The Effect of Obliquity-Driven Changes on Paleoclimate Sensitivity During the Late Pleistocene

We reanalyze existing paleodata of global mean surface temperature ΔT_g and radiative forcing ΔR of CO₂ and land ice albedo for the last 800,000 years to show that a state-dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔT_g is based on reconstructions, and not when ΔT_g is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land ice sheet growth and sea level fall) the multimillennial component of reconstructed ΔT_g diverges from CO₂, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction-based extrapolation of S for the future, we exclude intervals with strong ΔT_g-CO₂ divergence and find that S is less state-dependent, or even constant state-independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO₂.