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conditions. Furthermore, the time required to implement these mitigation options is not considered. Some options, such as those in energy efficiency, can be implemented immediately if the noneconomic obstacles are overcome. Others, such as changes in electricity production, might take considerably longer, on the order of decades. The rates at which these mitigation options are implemented depends on the decision makers in a wide range of firms, households, and governmental units throughout the United States.

Once the cost-effectiveness and mitigation potential of each option were determined, the Mitigation Panel categorized these options. The best-practice (Category 1) options have significant potential for mitigating greenhouse warming at negative or low net implementation cost; however, information and incentive mechanisms are needed to hasten these reductions. Although no firm quantitative estimates of the net contribution of these policies can be given, it is not unreasonable to believe that U.S. greenhouse gas equivalent emissions could be reduced 25 percent from 1990 levels through use of these relatively low cost options alone. The second category of options (Category 2) entails additional costs and benefits not included in the cost-effectiveness estimate. The United States and other countries are already working to reduce CFC emissions—providing a major contribution to the reduction of greenhouse gas emissions at a relatively low cost (in addition to the benefits to the stratospheric ozone layer). Perhaps one of the surprises of this analysis is the relatively low cost at which some of geoengineering options (Category 3) might be implemented. However, it will require further inquiry to decide if geoengineering options can produce the targeted responses without unacceptable additional efforts. The level at which science is currently able to evaluate the cost-effectiveness of engineering the global mean radiation balance leaves great uncertainty in both the areas of technical feasibility and environmental consequences. This analysis does suggest that further inquiry is appropriate.

Finally, greenhouse warming is an international problem that the United States cannot solve alone. Slowing worldwide population growth may be necessary to achieve a significant change in worldwide emissions of greenhouse gases. However, the panel's analysis indicates that reducing population growth alone may not reduce emissions of greenhouse gases if there is continued economic growth. Reduction of deforestation may provide another significant contribution to mitigating greenhouse gas emissions. Due to domestic concerns, however, candidate countries may find these options difficult to implement. The United States can make contributions to international efforts, and such action might significantly slow greenhouse warming at a cost that is less expensive than the cost of options implemented in the United States.

The uncertainties in all of the mitigation alternatives underscore the central role of learning. This is not the usual academic call for more research. It is instead a recommendation that policy actions be treated as opportunities