income countries are projected to account for the bulk of cumulative global GHG emissions.

Together, these factors frame the international context in which the United States will need to decide on its domestic emissions-reduction goals, and also on its support for and involvement in international actions. Even today, as one of the largest individual GHG emitters, the United States cannot substantially reduce global emissions through unilateral action. With its shrinking relative contribution to global emissions, unilateral action by the United States would be decreasingly effective from a quantitative perspective. In some sense, then, a primary role of U.S. action in climate change is to provide global leadership and to motivate effective international action. See Chapter 7 for a fuller discussion of these issues.


International policy goals for limiting climate change were established in 1992 under the UNFCCC, in which the United States and more than 190 other nations set the goal of “stabilization of GHG concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” Subsequent scientific research has sought to better understand and quantify the links among GHG emissions, atmospheric GHG concentrations, changes in global climate, and the impacts of those changes on human and environmental systems. Based on this research, many policy makers in the international community recognize limiting the increase in global mean surface temperature to 2°C above preindustrial levels as an important benchmark; this goal was embodied in the Copenhagen Accords, at a 2009 meeting of the G-8, and in other policy forums.

Although these temperature and concentration goals are essential metrics for limiting global climate change over time, they are not sufficient to guide near-term, domestic policy goals. Policy requires a goal linked to outcomes that domestic action can directly affect and that can be measured contemporaneously. Global temperature and concentration goals lack this attribute, because they are the consequence of global, and not just domestic, actions to limit GHG emissions. To avoid this problem, a limit on cumulative emissions from domestic sources, measured in physical quantities of GHGs allowed over a specified time period, is in the panel’s view a more useful domestic policy goal. Policy can affect emissions directly, and actual emissions can be measured reasonably accurately on a current basis.

Calculating the U.S. emissions budget is conceptually straightforward, but it involves a number of uncertainties and judgments that are complex and potentially contro-

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