It is a still open question how equilibrium warming in response to increasing radiative forcing (specific equilibrium climate sensitivity S) is depending on background climate. We here bring palaeodata-based evidence on the state dependency of S by using CO2 proxy data together with 3-D ice-sheet-model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land-ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. In our setup, in which the radiative forcing of CO2 and of the land-ice albedo (LI) is combined (dR_[CO2,LI]), we find a state dependency in the calculated specific equilibrium climate sensitivity S_[CO2,LI] for most of the Pleistocene (last 2.1 Myr). In the Pliocene part (2.6–5 Myr BP) the CO2 data uncertainties prevents a well-supported calculation for S_[CO2,LI], but our analysis suggests that during times without large land-ice area in the northern hemisphere (before 2.82 Myr BP) the specific equilibrium climate sensitivity S_[CO2,LI] was smaller than during interglacials of the Pleistocene. If we develop for S an equation as a function of dR_[CO2,LI] we find S_[CO2,LI] in interglacials to be 2–2.7x larger than during glacial maxima, potentially indicating that equilibrium warming for CO2 doubling might be in the upper range of results compiled in the IPCC AR4.
EPIC Alfred Wegener Institut