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...

SignificanceConflicting sets of hypotheses highlight either the role of ice sheets or atmospheric carbon dioxide (CO2) in causing the increase in duration and severity of ice age cycles ∼1 Mya during the Mid-Pleistocene Transition (MPT). We document early MPT CO2 cycles that were smaller than during recent ice age cycles. Using model simulations, we attribute this to post-MPT increase in glacial-stage dustiness and its effect on Southern Ocean productivity. Detailed analysis reveals the importance of CO2 climate forcing as a powerful positive feedback that magnified MPT climate change originally triggered by a change in ice sheet dynamics. These findings offer insights into the close coupling of climate, oceans, and ice sheets within the Earth System.

Abstract. Understanding natural and anthropogenic climate change processes involves using computational models that represent the main components of the Earth system: the atmosphere, ocean, sea ice, and land surface. These models have become increasingly computationally expensive as resolution is increased and more complex process representations are included. However, to gain robust insight into how climate may respond to a given forcing, and to meaningfully quantify the associated uncertainty, it is often required to use either or both ensemble approaches and very long integrations. For this reason, more computationally efficient models can be very valuable tools. Here we provide a comprehensive overview of the suite of climate models based around the HadCM3 coupled general circulation model. This model was developed at the UK Met Office and has been heavily used during the last 15 years for a range of future (and past) climate change studies, but has now been largely superseded for many scientific studies by more recently developed models. However, it continues to be extensively used by various institutions, including the BRIDGE (Bristol Research Initiative for the Dynamic Global Environment) research group at the University of Bristol, who have made modest adaptations to the base HadCM3 model over time. These adaptations mean that the original documentation is not entirely representative, and several other relatively undocumented configurations are in use. We therefore describe the key features of a number of configurations of the HadCM3 climate model family, which together make up HadCM3@Bristol version 1.0. In order to differentiate variants that have undergone development at BRIDGE, we have introduced the letter B into the model nomenclature. We include descriptions of the atmosphere-only model (HadAM3B), the coupled model with a low-resolution ocean (HadCM3BL), the high-resolution atmosphere-only model (HadAM3BH), and the regional model (HadRM3B). These also include three versions of the land surface scheme. By comparing withPublished by Copernicus Publications on behalf of the European Geosciences Union. observational datasets, we show that these models produce a good representation of many aspects of the climate system, including the land and sea surface temperatures, precipitation, ocean circulation, and vegetation. This evaluation, combined with the relatively fast computational speed (up to 1000 times faster than some CMIP6 models), motivates continued development and scientific use of the HadCM3B family of coupled climate models, predominantly for quantifying uncertainty and for long multi-millennial-scale simulations.