complicated and possibly important consequences for regional and global climate change as a result of changes in the surface energy budget.

RECOMMENDATIONS:

• Improve projections of future emissions of aerosols, aerosol precursors, and ozone precursors.

• Improve projections of future land-use changes.

Apply climate models to the investigation of scenarios in which aerosols are significantly reduced over the next 10 to 20 years and for a range of cloud microphysics parameterizations.

Integrate climate forcing criteria in the development of future policies for air pollution control and land management.

Provide Improved Guidance to the Policy Community

The radiative forcing concept is used to inform climate policy discussions, in particular to compare the expected relative impacts of forcing agents. For example, integrated assessment models use radiative forcing as input to simple climate models, which are linked with socioeconomic models that predict the economic damages from climate impacts and the costs of various response strategies. This approach has been used to evaluate potential greenhouse gas emissions control strategies for meeting the Kyoto Protocol targets, as well as other policy questions. Many simplified climate models have focused on global mean surface temperature as the primary climate response to forcings, although more recently they have considered regional temperature changes and other societally relevant aspects of climate, such as sea level. It is important that models used for policy analysis incorporate further complexities in the radiative and nonradiative forcing concepts, as identified in this chapter. It is important also to communicate the expanded forcing concepts as described in this report to the policy community and to develop the tools that will make their application useful in a policy context.

Many climate policy questions require comparing the climate change effects of different greenhouse gases, aerosols, and other forcings. The concept of global warming potential (GWP) was developed to address this need. Application of the GWP concept has been restricted mainly to the long-lived greenhouse gases. In principle, it could be applied to short-lived forcing agents such as ozone and aerosols or, more specifically, to the emissions of their precursors, but there are a number of complicating factors including (1) the often poorly defined relationship between a precursor and a radiative forcing agent; (2) the inhomogeneity of the forcing; and (3) the much shorter time horizons (decades or less) relevant to the radiative



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