• How should different preferences across different sections of society be weighed? Who stands to gain and who stands to lose under different kinds of climate policies? How will climate policies interact with other policy objectives, such as moving toward sustainability?

• What does science tell us about building political support for policy implementation?

This chapter summarizes the scientific aspects of climate policy, including how science can contribute to policy design as well as its implementation and evaluation. Strengths and weaknesses of different policy approaches have been examined substantially in the scientific literature, and this chapter provides an overview of the general conclusions that have been reached by the IPCC and others as a prelude to identifying key areas for further research. For an actual assessment of current policies being considered in the United States to limit the magnitude of future climate change, see the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c); for a more detailed description of potential policy approaches related to adaptation to climate change, see the companion report Adapting to the Impacts of Climate Change (NRC, 2010a). The companion report Informing Effective Decisions Related to Climate Change (NRC, 2010b) also contains a detailed treatment and analysis of various policy mechanisms, as well as other approaches for improving climate-related decision making. The last section of the chapter summarizes research that is needed to support understanding of the interaction of climate change with natural and social systems, as well as policy design and implementation.

While there is a great deal of complexity and nuance involved in policy assessment, the IPCC (Gupta et al., 2007) concludes that there is “high agreement” and “much evidence” to support a number of conclusions about the major kinds of national policies that have been proposed and in some cases implemented to limit climate change. The IPCC also points out (see Table 17.1):

• Direct regulation, when enforced, can reduce emissions.

• Taxes are cost effective but do not guarantee a particular level of emissions reductions and are hard to adapt and adjust.

• The environmental effectiveness and cost effectiveness of tradable permits depend on the structure of the policy, including the number of permits issued, how they are distributed, and whether permits can be banked.