Gordon Neil Inglis, Fran Bragg, Natalie J. Burls, David J.A. Evans, Gavin Foster, Matthew Huber, Dan Lunt, Nicholas Siler, Sebastian Steinig, Richard Wilkinson

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 during the geological record. GMST estimates from 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. Here, we develop a multi-method experimental framework to calculate GMST during three target intervals: 1) the latest Paleocene (~57 Ma), 2) the Paleocene-Eocene Thermal Maximum (56 Ma) and 3) the early Eocene Climatic Optimum (EECO; 49.4 to 53.3 Ma). Using six independent methodologies, we find that average GMST estimates during the latest Paleocene and PETM are 11.7°C (± 0.6°C) and 18.7°C (± 0.8°C) higher than pre-industrial, respectively. GMST estimates from the EECO are 13.3°C (±0.5°C) warmer than pre-industrial and comparable to previous IPCC AR5 estimates (12.7°C higher than pre-industrial). Leveraging the extremely large ‘signal’ associated with these extreme warm climates, we combine estimates of GMST and CO2 from the latest Paleocene, PETM and EECO to calculate a gross estimate of the average climate sensitivity between the early Paleogene and today. This yields gross climate sensitivity estimates for the latest Paleocene, PETM and EECO which range between 2.8 to 4.8°C (66% confidence). These largely fall within the range predicted by the IPCC (1.5 to 4.5°C per doubling CO2), but appear incompatible with low values (between 1.5 and 2.8°C per doubling CO2).