• recent European study (the adaptation and mitigation strategies ADAM project (Edenhofer et al., 2010) evaluated stabilization targets with a special focus on the technical feasibility and economic viability of low stabilization scenarios but still with full global participation. In contrast, EMF22 explores international scenarios in which many developing countries delay emissions reductions, providing a broader context for understanding U.S. goals.

  • EMF22 is the most recent large-scale, multimodel exercise exploring both U.S. and international emissions and concentrations goals. It therefore is based on the most recent understanding and historical data. In this way, it contains more recent assessments of underlying drivers and improvements to models used for analysis than previous work such as the reference scenarios from the IPCC’s Special Report on Renewable Energy (Nakicenovic et al., 2000) or the stabilization scenarios in the U.S. Climate Change Science Program report on scenarios (Clarke, 2007).

  • Many of the EMF22 models allow the examination of technology deployment requirements for meeting domestic emissions budgets. In Chapter 3, we test the feasibility of the proposed domestic emissions budget by comparing these deployment requirements against the technical potential of the relevant technologies to meet them.

We know of no other recent modeling exercise that exhibits all these useful features. Nevertheless, we also recognize some important limitations of the EMF22 study. These include, for instance, the question of how aerosols and land use impacts are represented in the models. We note in the subsequent sections where these limitations affect our analysis, but overall we do not believe these effects significantly change our main conclusions and recommendations.

167 gigatons (Gt) CO2-eq9 (and a 50 percent reduction corresponds to a budget of 203 Gt CO2-eq10). Table 2.1 illustrates that the budget values do not change a great deal if different baseline years are selected. This is a useful characteristic of a cumulative budget, since the choice of baseline year is often an issue of contentious debate in policy negotiations.

We chose to round these EMF numbers and adopt a cumulative emissions budget in a range of 170 to 200 Gt CO2-eq over the period 2012-2050 as a reasonably representative U.S. budget target, which can serve as a benchmark for developing policy recommendations and testing their feasibility. This budget range, which represents a significant change from business-as-usual U.S. emissions out to 2050, is roughly in line with the types of emissions-reduction goals found in many recent policy propos-


A gigaton equals a billion (109) tons.


These cumulative emissions levels are based on linear pathways from 2012 through 2050. Note, for comparison, that recent (2008) annual U.S. emissions were approximately 7 Gt.

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