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It is still an open question how equilibrium warming\udin response to increasing radiative forcing – the specific\udequilibrium climate sensitivity S – depends on background\udclimate. We here present palaeodata-based evidence\udon the state dependency of S, by using CO2 proxy data together\udwith a 3-D ice-sheet-model-based reconstruction of\udland ice albedo over the last 5 million years (Myr). We find\udthat the land ice albedo forcing depends non-linearly on the\udbackground climate, while any non-linearity of CO2 radiative\udforcing depends on the CO2 data set used. This nonlinearity\udhas not, so far, been accounted for in similar approaches\uddue to previously more simplistic approximations,\udin which land ice albedo radiative forcing was a linear function\udof sea level change. The latitudinal dependency of icesheet\udarea changes is important for the non-linearity between\udland ice albedo and sea level. In our set-up, in which the\udradiative forcing of CO2 and of the land ice albedo (LI) is\udcombined, we find a state dependence in the calculated specific\udequilibrium climate sensitivity, STCO2,LIU, for most of the\udPleistocene (last 2.1 Myr). During Pleistocene intermediate\udglaciated climates and interglacial periods, STCO2,LIU is on average\ud 45% larger than during Pleistocene full glacial conditions.\udIn the Pliocene part of our analysis (2.6–5 MyrBP)\udthe CO2 data uncertainties prevent a well-supported calculation\udfor STCO2,LIU, but our analysis suggests that during times\udwithout a large land ice area in the Northern Hemisphere\ud(e.g. before 2.82 MyrBP), the specific equilibrium climate\udsensitivity, STCO2,LIU, was smaller than during interglacials of\udthe Pleistocene. We thus find support for a previously proposed\udstate change in the climate system with the widespread\udappearance of northern hemispheric ice sheets. This study\udpoints for the first time to a so far overlooked non-linearity\udin the land ice albedo radiative forcing, which is important\udfor similar palaeodata-based approaches to calculate climate\udsensitivity. However, the implications of this study for a suggested\udwarming under CO2 doubling are not yet entirely clear\udsince the details of necessary corrections for other slow feedbacks\udare not fully known and the uncertainties that exist in\udthe ice-sheet simulations and global temperature reconstructions\udare large

On the state dependency of the equilibrium climate sensitivity during the last 5 million years