during the winter of 1994/1995. The summer of 1995 produced record heat waves throughout the United States and an unusually large number of tropical hurricanes. Are these weather events harbingers of long-term climate change that many experts predict based on their assessments of changes in the concentration of ''greenhouse gases" in the Earth's atmosphere? Do they portend more frequent climate-related disasters than in the past?

Addressing these questions requires an understanding of the nature and dynamics of climate change. Climate change involves enormously complex interactions among the atmosphere, hydrosphere, and biosphere (see Figure 2.4). These interactions vary significantly across spatial scales. Thus, geographic perspectives that consider place and scale are essential for understanding potential climate changes. For example, geographers have been leaders in contributing to our understanding of large-scale climate patterns, especially those associated with the hydrologic cycle. As one instance, geographic research has shown that considerably more precipitation reaches the Earth's surface than most previous estimates suggest—and many climate models would indicate (Willmott and Legates, 1991).

One important facet of understanding global climate change is appreciating the nature of climatic variations since the last glacial maximum. By mapping past climate variations, identifying regional continuities, and focusing on the spatial relationships between climate and vegetation patterns, geographic analysis contributes to the larger interdisciplinary efforts to understand the operation of the climate system—past, present, and future. These contributions, in turn, are critical to the development of numerical models that are needed if scientists are to understand the extent to which humans may be modifying the climate system and the implications of those modifications.

COHMAP—the Cooperative Holocene Mapping Project—is an example of a recent interdisciplinary climate change research project with a strong geographic component (COHMAP, 1988; Wright et al., 1993). The simulations developed by COHMAP showed how variations in macroscale controls of climate—for example, the size of ice sheets, ocean temperatures, composition of the atmosphere, and the latitudinal and seasonal distributions of solar radiation—govern regional patterns of climate change (see Figure 2.5).

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