The strengths of the traditional radiative forcing concept warrant its continued use in scientific investigations, climate change assessments, and policy applications. At the same time, its limitations call for using additional metrics that account more fully for the nonradiative effects of forcing, the spatial and temporal heterogeneity of forcing, and nonlinearities. The committee believes that these limitations can be addressed effectively through the introduction of additional forcing metrics in climate change research and policy. This chapter provides several recommendations for extending the traditional radiative forcing concept in the scientific and policy arenas. It identifies research needed to improve quantification and understanding of different forcings and their impacts on climate, to better inform climate policy discussions, and to obtain reliable observations of climate forcings and responses in the past and future. A large number of recommendations are provided because many research avenues need to be explored in order to improve understanding of climate forcings. The recommendations that should be undertaken immediately with high priority are identified with the symbol.
Recent observations show that radiative forcing calculated at the top of the atmosphere is not always a good index for changes in surface temperature. Indeed, the relationship between TOA radiative forcing and surface temperature is not valid if there is significant variation in the vertical distribution of radiative forcing. For example, the direct radiative forcing of black carbon and other absorbing aerosols leads to a reduction in surface heat input while increasing atmospheric heating. Likewise, land-use changes can modify latent and sensible heat fluxes at the surface. Considering the surface radiative forcing along with the comparable value at the top of the atmosphere would enable quantification of the effects of aerosols and other forcings on the surface energy balance and the net forcing of the atmosphere. It would provide information about the extent to which forcings affect the atmospheric lapse rate, with implications for precipitation and mixing.
Associated with expanding the treatment of radiative forcing in this way are several new research needs. In general, climate models have been unable to reproduce the vertical distribution of forcing due to aerosols observed during aircraft campaigns. Nor, in general, do general circulation models (GCMs) have the needed stratospheric processes to adequately model volcanic and solar ultraviolet radiation effects. Little research has addressed how climate response might depend on the vertical structure of the radiative forcing.