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LGM Review
« on: November 15, 2014, 03:08:51 PM »

A perspective on model-data surface temperature comparison at the Last Glacial Maximum (LGM)
Annan and Hargreaves 2015?

3.3. Climate sensitivity
One major reason for the interest in the climate of the LGM — though certainly not the only one — is the hope that it might also provide insight into the response of the climate to external forcing in the future. This is commonly summarised by the concept of (equilibrium) climate sensitivity, expressed as the long-term change in temperature in response to a doubling of the atmospheric CO2concentration (roughly 3.7 Wm−2). While it is doubtful that the full earth system is ever truly in equilibrium, the atmosphere-ocean system can be considered to be close to this condition over the millennial time scale associated with the LGM state (since a sustained and significant radiative imbalance over this long a time interval would result in major changes in temperature and/or ice volume). The stability of the climate system at that time, together with the reasonably well-known temperature anomaly and radiative forcing, suggests that the LGM may be a particularly useful interval to constrain the possibility of a very high equilibrium climate sensitivity (Annan and Hargreaves, 2006).

Simple calculations in which the global temperature anomaly at the LGM is divided by the total estimated forcing relative to the pre-industrial state have long been used to generate estimates of the equilibrium climate sensitivity. These estimates have remained close to 3 °C throughout changes in estimates of both components (Hansen and Lacis, 1990, Hoffert and Covey, 1992, Annan and Hargreaves, 2006 and Rohling et al., 2012). The most modern estimates for the (negative) forcing of 8 W m−2 (Annan and Hargreaves, 2006 and Jansen et al., 2007) and temperature anomaly of 4 °C (Annan and Hargreaves, 2013) would suggest a figure of just under 2 °C, which is at the lower end of the previous range of values. However, there are substantial uncertainties and perhaps biases associated with this approach. It is not expected that the response of the climate system to large negative and positive forcings will be perfectly linear, even at the global scale. In fact, model simulations show significant (and model-dependent) nonlinearity (Hargreaves et al., 2007). Moreover, the response to different forcings is not linearly additive. Thus, the climatic effect of large ice sheets, when combined with a reduction in greenhouse gas concentrations, is not equal to that of the same forcing when produced by changes in GHGs alone (Yoshimori et al., 2011). Various researchers have attempted to account for these factors by using climate models to simulate the past and future climates (Hansen et al., 1984 and Manabe and Broccoli, 1985). Annan et al., 2005, Schneider von Deimling et al., 2006b and Holden et al., 2009 and Schmittner et al. (2011) all used ensembles of models with a range of parameter values. In all cases, a strong response between the LGM cooling and 2 × CO2 warming was found across the ensemble, but the relationship itself was considerably model-dependent. When a range of GCMs was examined, the relationship between past and future was much weaker. Crucifix (2006) found no significant relationship at all across a small ensemble of PMIP2 models, but, in a larger ensemble, Hargreaves et al. (2012) did find a significant relationship between tropical SST at the LGM, and climate sensitivity. When constrained with the proxy-based observation of LGM cooling, this implies an equilibrium sensitivity of around 2.5 °C with a 90% confidence interval of about 0.5–4 °C. However, this result must be considered somewhat provisional, due to the small ensemble size and the previously mentioned uncertainties in forcings and proxy data.

0.5C to 4C sensitivity but this is only from LGM. Other sensitivity work makes 0.5C to 1.5C sensitivity unlikely leaving a range of 1.5C to 4C ... hmmm sounds familiar.