The major types of perturbations influencing the climate system are briefly reviewed in this chapter. The main focus is on the notions of forcing and feedback which provide a widely used framework to interpret the response of the system to changes in external conditions. The standard physical feedbacks are first presented before describing interactions implying jointly the energy-balance, hydrological and biogeochemical cycles.
Climate Forcing and Climate Response
4.1.1 Notion of Radiative Forcing
The climate system is influenced by different types of perturbations: changes in the amount of incoming solar radiation, in the composition of the atmosphere, and so on. To compare the magnitude of these perturbations and to evaluate their effect on the climate, it is convenient to analyse their impact on the radiative balance of the Earth. In this framework, a positive radiative forcing corresponds to more energy input to the system (or less output).
The climate changes in response to perturbations ultimately reach a new equilibrium (see Section 4.1.3). To have a clearer view of the dynamics of the system, it is important to separate as objectively as possible the forcing from this response. Consequently, the radiative forcing should be evaluated whilst maintaining the state variables of the system at their values before the perturbation is applied. In the case of an instantaneous forcing, this can be interpreted as the immediate changes in radiative budget of the Earth imposed by the perturbation before any adjustment of the climate. For some agents, such as the solar forcing (see Section 22.214.171.124), evaluation of the radiative forcing is relatively direct, as this can be done by measuring the variations in incoming solar radiation at the top of the atmosphere. For other agents, such as greenhouse gases, estimates are less straightforward as they should be based on computation of the impact of the changes in composition on the radiative transfer in the whole atmosphere.
The instantaneous forcing is not necessarily representative of the perturbation of the heat budget of the troposphere (which is generally the part of the Earth that is of interest for climate) on the time scale needed for its adjustment. In particular, the stratosphere is nearly in equilibrium with the perturbation after a few months, whilst the surface takes several decades at least to achieve equilibrium (see Section 4.1.5).