9.6.3.1 Estimates of Climate Sensitivity Based on Data for the Last Millennium

The relationship between forcing and response based on a long time horizon can be studied using palaeoclimatic reconstructions of temperature and radiative forcing, particularly volcanism and solar forcing, for the last millennium. However, both forcing and temperature reconstructions are subject to large uncertainties (Chapter 6). To account for the uncertainty in reconstructions, Hegerl et al. (2006a) use several proxy data reconstructions of NH extratropical temperature for the past millennium (Briffa et al., 2001; Esper et al., 2002; Mann and Jones, 2003; Hegerl et al., 2007) to constrain ECS estimates for the pre-industrial period up to 1850. This study used a large ensemble of simulations of the last millennium performed with an energy balance model forced with reconstructions of volcanic (Crowley, 2000, updated), solar (Lean et al., 2002) and greenhouse gas forcing (see Section 9.3.3 for results on the detection of these external influences). Their estimated PDFs for ECS incorporate an estimate of uncertainty in the overall amplitude (including an attempt to account for uncertainty in efficacy), but not the time evolution, of volcanic and solar forcing. They also attempt to account for uncertainty in the amplitude of reconstructed temperatures in one reconstruction (Hegerl et al., 2007), and assess the sensitivity of their results to changes in amplitude for others. All reconstructions combined yield a median climate sensitivity of 3.4°C and a 5 to 95% range of 1.2°C to 8.6°C (Figure 9.20). Reconstructions with a higher amplitude of past climate variations (e.g., Esper et al., 2002; Hegerl et al., 2007) are found to support higher ECS estimates than reconstructions with lower amplitude (e.g., Mann and Jones, 2003). Note that the constraint on ECS originates mainly from low-frequency temperature variations associated with changes in the frequency and intensity of volcanism which lead to a highly significant detection of volcanic response (Section 9.3.3) in all records used in the study.

The results of Andronova et al. (2004) are broadly consistent with these estimates. Andronova et al. (2004) demonstrate that climate sensitivities in the range of 2.3°C to 3.4°C yield reasonable simulations of both the NH mean temperature from 1500 onward when compared to the Mann and Jones (2003) reconstruction, and for the instrumental period. The agreement is less good for reconstructed SH temperature, where reconstructions are substantially more uncertain (Chapter 6).

Rind et al. (2004) studied the period from about 1675 to 1715 to attempt a direct estimate of climate sensitivity. This period has reduced radiative forcing relative to the present due to decreased solar radiation, decreased greenhouse gas and possibly increased volcanic forcing (Section 9.2.1.3). Different NH temperature reconstructions (Figure 6.10) have a wide range of cooling estimates relative to the late 20th century that is broadly reproduced by climate model simulations. While climate in this cold period may have been close to radiative balance (Rind et al., 2004), some of the forcing during the present period is not yet realised in the system (estimated as 0.85 W m^–2; Hansen et al., 2005). Thus, ECS estimates based on a comparison between radiative forcing and climate response are subject to large uncertainties, but are broadly similar to estimates discussed above. Again, reconstructions with stronger cooling in this period imply higher climate sensitivities than those with weaker cooling (results updated from Rind et al., 2004).