Plio-Pleistocene climate sensitivity evaluated using high-resolution CO₂ records

Theory and climate modelling suggest that the sensitivity of Earth's climate to changes in radiative forcing could depend on the background climate. However, palaeoclimate data have thus far been insufficient to provide a conclusive test of this prediction. Here we present atmospheric carbon dioxide (CO2) reconstructions based on multi-site boron-isotope records from the late Pliocene epoch (3.3 to 2.3 million years ago). We find that Earth's climate sensitivity to CO2-based radiative forcing (Earth system sensitivity) was half as strong during the warm Pliocene as during the cold late Pleistocene epoch (0.8 to 0.01 million years ago). We attribute this difference to the radiative impacts of continental ice-volume changes (the ice-albedo feedback) during the late Pleistocene, because equilibrium climate sensitivity is identical for the two intervals when we account for such impacts using sea-level reconstructions. We conclude that, on a global scale, no unexpected climate feedbacks operated during the warm Pliocene, and that predictions of equilibrium climate sensitivity (excluding long-term ice-albedo feedbacks) for our Pliocene-like future (with CO2 levels up to maximum Pliocene levels of 450 parts per million) are well described by the currently accepted range of an increase of 1.5 K to 4.5 K per doubling of CO2.This study was funded by NERC grants NE/H006273/1 to R.D.P., G.L.F., D.J.L. and D.N.S. (which supported M.A.M.-B. and M.P.S.B.) and NE/I006346/1 to P.F.S. and G.L.F. M.A.M.-B. was also supported by the European Community through a Marie Curie Fellowship and E.J.R. was supported by 2012 Australian Laureate Fellowship FL120100050. G.L.F. also wishes to acknowledge the support of Yale University (as Visiting Flint Lecturer)