Radiative forcing

For people who check facts

The term ÃÂÃÂradiative forcingÃÂÃÂ has been employed in the IPCC Assessments to denote an externally imposed perturbation in the radiative energy budget of the EarthÃÂÃÂs climate system, which may lead to changes in climate parameters [1]. The exact definition used is:

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. [1]

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 radiative forcing is intended as a useful mechanism to compare and explain different (CO2; land-use changes) perturbations to the climate system. Other possible tools can be constructed for the same purpose: for example Shine et al, An alternative to radiative forcing for estimating the relative importance of climate change mechanisms, GEOPHYSICAL RESEARCH LETTERS, 2003 say "...recent experiments indicate that for changes in absorbing aerosols and ozone, the predictive ability of radiative forcing is much worse... we propose an alternative, the "adjusted troposphere and stratosphere forcing". We present GCM calculations showing that it is a significantly more reliable predictor of this GCM's surface temperature change than radiative forcing. It is a candidate to supplement radiative forcing as a metric for comparing different mechanisms...". In this quote, the word "predictive" may be confusing: it refers to the ability of the tool to help explain the response, not to the ability of GCMs to forecast climate change.