On the state dependency of fast feedback processes in (paleo) climate sensitivity

Paleo data have been frequently used to determine the equilibrium (Charney) climate sensitivity S^a, and - if slow feedback processes (e.g., land-ice albedo) are adequately taken into account - they indicate a similar range as estimates based on instrumental data and climate model results. Many studies assume the (fast) feedback processes to be independent of the background climate state, e.g., equally strong during warm and cold periods. Here we assess the dependency of the fast feedback processes on the background climate state using data of the last 800 kyr and a box model of the climate system for interpretation. Applying a new method to account for background state dependency, we find S^a=0.61±0.07 K (W m^-2)^-1(±1σ) using a reconstruction of Last Glacial Maximum (LGM) cooling of -4.0 K and significantly lower climate sensitivity during glacial climates. Due to uncertainties in reconstructing the LGM temperature anomaly, S^a is estimated in the range S^a = 0.54-0.95 K (W m^-2)^-1. Key PointsWe analyzed data of radiative forcing and temperature from the last 800,000 yearsThe equilibrium climate sensitivity depends on the background climate stateEquilibrium climate sensitivity is higher in warmer than in colder climates