mated in the most recent synthesis report of the Intergovernmental Panel on Climate Change (IPCC, 2001). The largest positive forcing (warming) in Figure ES-2 is from the increase of well-mixed greenhouse gases (CO2, nitrous oxide [N2O], methane [CH4], and chlorofluorocarbons [CFCs]) and amounted to 2.4 W m−2 (watts per square meter) between the years 1750 and 2000. Of the forcings shown in the figure, the radiative impact of aerosols is the greatest uncertainty.
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. This allows quantitative and expedient comparison of the effects of different forcings in the past and of various possible future forcing scenarios. TOA radiative forcing is relatively easy to compute, generally robust across models, straightforward to use in policy applications, directly observable from space, and also inferable from observed changes in ocean heat content. It provides an extremely useful metric for climate change research and policy.
Despite all these advantages, the traditional global mean TOA radiative forcing concept has some important limitations, which have come increasingly to light over the past decade. The concept is inadequate for some forcing agents, such as absorbing aerosols and land-use changes, that may have regional climate impacts much greater than would be predicted from TOA radiative forcing. Also, it diagnoses only one measure of climate change—global mean surface temperature response—while offering little information on regional climate change or precipitation. These limitations can be addressed by expanding the radiative forcing concept and through the introduction of additional forcing metrics. In particular, the concept needs to be extended to account for (1) the vertical structure of radiative forcing, (2) regional variability in radiative forcing, and (3) nonradiative forcing. A new metric to account for the vertical structure of radiative forcing is recommended below. Understanding of regional and nonradiative forcings is too premature to recommend specific metrics at this time. Instead, the committee identifies specific research needs to improve quantification and understanding of these forcings.