The term "radiative forcing" has been employed in the IPCC Assessments to denote an externally imposed perturbation in the radiative energy budget of the Earth's climate system. Such a perturbation can be brought about by secular changes in the concentrations of radiatively active species (e.g., CO2, aerosols), changes in the solar irradiance incident upon the planet, or other changes that affect the radiative energy absorbed by the surface (e.g., changes in surface reflection properties). This imbalance in the radiation budget has the potential to lead to changes in climate parameters and thus result in a new equilibrium state of the climate system. In particular, IPCC (1990, 1992, 1994) and the Second Assessment Report (IPCC, 1996) (hereafter SAR) used the following definition for the radiative forcing of the climate system: "The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent (say, a change in greenhouse gas concentrations) is the change in net (down minus up) irradiance (solar plus long-wave; in Wm-2) at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropo-spheric temperatures and state held fixed at the unperturbed values". In the context of climate change, the term forcing is restricted to changes in the radiation balance of the surface-troposphere system imposed by external factors, with no changes in stratospheric dynamics, without any surface and tropospheric feedbacks in operation (i.e., no secondary effects induced because of changes in tropospheric motions or its thermodynamic state), and with no dynamically-induced changes in the amount and distribution of atmospheric water (vapour, liquid, and solid forms). Note that one potential forcing type, the second indirect effect of aerosols (Chapter 5 and Section 6.8), comprises microphysically-induced changes in the water substance. The IPCC usage of the "global mean" forcing refers to the globally and annually averaged estimate of the forcing.
The prior IPCC Assessments as well as other recent studies (notably the SAR; see also Hansen et al. (1997a) and Shine and Forster (1999)) have discussed the rationale for this definition and its application to the issue of forcing of climate change. The salient elements of the radiative forcing concept that characterise its eventual applicability as a tool are summarised in Appendix 6.1 (see also WMO, 1986; SAR). Defined in the above manner, radiative forcing of climate change is a modelling concept that constitutes a simple but important means of estimating the relative impacts due to different natural and anthropogenic radiative causes upon the surface-troposphere system (see Section 6.2.1). The IPCC Assessments have, in particular, focused on the forcings between pre-industrial times (taken here to be 1750) and the present (1990s, and approaching 2000). Another period of interest in recent literature has been the 1980 to 2000 period, which corresponds to a time frame when a global coverage of the climate system from satellites has become possible.
We find no reason to alter our view of any aspect of the basis, concept, formulation, and application of radiative forcing, as laid down in the IPCC Assessments to date and as applicable to the forcing of climate change. Indeed, we reiterate the view of previous IPCC reports and recommend a continued usage of the forcing concept to gauge the relative strengths of various perturbation agents, but, as discussed below in Section 6.2, urge that the constraints on the interpretation of the forcing estimates and the limitations in its utility be noted.
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