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of greenhouse gases. As such, should greenhouse warming require active intervention, the United States has a responsibility to do its part to reduce greenhouse emissions, and unilateral action could contribute significantly to a reduction in the rate of emission growth. However, the U.S. role in greenhouse warming, although large (approximately 20 percent of worldwide CO2-equivalent emissions), is not so large that unilateral action could stabilize global climate. At least a 60 percent reduction in current worldwide CO2-equivalent emissions would be needed for stabilization, according to the Intergovernmental Panel on Climate Change (Intergovernmental Panel on Climate Change, 1991). As discussed in Chapter 28, geoengineering options may be able to reduce the amount of reduction required, but international agreement and participation in such actions would be necessary in order to undertake such action on a planetary scale.

Second, U.S. policy and technology will affect the inclination and capability of other nations to respond to greenhouse warming. The U.S. policy is of instrumental importance, both in meeting our potential national responsibilities within the world community and in leading constructive change in that community. The large magnitude and long time scale of potential adjustments imply that any response will require a coherent and sustained commitment on a global scale. What is needed is not a single national policy, but a long-term strategic perspective on greenhouse warming and its implications for the world economy.

Third, developing countries are unlikely to be able to respond to the potential threat of greenhouse warming at the same level as industrialized countries. The United States should not focus exclusively on interventions within its own boundaries because greenhouse warming is a global issue and emission reduction in one country could be as beneficial as in another. It may be appropriate for the United States and other industrial economies to seek low-cost opportunities for reducing greenhouse gas emissions in developing countries, or to provide economic and technological support, through the political process, should these countries decide that such actions are warranted.

Three basic premises are central to the panel's comparison of different mitigation policy options.

• First, possible responses to greenhouse warming should be regarded as investments in the future of the nation and the planet. That is, the actions needed would have to be implemented over a long time. They should be evaluated as investments, in comparison with other claims on the nation's resources, bearing in mind their often widespread implications for the economy.

• Second, cost-effectiveness is an essential guideline. The changes in energy, industrial practice, land use, agriculture, and forestry that might be implemented to limit greenhouse gas emissions, or the use of geoengineering options, imply an investment effort lasting several generations and large enough to affect the macroeconomic profile of the country. Costs of climate policy therefore need to be considered as a central element. A sensible guideline is cost-effectiveness: obtaining the largest reductions in greenhouse gas emissions at the lowest cost to society. Positive or negative effects of any mitigation option on societal factors not related to greenhouse warming must also be taken into account.