the environment at the global scale, and for theory to relate smaller-scale activities to the global scale.

Global and continental-scale analyses are needed because social science has done relatively little work at those scales and because so many important human-environment processes have global causes. The theoretical needs can be illustrated with the example of the causes of aggregate global deforestation. Relevant variables surely include population and technological change, levels of and inadequacies in market development, numerous aspects of socioeconomic organization, and national policies (Clark, 1988; Turner, 1989). These variables are typically measured at the national level and globally aggregated. But other variables at the truly global level of analysis may also be related to deforestation: global industrialization, market penetration, and flows of investment and information are examples.

Analysis at the global scale may be needed even for local responses to environmental problems. For example, regulation of localized industrial pollution can diminish a country's attractiveness to international investment; the depletion of resources at one locale can increase pressures on other sources of supply; local environmental disasters or degradation may prompt migration to other areas.

Events at lower levels of spatial aggregation are also significant for global change studies. Human-environment studies at the scales of regions or places, focusing on nation-states, firms, social groups, and individuals can show how specific sites and situations affect the ways in which the earth is sustained, altered, or transformed and the ways humans are affected by global change. They illustrate the unevenness of the processes and impacts of change, even systemic change.

The regional approach is important for linking analyses at lower levels to global processes. For example, population pressures and market-based demand, which vary in strength across the globe and by resource and environmental setting, can have global environmental effects. Albedo changes in the North American Great Plains are, in part, a response to agricultural land use changes that are, in turn, influenced by national and international (but not local) agricultural demands. Albedo changes in the West African Sahel, by contrast, are a response to land use changes created by the dynamics of international markets and local subsistence needs, but not national agricultural demand. The pressure of growing populations on local resources can often be traced to the diffusion of medical and public health technologies that have lowered birth