energy use. But there is also a growing body of work on the fundamental social processes that drive human use of the environment.

Human Activity and Land Use Change

Interest in the causes of local and regional land use changes is long standing in the social sciences.20 Significant steps have been made in documenting the long history of human transformation of land cover and in explaining the major forces that drive land use. These studies are of interest to a wide range of social and environmental scientists because land is a key factor in social relationships and resource use. But these studies also provide specific contributions to scientific understanding of biogeochemical cycles (especially the carbon cycle), regional climate modification, and alterations in natural ecosystems and are a critical basis for policies to mitigate and adapt to climate change, conserve biodiversity, and reduce land degradation.21 Land use studies provide a powerful rationale for maintaining land and marine remote sensing satellite systems and suggest ways in which these technologies can be made more germane to decision making.

The global change research community has made considerable progress in recent years on several important questions, such as the social causes of deforestation in regions like the Amazon River basin and Southeast Asia; the role of social, political, and economic institutions in land use decisions; and the relationships between population and land use (and land cover) change.22 There have also been tremendous improvements in the ability to combine social, physical, and remote sensing data within geographic information systems, often with the explicit purpose of understanding how processes at local scales are nested in regional, national, and global scales. 23

Additionally, human dimensions research has highlighted the important distinction between land use and land cover. Whereas land cover refers to the land's physical attributes (e.g., forest, grassland), land use expresses the way such attributes have been transformed by human action (e.g., ranching, crop production, logging); that is, land use measures provide a socioeconomic portrait of a landscape.24 Land cover is directly represented in global climate models. Land use links land cover to the human activities that transform the land.

The emerging field of environmental history has provided important data on the trajectories and causes of land use changes in the past. For example, historical studies of the U.S. Great Plains have shown how changes in the use and management of grazing and croplands relate to government policy and economics and in turn influence the cycling of carbon and nutrients.25 Historians and geographers have also reconstructed the history of human use of such regions as the Mediterranean, Caribbean, and Latin America.26

Historical studies of land use have altered scientific thinking on the past and the present in a variety of ways. For instance, many observers have presumed that much of the humid tropical forests is pristine or that human impacts on the

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