9.6.3 Estimates of Climate Sensitivity Based on Palaeoclimatic Data
The palaeoclimate record offers a range of opportunities to assess the response of climate models to changes in external forcing. This section discusses estimates from both the palaeoclimatic record of the last millennium, and from the climate of the LGM. The latter gives a different perspective on feedbacks than anticipated with greenhouse warming, and thus provides a test bed for the physics in climate models. There also appears to be a likely positive relationship between temperature and CO2 prior to the 650 kyr period covered by ice core measurements of CO2 (Section 6.3).
As with analyses of the instrumental record discussed in Section 9.6.2, some studies using palaeoclimatic data have also estimated PDFs for ECS by varying model parameters. Inferences about ECS made through direct comparisons between radiative forcing and climate response, without using climate models, show large uncertainties since climate feedbacks, and thus sensitivity, may be different for different climatic background states and for different seasonal characteristics of forcing (e.g., Montoya et al., 2000). Thus, sensitivity to forcing during these periods cannot be directly compared to that for atmospheric CO2 doubling.