From a quantitative perspective, significant U.S. emissions reductions will not by themselves substantially alter the rate of climate change. Although the United States has the largest share of historic contributions to global GHG concentrations, this relative share will decrease over time. All major economies will need to reduce emissions substantially in concert with the United States.

Although long-term global mean temperature change and global atmospheric GHG concentrations are essential outcomes for policies to limit future climate change, they are not sufficient metrics for setting a domestic policy goal. The domestic goal will need to be one that policy can affect directly and for which progress can be measured directly. We recommend that the U.S. goal be framed as a cumulative emissions budget over a set period of time.

Identifying global temperature and atmospheric GHG concentration targets, and linking these to global and U.S. emissions-reduction goals, involves numerous scientific uncertainties as well as ethical and political judgments. We thus do not attempt to recommend definitive U.S. emissions-reduction goals here. However, as a benchmark for the analyses in this study, we conclude that a reasonable range for representative budget goals is 170 to 200 Gt CO2-eq for the period 2012 to 2050. These numbers were chosen because they roughly correspond to the goals of reducing U.S. emissions by 80 and 50 percent, respectively, by 2050—targets that have been used in many recent policy proposals—and because studies indicate that they are roughly consistent with the goals of limiting global GHG concentrations to 450 and 550 ppm, respectively (using global least-cost criteria for allocating a global emissions budget).

This representative U.S. emissions budget range is suggested as actual reductions in domestic emissions rather than a goal to be met through international offsets. A commitment to deeper emissions reductions, as some suggest is warranted on precautionary or fairness grounds, could possibly be achieved through mechanisms for investing in emissions reductions internationally, including the purchase of international offsets if they are truly additional and verifiable (which is discussed further in Chapter 4).

The costs of meeting these emissions-reduction goals are highly uncertain and depend heavily on the available technological options. Recent research estimates the prices of CO2 per ton that would result from the emissions budget scenarios mentioned above; across these scenarios, however, climate action reduces U.S. GDP by between 0.5 and 4.5 percent in 2030. In all the scenarios, however, GDP continues to grow substantially through midcentury. None of these GDP impacts include estimates of the benefits that would be associated with reducing GHG emissions. Also note that these studies assume well-constructed, efficient national mitigation policies. Less efficient approaches could substantially increase the costs of mitigation.

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