4.1.1 Notion of radiative forcing

The climate system is influenced by different types of perturbation: changes in the amount of incoming solar radiation, in the composition of the atmosphere, in the topography of the surface, etc. In order to compare the magnitude of those perturbations and to evaluate their effect on the climate, it is often convenient to analyse their impact on the radiative balance of the Earth. More precisely, radiative forcing is commonly defined as the net change in the Earth's radiative budget at the tropopause caused by the perturbation. According to this definition, when the modification of the upward and downward radiatives fluxes is evaluated, the stratospheric temperatures should be allowed to reach a new radiative equilibrium while the surface and tropospheric temperatures are fixed at their unperturbed values (Forster et al., 2007, Fig 4.1). The reason for allowing stratospheric temperature changes is the rapid adjustment of the stratosphere to the perturbation (of the order of a month) compared to that of the troposphere (one or more decades). The forcing at the tropopause thus represents the influence of the perturbation on timescales longer than a year.

The radiative forcing ($\Delta$Q) associated with a perturbation is generally given in annual and global mean (e.g. Forster et al., 2007, see Chapter 6). The interest of this measure is dependent on the nature of the perturbation: it is generally more useful when the perturbation is relatively constant in space and over the seasons than for strongly seasonal or spatially variable changes.

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