Radiative Forcing of Climate Change Expanding the Concept and Addressing Uncertainties (2005) / Chapter Skim
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Executive Summary
Executive Summary
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In order to advance understanding of this issue, the U.S. Climate Change Science Program asked the National Academies to examine the current state of knowledge of how the energy balance regulating Earth's climate is modified by "forcings" including gases and aerosols, land use, and solar variability and to identify relevant research needs (see Appendix B for the full statement of task)
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Nonradiative forcings create an energy imbalance that does not directly involve radiation; an example is the increasing evapotranspiration flux resulting from agricultural irrigation. Studies of long-term changes in climate have emphasized global mean surface temperature as the primary index for climate change.
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Figure ES-2 shows the magnitude of several important forcings as esti FIGURE ES-2 Estimated radiative forcing since preindustrial times for the Earth and troposphere system (TOA radiative forcing with adjusted stratospheric temperatures)
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The radiative forcing concept has been used extensively in the climate research literature over the past few decades and has also become a standard tool for policy analysis endorsed by the Intergovernmental Panel on Climate Change. For a wide range of forcings, there is a nearly linear relationship between the TOA radiative forcing and the resulting equilibrium response of global mean surface temperature as simulated in general circulation models.
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Regional diabatic heating can also cause atmospheric teleconnections that influence regional climate thousands of kilometers away from the point of forcing. Improving societally relevant projections of regional climate impacts will require a better understanding of the magnitudes of regional forcings and the associated climate responses.
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For example, integrated assessment models use radiative forcing as input to simple climate models, which are linked with socioeconomic models that predict economic damages from climate impacts and costs of various response strategies. The simplified climate models generally focus on global mean surface temperature, ignoring regional temperature changes
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It is important 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. PRIORITY RECOMMENDATION: Encourage policy analysts and integrated assessment modelers to move beyond simple climate models based entirely on global mean TOA radiative forcing and incorporate new global and regional radiative and nonradiative forcing metrics as they become available.
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Using an ensemble of climate models, simulate the regional and global climate response to the best-estimate forcings and compare to the observed climate record. Reduce Uncertainties Associated with Indirect Aerosol Radiative Forcing The interaction between aerosols and clouds can lead to a number of indirect radiative effects that arguably represent the greatest uncertainty in current radiative forcing assessments.
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These include fundamental research on the physical and chemical composition of aerosols, aerosol activation, cloud microphysics, cloud dynamics, and subgrid-scale variability in relative humidity and vertical velocity. PRIORITY RECOMMENDATION: Improve understanding and parameterizations of the indirect aerosol radiative and nonradiative effects in general circulation models using process models, laboratory measurements, field campaigns, and satellite measurements.
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The continued conversion of landscapes by human activity, particularly in the humid tropics, has complex and possibly important consequences for regional and global climate change as a result of changes in the surface energy budget. PRIORITY RECOMMENDATIONS: 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.
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