2.2 Concept of Radiative Forcing
The definition of RF from the TAR and earlier IPCC assessment reports is retained. Ramaswamy et al. (2001) define it as ‘the change in net (down minus up) irradiance (solar plus longwave; in W m–2) at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values’. Radiative forcing is used to assess and compare the anthropogenic and natural drivers of climate change. The concept arose from early studies of the climate response to changes in solar insolation and CO2, using simple radiative-convective models. However, it has proven to be particularly applicable for the assessment of the climate impact of LLGHGs (Ramaswamy et al., 2001). Radiative forcing can be related through a linear relationship to the global mean equilibrium temperature change at the surface (ΔTs): ΔTs = λRF, where λ is the climate sensitivity parameter (e.g., Ramaswamy et al., 2001). This equation, developed from these early climate studies, represents a linear view of global mean climate change between two equilibrium climate states. Radiative forcing is a simple measure for both quantifying and ranking the many different influences on climate change; it provides a limited measure of climate change as it does not attempt to represent the overall climate response. However, as climate sensitivity and other aspects of the climate response to external forcings remain inadequately quantified, it has the advantage of being more readily calculable and comparable than estimates of the climate response. Figure 2.1 shows how the RF concept fits within a general understanding of climate change comprised of ‘forcing’ and ‘response’. This chapter also uses the term ‘surface forcing’ to refer to the instantaneous perturbation of the surface radiative balance by a forcing agent. Surface forcing has quite different properties than RF and should not be used to compare forcing agents (see Section 2.8.1). Nevertheless, it is a useful diagnostic, particularly for aerosols (see Sections 2.4 and 2.9).
Since the TAR a number of studies have investigated the relationship between RF and climate response, assessing the limitations of the RF concept; related to this there has been considerable debate whether some climate change drivers are better considered as a ‘forcing’ or a ‘response’ (Hansen et al., 2005; Jacob et al., 2005; Section 2.8). Emissions of forcing agents, such as LLGHGs, aerosols and aerosol precursors, ozone precursors and ozone-depleting substances, are the more fundamental drivers of climate change and these emissions can be used in state-of-the-art climate models to interactively evolve forcing agent fields along with their associated climate change. In such models, some ‘response’ is necessary to evaluate the RF. This ‘response’ is most significant for aerosol-related cloud changes, where the tropospheric state needs to change significantly in order to create a radiative perturbation of the climate system (Jacob et al., 2005).
Over the palaeoclimate time scales that are discussed in Chapter 6, long-term changes in forcing agents arise due to so-called ‘boundary condition’ changes to the Earth’s climate system (such as changes in orbital parameters, ice sheets and continents). For the purposes of this chapter, these ‘boundary conditions’ are assumed to be invariant and forcing agent changes are considered to be external to the climate system. The natural RFs considered are solar changes and volcanoes; the other RF agents are all attributed to humans. For the LLGHGs it is appropriate to assume that forcing agent concentrations have not been significantly altered by biogeochemical responses (see Sections 7.3 and 7.4), and RF is typically calculated in off-line radiative transfer schemes, using observed changes in concentration (i.e., humans are considered solely responsible for their increase). For the other climate change drivers, RF is often estimated using general circulation model (GCM) data employing a variety of methodologies (Ramaswamy et al., 2001; Stuber et al., 2001b; Tett et al., 2002; Shine et al., 2003; Hansen et al., 2005; Section 2.8.3). Often, alternative RF calculation methodologies that do not directly follow the TAR definition of a stratospheric-adjusted RF are used; the most important ones are illustrated in Figure 2.2. For most aerosol constituents (see Section 2.4), stratospheric adjustment has little effect on the RF, and the instantaneous RF at either the top of the atmosphere or the tropopause can be substituted. For the climate change drivers discussed in Sections 7.5 and 2.5, that are not initially radiative in nature, an RF-like quantity can be evaluated by allowing the tropospheric state to change: this is the zero-surface-temperature-change RF in Figure 2.2 (see Shine et al., 2003; Hansen et al., 2005; Section 2.8.3). Other water vapour and cloud changes are considered climate feedbacks and are evaluated in Section 8.6.
Climate change agents that require changes in the tropospheric state (temperature and/or water vapour amounts) prior to causing a radiative perturbation are aerosol-cloud lifetime effects, aerosol semi-direct effects and some surface change effects. They need to be accounted for when evaluating the overall effect of humans on climate and their radiative effects as discussed in Sections 7.2 and 7.5. However, in both this chapter and the Fourth Assessment Report they are not considered to be RFs, although the RF definition could be altered to accommodate them. Reasons for this are twofold and concern the need to be simple and pragmatic. Firstly, many GCMs have some representation of these effects inherent in their climate response and evaluation of variation in climate sensitivity between mechanisms already accounts for them (see ‘efficacy’, Section 2.8.5). Secondly, the evaluation of these tropospheric state changes rely on some of the most uncertain aspects of a climate model’s response (e.g., the hydrologic cycle); their radiative effects are very climate-model dependent and such a dependence is what the RF concept was designed to avoid. In practice these effects can also be excluded on practical grounds – they are simply too uncertain to be adequately quantified (see Sections 7.5, 2.4.5 and 2.5.6).
The RF relationship to transient climate change is not straightforward. To evaluate the overall climate response associated with a forcing agent, its temporal evolution and its spatial and vertical structure need to be taken into account. Further, RF alone cannot be used to assess the potential climate change associated with emissions, as it does not take into account the different atmospheric lifetimes of the forcing agents. Global Warming Potentials (GWPs) are one way to assess these emissions. They compare the integrated RF over a specified period (e.g., 100 years) from a unit mass pulse emission relative to CO2 (see Section 2.10).