respond effectively to the impacts of global change as they become apparent. Many governments are already beginning to take actions to mitigate ozone depletion and other forms of global change by limiting CFC production, promoting energy conservation, and other means. As with other major public policy initiatives, it is important to evaluate both the intended and the unintended consequences of the policies. What works and what does not work in efforts to control the social driving forces underlying global environmental change? What can we learn from the experience with ozone depletion about effective responses to other types of global change? Can public policy play a major role in reducing the energy intensity of advanced, industrial economies? Are some policy instruments (for example, regulations, charges, transferable permits) more effective than others in curbing the anthropogenic sources of global change?

It has been customary in U.S. social policy to evaluate the effects of policy initiatives and to plan for the evaluation in the budgetary process. But post hoc studies of interventions in the environment have not received nearly sufficient government resources or sufficient involvement from the academic centers of social science. To plan for policy, post hoc studies of human responses to past environmental changes are also essential. Agencies and foundations that support basic research on global change should play a key role in fostering rigorous, theoretically informed, and methodologically well-designed empirical studies assessing the effects of human responses to global change. In particular, we recommend that all federal agencies that sponsor programs anticipated to affect processes of global environmental change should routinely include in their budgets funds to evaluate the effects of those programs after they have been enacted.

This recommendation is not a proposal for a new form of environmental impact statements. It is a proposal for data gathering after a policy is in place, because for adjusting policies and for long-term gains in understanding, it is critically important to assess actual impacts after the fact. Over time, post hoc studies can lead to the accumulation of a sizable body of empirically grounded propositions.

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