Accurate estimates of past global mean surface temperature (GMST) help to contextualise future climate change and are required to estimate the sensitivity of the climate system to CO2 forcing through Earth s history. Previous GMST estimates for the latest Paleocene and early Eocene (57 to 48 million years ago) span a wide range (9 to 23 C higher than pre-industrial) and prevent an accurate assessment of climate sensitivity during this extreme greenhouse climate interval. Using the most recent data compilations, we employ a multi-method experimental framework to calculate GMST during the three DeepMIP target intervals: (1) the latest Paleocene (57 Ma), (2) the Paleocene Eocene Thermal Maximum (PETM; 56 Ma), and (3) the early Eocene Climatic Optimum (EECO; 53.3 to 49.1 Ma). Using six different methodologies, we find that the average GMST estimate (66% confidence) during the latest Paleocene, PETM, and EECO was 26.3 C (22.3 to 28.3 C), 31.6 C (27.2 to 34.5 C), and 27.0 C (23.2 to 29.7 C), respectively. GMST estimates from the EECO are 10 to 16 C warmer than pre-industrial, higher than the estimate given by the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (9 to 14 C higher than pre-industrial). Leveraging the large "signal" associated with these extreme warm climates, we combine estimates of GMST and CO2 from the latest Paleocene, PETM, and EECO to calculate gross estimates of the average climate sensitivity between the early Paleogene and today. We demonstrate that "bulk" equilibrium climate sensitivity (ECS; 66% confidence) during the latest Paleocene, PETM, and EECO is 4.5 C (2.4 to 6.8 C), 3.6 C (2.3 to 4.7 C), and 3.1 C (1.8 to 4.4 C) per doubling of CO2. These values are generally similar to those assessed by the IPCC (1.5 to 4.5 C per doubling CO2) but appear incompatible with low ECS values (1:5 per doubling CO2).
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Acknowledgements. We thank two anonymous reviewers whose thoughtful comments significantly improved the paper. This research was funded by NERC through NE/P01903X/1 and NE/N006828/1, both of which supported Gordon N. Inglis, Daniel J. Lunt, Sebastian Steinig, and Richard D. Pancost. Gordon N. In-glis was also supported by a GCRF Royal Society Dorothy Hodgkin Fellowship. Natalie J. Burls is supported by NSF AGS-1844380 and the Alfred P. Sloan Foundation as a Research Fellow. Fran Bragg, Daniel J. Lunt, and Richard Wilkinson were funded by the EPSRC Past Earth Network (EP/M008363/1). Matthew Huber was funded by NSF OPP 1842059. Tom Dunkley Jones, Kirsty M. Edgar, and Gavin L. Foster were supported by NERC grant NE/P013112/1. Agatha De Boer and David Hutchinson acknowledge support from the Swedish Research Council under project 2016-03912. GFDL numerical simulations were performed using resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC, Linköping. David Hutchinson was also supported by FOR- MAS project 2018-01621. The authors also thank Chris Poulsen and Jiang Zhu for assistance with the CESM1.2 model simulations.
Financial support. This research has been supported by the Natural Environment Research Council (grant nos. NE/P01903X/1 and NE/N006828/1), the National Science Foundation (grant nos. AGS-1844380 and OPP 1842059), the EPSRC Past Earth Network (EP/M008363/1), NERC (NE/P013112/1), the Swedish Research Council (grant nos. 2016-03912 and 2018-01621), and the Royal Society (grant no. DHF\R1\191178).
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ASJC Scopus subject areas
- Global and Planetary Change