2.8.5.3 Solar

Solar changes, compared to CO₂, have less high-latitude RF and more of the RF realised at the surface. Established but incomplete knowledge suggests that there is partial compensation between these effects, at least in some models, which leads to solar efficacies close to 1.0. All models with a positive solar RF find efficacies of 1.0 or smaller. One study finds a smaller efficacy than other models (0.63: Gregory et al., 2004). However, their unique methodology for calculating climate sensitivity has large uncertainties (see Section 2.8.4). These studies have only examined solar RF from total solar irradiance change; any indirect solar effects (see Section 2.7.1.3) are not included in this efficacy estimate. Overall, there is medium confidence that the direct solar efficacy is within the 0.7 to 1.0 range.

2.8.5.4 Ozone

Stratospheric ozone efficacies have normally been calculated from idealised ozone increases. Experiments with three models (Stuber et al., 2001a; Joshi et al., 2003; Stuber et al., 2005) found higher efficacies for such changes; these were due to larger than otherwise tropical tropopause temperature changes which led to a positive stratospheric water vapour feedback. However, this mechanism may not operate in the two versions of the GISS model, which found smaller efficacies. Only one study has used realistic stratospheric ozone changes (see Figure 2.19); thus, knowledge is still incomplete. Conclusions are only drawn from the idealised studies where there is (1) medium confidence that the efficacy is within a 0.5 to 2.0 range and (2) established but incomplete physical understanding of how and why the efficacy could be larger than 1.0. There is medium confidence that for realistic tropospheric ozone perturbations the efficacy is within the 0.6 to 1.1 range.