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a These authors contributed equally to this work. 14 Theory and climate modelling suggest that the sensitivity of Earth's climate to changes in radiative 15forcing could depend on background climate. However, palaeoclimate data have thus far been 16 insufficient to provide a conclusive test of this prediction. Here we present new atmospheric CO 2 17 reconstructions based on multi-site boron-isotope records through the late Pliocene (3.3 to 2.3 18 Myr ago). We find that Earth's climate sensitivity to CO 2 -based radiative forcing (Earth System 19 Sensitivity) was half as strong during the warm Pliocene as during the cold late Pleistocene (0.8 to 20 0 Myr ago). We attribute this difference to the radiative impacts of continental ice-volume 21 changes (ice-albedo feedback) during the late Pleistocene, because equilibrium climate sensitivity 22 is identical for the two intervals when we account for such impacts using sea-level reconstructions. 23 We conclude that, on a global scale, no unexpected climate feedbacks operated during the warm 24Pliocene, and that predictions of equilibrium climate sensitivity (excluding long-term ice-albedo 25 feedbacks) for our Pliocene-like future (with CO 2 levels up to maximum Pliocene levels of 450 26 ppm) are well described by the currently accepted range of 1.5 to 4.5 K per CO 2 doubling. 27Since the start of the industrial revolution, the concentration of atmospheric CO 2 (and other 28 greenhouse gases; GHGs) has increased dramatically (from ~280 to ~400 ppm) 1 . It has been known 29 for over 100 years that changes in GHG concentration will cause the surface temperature of the 30 Earth to vary 2 . A wide range of observations reveals that the sensitivity of Earth's surface 31 temperature to radiative forcing amounts to ~3 K warming per doubling of atmospheric CO 2 32 concentration (with a 66% confidence range of 1.5 to 4.5 K; e.g. ref. 1,3), due to direct radiative 33 forcing by CO 2 plus the action of a number of fast-acting positive feedback mechanisms, mainly 34 related to atmospheric water vapour content and sea-ice and cloud albedo. Uncertainty in the 35 magnitude of these feedbacks confounds our ability to determine the exact equilibrium climate 36 sensitivity (ECS; the equilibrium global temperature change for a doubling of CO 2 on timescales of 37 about a century, when all 'fast' feedbacks have had time to operate; see ref. 3 for more detail). 38Although the likely range of values for ECS is 1.5 to 4.5 K per CO 2 doubling, there is a small but finite 39 possibility that climate sensitivity may exceed 5 K (e.g. ref. 1). Understanding the likely value of ECS 40 clearly has important implications for the magnitude, eventual impact and potential mitigation of 41 future climate change. 42Any long-range forecast of global temperature (i.e. beyond the next 100 years) must also consider 43 the possibility that ECS could depend on the background state of the climate 4,5 . That is, in a warmer 44 world, some feedbacks that determine ECS could become more efficient and/or new feed...

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