and services, the increasing importance of human population migrations as an environmental threat, and the environmental significance of major technological trends affecting the rates of substitution of inexhaustible resources for depletable ones.
Research on the impacts of past climatic variability on societies and economies shows that these impacts depend as much on the social systems that produce vulnerability as on the biophysical systems that cause environmental change. Vulnerability depends on a number of factors, including intensity of land and water use and population immigration in marginal areas, access to economic resources, infrastructure for hazard response, the health status of potentially affected populations, and the structure of the hazard management systems a society has in place to prepare for and manage environmental events. Vulnerability analyses are essential for estimating the human impacts of environmental change and variation. For instance, climate models can be linked to crop models and estimates of vulnerability to provide early warnings of famine, and ecological models can be linked to vulnerability analyses to estimate the effects of global change and climate variability on human health.
Research on human use of common-pool resources has shown that the "tragedy of the commons" scenario is not inevitable. Tragedy has often been prevented and resources sustained over periods of generations to centuries by the design of institutions that monitor the conditions of resources locally, effectively govern access to them, establish norms of resource use and sanctions for overexploitation, and appropriately link local institutions with those at higher levels. A key to implementing effective responses to global change is to design incentive-compatible institutions, that is, institutions capable of internalizing the overall costs of environmental degradation for the individuals, private firms, and public organizations whose actions create environmental stress. Ongoing research on existing institutions and on the theory of institutional design is clarifying the conditions for successful long-term environmental resource management and the institutional structures that have been successful with particular types of resources. Better understanding of institutions that shape human interactions with the environment, these institutions' functioning, and their linkages is essential to forecasting global change and developing policy responses that reduce vulnerability as well as to effective long-term resource management.