ergy demand and therefore the global environment, not only in the United States, but also in other countries that aspire to an American style of economy.
It is important to learn about how major, long-term social transformations affect the global environment; the extent to which they can be reversed, slowed, or redirected; and the conditions under which such changes in trajectory are possible.
Analysis of the human dimensions of global change requires a theoretical structure capable of addressing varying time scales, particularly the longer ones that correspond to processes of physical and ecological change (Clark, 1987). However, as a rule, the behavioral and social sciences have focused on phenomena occurring on time scales from milliseconds (e.g., processes of human visual perception) through decades (e.g., adjustments to capital stock, changes in governmental institutions). They have devoted less systematic effort to explaining events on the time scale of decades to centuries, although there are important exceptions in anthropology (e.g., White, 1959; Steward, 1955, 1977), history (e.g., Braudel, 1983, 1984, 1985; Goldstein, 1988; Tilly, 1989, 1990), economics (e.g., Rostow, 1978; Kuznets, 1983; North and Thomas, 1973), geography (e.g., Chisholm, 1982) and political science (e.g., Modelski, 1987). The explanatory variables typically used in the social sciences are not necessarily applicable to the longer time scales. A focus on decades to centuries appears at first glance to favor some explanatory variables over others, and possibly some social science disciplines over others. Several explanatory variables in social science seem immediately applicable to the time scale of decades to centuries:
. Demographic shifts Fertility, mortality (before completion of childbearing), and migration have predictable effects on the sizes of populations over many decades (Lee, 1978; Lindert, 1978). Urbanization and suburbanization take decades to occur and may be stable over much longer periods if they become embodied in long-lived buildings and supporting economic and political structures.