Global mean surface temperature and climate sensitivity of the early Eocene Climatic Optimum (EECO), Paleocene–Eocene Thermal Maximum (PETM), and latest Paleocene

Gordon N. Inglis1,2, Fran Bragg3, Natalie J. Burls4, Margot J. Cramwinckel5,a, David Evans6, Gavin L. Foster1, Matthew Huber7, Daniel J. Lunt3, Nicholas Siler8, Sebastian Steinig3, Jessica E. Tierney9, Richard Wilkinson10,b, Eleni Anagnostou11, Agatha M. de Boer12, Tom Dunkley Jones13, Kirsty M. Edgar13, Christopher J. Hollis14, David K. Hutchinson12, and Richard D. Pancost2

1School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, UK
2Organic Geochemistry Unit, School of Chemistry, School of Earth Sciences, Cabot Institute for the Environment, University of Bristol, Bristol, UK
3School of Geographical Sciences, University of Bristol, Bristol, UK
4Department of Atmospheric, Oceanic and Earth Sciences, George Mason University, Fairfax, USA
5 Department of Earth Sciences, Utrecht University, Utrecht, the Netherlands
6 Institute of Geosciences, Goethe University Frankfurt, Frankfurt am Main, Germany
7Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, USA
8College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, USA
9Department of Geosciences, The University of Arizona, 1040 E 4th St., Tucson, USA
10School of Mathematics and Statistics, University of Sheffield, Sheffield, UK
11GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
12Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
13School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK
14GNS Science, Lower Hutt, New Zealand
acurrently at: School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, UK
bcurrently at: School of Mathematical Sciences, University of Nottingham, Nottingham, UK

https://cp.copernicus.org/articles/16/1953/2020/

Abstract
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).