targets is, however, an inevitably contentious process, and a failure to reach consensus on such targets can become a barrier to moving ahead with meaningful actions. It is of course possible to proceed with meaningful actions to limit GHG emissions in the absence of universally-accepted quantitative goals. Nonetheless, it is important to understand the different types of goals that are being actively debated at national and international levels.
At the international level, a commonly discussed goal is the tolerable increase in global average surface temperature relative to pre-industrial times. (The goal of limiting global average temperature rise to 2°C (3.6°F) has been agreed to in a number of major international platforms,1 although there is ongoing scientific debate about whether that actually represents a “safe” threshold for limiting climate change.2) For any given global temperature goal, corresponding goals can then be derived for atmospheric GHG concentrations that would give a reasonable chance of meeting the temperature goal, for global GHG emissions limits that would give a reasonable chance of meeting those GHG concentration goals, and, finally, for national GHG emission limits that would collectively achieve the needed global emission reductions. These relationships are complicated, however, by a variety of scientific uncertainties and value judgments (see Figure 5.1).3
A global mean temperature limit is not a goal that can be directly controlled, but rather, is an emergent property of the decisions made by countless governments, private sector actors, and individuals around the world, and of the earth system processes that determine how emissions affect the earth’s climate. Operationally, domestic-level response strategies require metrics that can be directly tracked and controlled at the national level. For the U.S. national goal, the America’s Climate Choices (ACC) panel report Limiting the Magnitude of Future Climate Change recommends setting a “budget” for cumulative domestic GHG emissions over a set period of time—a recommendation the committee supports. The budget concept has also been proposed in the context of global emissions.4
It is beyond the mandate of this committee to recommend specific global or national emission budget goals because such goals are based in large part on value judgments about what is an acceptable degree of risk, and what is a fair U.S. share of the global emissions-reduction burden. Nor do we try to evaluate the risks of adverse climate impacts associated with different possible U.S. emission goals, because such risks ultimately depend on global emissions, not U.S. emissions alone. We do suggest, however, that in the context of iterative risk management, any such goals need to be periodically revisited and revised over time, in response to new information and understanding.