BOX 2.2

Our Use of EMF22

To identify plausible goals for a U.S. GHG emissions budget, and to evaluate strategies for meeting this budget, we have relied largely on the work of Energy Modeling Forum Study 22 (EMF22). EMF22 included two components that are relevant here: an international component that engaged 10 of the world’s leading integrated assessment models to assess global climate regimes (Clarke et al., 2009) and a U.S. component that engaged six models to assess U.S. emissions goal options (Fawcett et al., 2009). There are other modeling studies and projections that one could consider, but we found EMF22 to be particularly useful for a number of reasons:

  • EMF22 relates global GHG concentration goals to global emissions reduction and to U.S. emissions reduction as well as reductions in other major countries (see details in subsequent sections of this chapter).

  • EMF22 is a multimodel analysis, which helps mitigate concerns about results being skewed by the assumptions (e.g., about the key drivers such as population, economic growth, and energy use) built into any one model. The resulting spread of model results provides a relatively robust way to identify estimates for a corresponding domestic emissions budget and to clarify the level of potential uncertainty.

  • Most previous multimodel evaluations of long-term climate goals (e.g., from the U.S. Climate Change Science Program [Clarke, 2007], earlier studies by the Energy Modeling Forum [de la Chesnaye and Weyant, 2006], and scenarios assessed by the IPCC [Fisher et al., 2007]) have assumed global, immediate action, and some even have a start date as far back as 2000. A

developing regions delay action by a few decades, then the 450 ppm CO2-eq goal could be met only by the end of the century if concentrations are allowed to temporarily overshoot this goal.

U.S. EMISSION TARGETS

U.S. GHG emissions reductions will not by themselves have a decisive impact on global atmospheric GHG concentrations, but actions that the United States might take over the coming decades to reduce domestic emissions do need to be considered within the context of the ultimate goal of stabilizing global climate. To this end, the EMF22 U.S. study (Fawcett et al., 2009) calculated different cumulative U.S. GHG emission budget goals over the period 2012 to 2050, based on the selection of a base year and a corresponding emissions-reduction target to be achieved by 2050 (Table 2.1). They calculated that an 80 percent reduction from 1990 levels corresponds to a budget of



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