tive forcing concept facilitates comparison of forcing calculations between climate models and with benchmark line-by-line radiative transfer calculations.

Radiative forcing is thus one of the more highly quantified methods of determining how the climate system is forced. In addition, observational records are available for surface temperature (space-based monitoring, in situ monitoring, and proxy data) and the radiation balance at the top of the atmosphere. These data provide an important observational constraint on estimates of radiative forcing and temperature response. Furthermore, numerous model and observation-based estimates of radiative forcing have been reported in the scientific literature over the past decades, providing an important historical reference for future calculations.

The radiative forcing concept has also been used effectively in policy applications. The concept is already entrained in the policy dialogue, particularly through the emphasis given it in the IPCC reports. Policy analysts have input radiative forcing into simple climate models, which are used to examine a wide range of scenarios of past, present, and future climate. Comparison between these simple models and the more complex fully coupled models also helps in interpreting causal mechanisms in the fully coupled models (e.g., Murphy, 1995; Raper et al., 2001).

Although the traditional TOA radiative forcing concept remains very useful, it is limited in several ways. It is inadequate to describe fully the radiative effects of several anthropogenic influences including

• absorbing aerosols, which lead to a positive radiative forcing of the troposphere with little net radiative effect at the top of the atmosphere;

• effects of aerosols on cloud properties (including cloud fraction, cloud microphysical parameters, and precipitation efficiency), which may modify the hydrological cycle without significant radiative impacts;

• perturbations of ozone in the upper troposphere and lower stratosphere, which challenge the manner in which the stratospheric temperature adjustment is done; and

• surface modification due to deforestation, urbanization, and agricultural practices and surface biogeochemical effects.

Land surface modification of heat fluxes and aerosol-induced changes to the precipitation efficiency modify not only the radiative fluxes but also the dynamical (turbulent heat flux) and thermodynamical fluxes (evaporation). These modifications to the climate system fall under the broader umbrella of climate forcings, which include radiative and nonradiative fluxes. Broadening the concept of radiative forcing in this way allows consideration of climate variables that may have more direct societal impacts, such as changes in precipitation. Indeed, the traditional radiative forcing