evolving models of the earth system (National Research Council, 1990b:111). The need to understand global change may well become a powerful force for change in the existing structure of scientific disciplines.

Research on the human dimensions of global change strives to understand the interactions between human systems and environmental systems, particularly global environmental systems, and to understand the aspects of human systems that affect those interactions. Human systems include economies, populations, cultures, governments, organizations that make technological choices, and so forth. Many of them are associated with disciplines that specialize in their study. Environmental systems include systems of atmospheric gas exchange, biogeochemical dynamics, ocean circulation, ecological interactions of populations of organisms, and so forth. These also tend to be associated with academic specializations.

Interactions between human systems and environmental systems have two critical interfaces, as shown in Figure 2-2. One is the subset of human actions that act as proximate causes of environmental change, that is, that directly alter aspects of the environment. The other is the subset of outcomes of environmental systems that proximally affect what humans value.

The example of anthropogenic climate change can clarify the relationships involved. Each human system has its own internal dynamics, and each also interacts with other human systems and the environment. Some of the activities of human systems, such as fossil fuel burning and agricultural conversion of wetlands, are significant proximate causes of global environmental change. That is, they directly alter aspects of the environment in ways that have global effects. These particular proximal causes add carbon dioxide and methane to the atmosphere and thus contribute to the greenhouse effect. The human causes of global environmental change, which are the focus of Chapter 3, include the human activities that proximally, or directly, alter the global environment and the aspects of human systems that explain those activities and therefore affect the global environment indirectly through their effects on the proximal causes. It is important to emphasize that the human causes of global environmental change quite often depend on decisions made and actions taken without any consideration of the global environment.