and individuals. The essential role of the federal government is to put in place an overarching, national policy framework designed to ensure that all of these actors are furthering the shared national goal of emissions reductions. In addition, a national policy framework that both generates and is underpinned by international cooperation is crucial if the risks of global climate change are to be substantially curtailed.

• Adaptable means for managing policy responses. It is inevitable that policies put in place now will need to be modified in the future as new scientific information emerges, providing new insights and understanding of the climate problem. Even well-conceived policies may experience unanticipated difficulties, while others may yield unexpectedly high levels of success. Moreover, the degree, rate, and direction of technological innovation will alter the array of response options available and the costs of emissions abatement. Quickly and nimbly responding to new scientific information, the state of technology, and evidence of policy effectiveness will be essential to successfully managing climate risks over the course of decades.

While recognizing that there is ongoing debate about the goals for international efforts to limit climate change, for this analysis we have focused on a range of global atmospheric GHG concentrations between 450 and 550 parts per million (ppm) CO₂-equivalent (eq), a range that has been extensively analyzed by the scientific and economic communities and is a focus of international climate policy discussions. In evaluating U.S. climate policy choices, it useful to set goals that are consistent with those in widespread international use, both for policy development and for making quantitative assessments of alternative strategies.

Global temperature and GHG concentration targets are needed to help guide long-term global action. Domestic policy, however, requires goals that are more directly linked to outcomes that can be measured and affected by domestic action. The panel thus recommends that the U.S. policy goal be stated as a quantitative limit on domestic GHG emissions over a specified time period—in other words, a GHG emissions budget.

The panel does not attempt to recommend a specific budget number, because there are many political and ethical judgments involved in determining an “appropriate” U.S. share of global emissions. As a basis for developing and assessing domestic strategies, however, the panel used recent integrated assessment modeling studies¹ to suggest