that a reasonable “representative” range for a domestic emissions budget would be 170 to 200 gigatons (Gt) of CO₂-eq for the period 2012 through 2050. This corresponds roughly to a reduction of emissions from 1990 levels by 80 to 50 percent, respectively. We note that this budget range is based on “global least cost” economic efficiency criteria for allocating global emissions among countries. Using other criteria, different budget numbers could be suggested. (For instance, some argue that, based on global “fairness” concerns, a more aggressive U.S. emission-reduction effort is warranted.)

As illustrated in Figure S.1, meeting an emissions budget in the range suggested above, especially the more stringent budget of 170 Gt CO₂-eq, will require a major departure from business-as-usual emission trends (in which U.S. emissions have been rising at a rate of ~1 percent per year for the past three decades). The main drivers of GHG emissions are population growth and economic activity, coupled with energy use per capita and per unit of economic output (“energy intensity”). Although the energy intensity of the U.S. economy has been improving for the past two decades, total emis-

FIGURE S.1 Illustration of the representative U.S. cumulative GHG emissions budget targets: 170 and 200 Gt CO₂-eq (for Kyoto gases) (Gt, gigatons, or billion tons; Mt, megatons, or million tons). The exact value of the reference budget is uncertain, but nonetheless illustrates a clear need for a major departure from business as usual.