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per residence) with compact fluorescent lights. Another option calls for improving on-road fuel economy to 25 miles per gallon (32.5 mpg in Corporate Average Fuel Economy (CAFE) terms) in light vehicles by implementing existing technologies that would not require changes in size or attributes of vehicles. Each option is also evaluated in terms of an optimistic "upper-bound" (100 percent achievement) or a pessimistic "lower-bound" (25 percent) level of implementation. A brief description of the mitigation options considered in this study is found in Table 6.1.

The third factor is the interconnectedness of the option to other issues in addition to greenhouse warming, for example, destruction of the ozone layer or biological extinction. These additional factors, however, were considered only in a qualitative manner and are part of the reason that recommendations are not based solely on the cost-effectiveness calculations developed in this study.

Table 6.2 shows selected mitigation options in order of cost-effectiveness. Some options, primarily in energy efficiency and conservation, have substantial potential to mitigate greenhouse warming with net savings or very low net cost. However, they have not been fully adopted because of various implementation obstacles.

Net savings does not mean that no expenditure is required to implement these options. Rather, it indicates that the total discounted cost of the option over the period of analysis is less than its discounted direct benefit, usually reduction in energy consumption, where the discount rate is 6 percent. At higher discount rates the relative cost would rise. These are options that ought to be, and probably will be, implemented, since they are in the interests of those who implement them. The decisions to start, however, can be hastened through better information and incentives.

Table 6.2 also includes some options that are more costly, face substantial obstacles to their implementation, or have other costs or benefits that are difficult to characterize. For example, reduction of CFC consumption is also beneficial in reducing stratospheric ozone depletion, and the combined benefit derived for greenhouse warming and ozone depletion would raise CFC control options in the ranking of preferred actions. Questions about the appropriateness of current technologies and public opposition to nuclear power, however, currently make this option difficult to implement. To the extent that concern about greenhouse warming replaces concern about nuclear energy and "inherently safe" nuclear plants are developed, this option increases its priority ranking.

Table 6.3 presents what the panel calls geoengineering options. The geoengineering options in this preliminary analysis include several ways of reducing temperature increases by screening sunlight (e.g., space mirrors, stratospheric dust, multiple balloons, stratospheric soot, and stimulating cloud condensation nuclei) as well as stimulation of ocean uptake of CO2. Several