2.9.5 Time Evolution of Radiative Forcing and Surface Forcing
There is a good understanding of the time evolution of the LLGHG concentrations from in situ measurements over the last few decades and extending further back using firn and ice core data (see Section 2.3, FAQ 2.1, Figure 1 and Chapter 6). Increases in RF are clearly dominated by CO2. Halocarbon RF has grown rapidly since 1950, but the RF growth has been cut dramatically by the Montreal Protocol (see Section 2.3.4). The RF of CFCs is declining; in addition, the combined RF of all ozone-depleting substances (ODS) appears to have peaked at 0.32 W m–2 during 2003. However, substitutes for ODS are growing at a slightly faster rate, so halocarbon RF growth is still positive (Table 2.1). Although the trend in halocarbon RF since the time of the TAR has been positive (see Table 2.1), the halocarbon RF in this report, as shown in Table 2.12, is the same as in the TAR; this is due to a re-evaluation of the TAR results.
Radiative forcing time series for the natural (solar, volcanic aerosol) forcings are reasonably well known for the past 25 years; estimates further back are prone to uncertainties (Section 2.7). Determining the time series for aerosol and ozone RF is far more difficult because of uncertainties in the knowledge of past emissions and chemical-microphysical modelling. Several time series for these and other RFs have been constructed (e.g., Myhre et al., 2001; Ramaswamy et al., 2001; Hansen et al., 2002). General Circulation Models develop their own time evolution of many forcings based on the temporal history of the relevant concentrations. As an example, the temporal evolution of the global and annual mean, instantaneous, all-sky RF and surface forcing due to the principal agents simulated by the Model for Interdisciplinary Research on Climate (MIROC) + Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS) GCM (Nozawa et al., 2005; Takemura et al., 2005) is illustrated in Figure 2.23. Although there are differences between models with regards to the temporal reconstructions and thus present-day forcing estimates, they typically have a qualitatively similar temporal evolution since they often base the temporal histories on similar emissions data.
General Circulation Models compute the climate response based on the knowledge of the forcing agents and their temporal evolution. While most current GCMs incorporate the trace gas RFs, aerosol direct effects, solar and volcanoes, a few have in addition incorporated land use change and cloud albedo effect. While LLGHGs have increased rapidly over the past 20 years and contribute the most to the present RF (refer also to Figure 2.20 and FAQ 2.1, Figure 1), Figure 2.23 also indicates that the combined positive RF of the greenhouse gases exceeds the contributions due to all other anthropogenic agents throughout the latter half of the 20th century.
The solar RF has a small positive value. The positive solar irradiance RF is likely to be at least five times smaller than the combined RF due to all anthropogenic agents, and about an order of magnitude less than the total greenhouse gas contribution (Figures 2.20 and 2.23 and Table 2.12; see also the Foukal et al., 2006 review). The combined natural RF consists of the solar RF plus the large but transitory negative RF from episodic, explosive volcanic eruptions of which there have been several over the past half century (see Figure 2.18). Over particularly the 1950 to 2005 period, the combined natural forcing has been either negative or slightly positive (less than approximately 0.2 W m–2), reaffirming and extending the conclusions in the TAR. Therefore, it is exceptionally unlikely that natural RFs could have contributed a positive RF of comparable magnitude to the combined anthropogenic RF term over the period 1950 to 2005 (Figure 2.23). Attribution studies with GCMs employ the available knowledge of the evolution of the forcing over the 20th century, and particularly the features distinguishing the anthropogenic from the natural agents (see also Section 9.2).
The surface forcing (Figure 2.23, top panel), in contrast to RF, is dominated by the strongly negative shortwave effect of the aerosols (tropospheric and the episodic volcanic ones), with the LLGHGs exerting a small positive effect. Quantitative values of the RFs and surface forcings by the agents differ across models in view of the differences in model physics and in the formulation of the forcings due to the short-lived species (see Section 10.2, Collins et al. (2006) and Forster and Taylor (2006) for further discussion on uncertainties in GCMs’ calculation of RF and surface forcing). As for RF, it is difficult to specify uncertainties in the temporal evolution, as emissions and concentrations for all but the LLGHGs are not well constrained.