for retirement commensurate with emission reductions credited to renewable-energy or energy-efficiency projects (see Keith et al. 2003; EPA 2004; Bluestein et al. 2006). At present there is no set-aside program for SO₂ allowances, although set-asides for NO_x allowances can be established by states affected by the CAIR. Emissions-displacement rates for NO_x set-asides of 1.5 lbs/MWh through 2015 and 1.25 lbs/MWh after 2015 have been proposed by the EPA (Bluestein et al. 2006). However, the potential for emissions-cap reductions due to wind development remains uncertain. In the six states that have established NO_x set-asides, only 1 to 5% of total NO_x allowances are reserved for set-asides, and this amount can be allocated to either renewable-energy or energy-efficiency projects. The National Research Council (NRC 2006) pointed out that cap-and-trade programs have potential pitfalls and that such programs can result in emission trades from one location to another and from one period to another with potentially detrimental consequences. However, analytical tools are not sufficient to assess the potential effect of cap-and-trade programs on local air quality or the extent to which wind-powered EGUs might alter those effects. In contrast to NO_x and SO₂, emissions of CO₂ from fossil-fuel-fired EGUs or other sources are not subject to national regulatory controls or emissions caps, although subregional control efforts have been initiated (RGGI 2006) and various national controls have been proposed (see Johnston et al. 2006). Thus, to the extent that CO₂-emitting sources of electricity generation are displaced, wind-energy development can achieve displacement of CO₂ emissions. As indicated above, however, CO₂ emissions from fossil-fuel-fired EGUs are projected to increase an average of 1.4% per year between 2004 and 2030 (reference-case forecast; EIA 2006a). Moreover, as also indicated above, fossil-fuel-fired EGUs accounted for about 39% of anthropogenic CO₂ emissions in the United States in 2005 (EIA 2006d). Compared with just the projections for CO₂ emissions from fossil-fuel-fired EGUs, the potential for offsetting emissions with wind-energy development is illustrated by Figure 2-8, which compares projected annual emissions of CO₂ from fossil-fuel-fired EGUs in the United States with offsets that might be achieved through wind-energy development. The estimated offsets are based on the maximum forecasts for wind-powered generation of electricity provided in Table 2-4 and on the system-average emission rates for CO₂ listed in Table 2.5. Based on this comparison, the effect of wind development by 2020 is expected to offset CO₂ emissions from fossil-fuel-fired EGUs in the United States by 4.5%. If fossil-fuel-fired EGUs continue to account for less than half of anthropogenic CO₂ emissions in the United States, then the effect of projected wind-energy development in 2020 would be to offset total anthropogenic CO₂ emissions by less than 2.25%. However, potential technological improvements in emission controls, and other factors that will affect total CO₂ emissions, are as hard to predict for the