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

Gordon N Inglis,Fran Bragg,Sebastian Steinig,Kirsty M Edgar,Jessica E Tierney,Nicholas Siler,Christopher J Hollis,David K Hutchinson,Tom Dunkley Jones,Agatha M De Boer,Margot J Cramwinckel,Matthew Huber,Richard Wilkinson,Natalie J Burls,Daniel J Lunt,Richard D Pancost,Eleni Anagnostou,Gavin L Foster,David Evans

doi:10.5194/cp-16-1953-2020

Abstract

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).

Highlights

Under high growth and low mitigation scenarios, atmospheric carbon dioxide (CO2) could exceed 1000 parts per million by the year 2100 (Stocker et al, 2013)
During the latest Paleocene and Paleocene–Eocene Thermal Maximum (PETM), global mean surface temperature (GMST) estimates derived from Dsurf-baseline average ∼ 27 and 33 ◦C, respectively (Table 3; Fig. 6)
We show that land air temperature (LAT)-only GMST estimates are up to 6 ◦C lower than our baseline (SST + LAT) calculations, suggesting that Eocene Climatic Optimum (EECO) GMST estimates (Dsurf-baseline) may represent a minimum temperature constraint

Summary

Introduction

Under high growth and low mitigation scenarios, atmospheric carbon dioxide (CO2) could exceed 1000 parts per million (ppm) by the year 2100 (Stocker et al, 2013). One way to better constrain these climate predictions is to examine intervals in the geological past during which greenhouse gas levels were similar to those predicted under future scenarios This is the rationale behind the Deep-time Model Intercomparison Project (DeepMIP; https://www.deepmip.org/, last access: 21 October 2020) which aims to investigate the behaviour of the Earth system in three high-CO2 climate states in the latest Paleocene and early Eocene (∼ 57–48 Ma) (Lunt et al, 2017; Hollis et al, 2019).

Methods

Results

Conclusion