TABLE 2.1 Cumulative U.S. GHG Emissions (Gt CO2-eq) for the Period 2012 to 2050, Assuming Linear Emissions Reductions Throughout That Time Period, Beginning at a 2008 Emissions Level, and Assuming 100 Percent Coverage of GHG Emissions in the Economy

Base Year

Percent Below Base Year Emissions in 2050

83%

80%

65%

50%

35%

20%

0%

1990

164

167

185

203

221

239

262

2005

167

171

192

213

234

254

282

2008

168

172

194

215

237

258

287

SOURCES: Fawcett et al. (2009). Emissions data from 1990 and 2005 are from EPA (2009), and 2008 emissions projections are based on Paltsev et al. (2007).

als. See Figure 2.10 for a graphical representation of these goals. Note that these goals are presented as limits for cumulative U.S. domestic emissions (rather than goals to be met through international offsets). Excluding some sectors from emissions reductions, or allowing international and domestic offsets, could substantially alter the actual U.S. emissions reductions.


This budget range also relates usefully to the EMF22 international scenarios, where modeling results for global emissions reduction were disaggregated (using the global least-cost criteria described earlier) to indicate the degree of U.S. action associated with global goals under differing cases of international action (the same set of cases illustrated in Figure 2.9). The results show a considerable range of possible U.S. contributions, reflecting the large uncertainties inherent in any long-range (midcentury) projections. In general, however, the EMF22 results indicate that a 200 Gt CO2-eq U.S. budget is roughly consistent with a long-term global goal of 550 ppm CO2-eq, particularly if there is full international participation. This same U.S. budget could also be consistent with long-term global goals of 450 ppm CO2-eq, but only if the option exists to overshoot the goal (with the attendant requirement for more aggressive actions beyond 2050) and if there are also immediate, aggressive, and comprehensive global GHG emissions-reduction efforts. The more stringent 170 Gt CO2-eq U.S. budget is roughly consistent with the 550 ppm CO2-eq global goals without overshoot or with delayed participation—or with the most idealized of the 450 ppm CO2-eq goals (immediate, comprehensive international action, and the opportunity to overshoot the long-term goal prior to 2100). See Clarke et al. (2009) and Fawcett et al. (2009) for further details on these calculations.



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