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

• Main Authors: G. N. Inglis, F. Bragg, N. J. Burls, M. J. Cramwinckel, D. Evans, G. L. Foster, M. Huber, D. J. Lunt, N. Siler, S. Steinig, J. E. Tierney, R. Wilkinson, E. Anagnostou, A. M. de Boer, T. Dunkley Jones, K. M. Edgar, C. J. Hollis, D. K. Hutchinson, R. D. Pancost
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-10-01
• Series: Climate of the Past
• Online Access: https://cp.copernicus.org/articles/16/1953/2020/cp-16-1953-2020.pdf

<p><span id="page1954"/>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 <span class="inline-formula">CO₂</span> forcing through Earth's history. Previous GMST estimates for the latest Paleocene and early Eocene (<span class="inline-formula">∼57</span> to 48 million years ago) span a wide range (<span class="inline-formula">∼9</span> to 23&thinsp;<span class="inline-formula">^∘</span>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 (<span class="inline-formula">∼57</span>&thinsp;Ma), (2) the Paleocene–Eocene Thermal Maximum (PETM; 56&thinsp;Ma), and (3) the early Eocene Climatic Optimum (EECO; 53.3 to 49.1&thinsp;Ma). Using six different methodologies, we find that the average GMST estimate (66&thinsp;% confidence) during the latest Paleocene, PETM, and EECO was 26.3&thinsp;<span class="inline-formula">^∘</span>C (22.3 to 28.3&thinsp;<span class="inline-formula">^∘</span>C), 31.6&thinsp;<span class="inline-formula">^∘</span>C (27.2 to 34.5&thinsp;<span class="inline-formula">^∘</span>C), and 27.0&thinsp;<span class="inline-formula">^∘</span>C (23.2 to 29.7&thinsp;<span class="inline-formula">^∘</span>C), respectively. GMST estimates from the EECO are <span class="inline-formula">∼10</span> to 16&thinsp;<span class="inline-formula">^∘</span>C warmer than pre-industrial, higher than the estimate given by the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (9 to 14&thinsp;<span class="inline-formula">^∘</span>C higher than pre-industrial). Leveraging the large “signal” associated with these extreme warm climates, we combine estimates of GMST and <span class="inline-formula">CO₂</span> 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&thinsp;% confidence) during the latest Paleocene, PETM, and EECO is 4.5&thinsp;<span class="inline-formula">^∘</span>C (2.4 to 6.8&thinsp;<span class="inline-formula">^∘</span>C), 3.6&thinsp;<span class="inline-formula">^∘</span>C (2.3 to 4.7&thinsp;<span class="inline-formula">^∘</span>C), and 3.1&thinsp;<span class="inline-formula">^∘</span>C (1.8 to 4.4&thinsp;<span class="inline-formula">^∘</span>C) per doubling of <span class="inline-formula">CO₂</span>. These values are generally similar to those assessed by the IPCC (1.5 to 4.5&thinsp;<span class="inline-formula">^∘</span>C per doubling <span class="inline-formula">CO₂</span>) but appear incompatible with low ECS values (<span class="inline-formula">&lt;1.5</span> per doubling <span class="inline-formula">CO₂</span>).</p>