on the interrelationships among these phenomena has significant potential to improve social science modeling over the long-term.

UNDERSTANDING NONLINEARITIES

On the time scale of centuries, major discontinuities in societies are the norm (e.g., North and Thomas, 1973; North, 1981). Very few nations have had the same type of government or the same national boundaries for the past 200 years. Many of these discontinuities can have implications for the global environment. Although this is obviously the case for major wars, discontinuous peaceful change can also have significant effects. Consider the recent example of political changes in Eastern Europe. Evidence presented in Chapter 3 suggests that market economies are much less energy-intensive than state-socialist ones; if this relationship is reliable, the rapid change in Eastern Europe may have significant implications for the global environment.

Research Needs

Although it is hard to imagine progress in social science sufficient to predict the timing of social revolutions, it is possible to imagine a growing ability to predict the probability of such changes. The issue of major social nonlinearities could be approached by working first to identify those major political-economic transitions that have had significant effects on the global environment and then to expand knowledge about the conditions under which such changes occur.

UNDERSTANDING SOCIAL IRREVERSIBILITY

Some changes in human systems seem to be irreversible. Societies that have developed a stable agricultural economy do not revert to a hunting-and-gathering system; industrial economies may decline but do not seem to revert to preindustrial forms; scientific discoveries rarely come undone; new crops or technologies, once proven, do not disappear from the scene. The rise of an automobilized society in the United States, with its infrastructure of roads, homes, and workplaces that depend on roads for access and powerful political interests organized to maintain and extend that infrastructure, may also be essentially irreversible. If the change is irreversible, the implications are profound for en-



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