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).

中文翻译：

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早期始新世气候最适（EECO），古新世-始新世热最大值（PETM）和最新古新世的全球平均表面温度和气候敏感性

对过去的全球平均地表温度（GMST）的准确估算有助于把握未来的气候变化，并需要估算气候系统对贯穿地球历史的CO ₂强迫的敏感性。上一页GMST估计最新的古新世和始新世早期（~57至48万年前）涉及广泛（〜9至23  ^∘温度比工业革命前高），并防止这种极端温室气候的间隔期间气候敏感性的准确评估。使用最新的数据汇编，我们采用了一种多方法实验框架来计算三个DeepMIP目标间隔内的GMST：（1）最新的古新世（〜57 Ma），（2）古新世-始新世最大温度（PETM； 56 Ma），以及（3）始新世早期气候最佳（EECO； 53.3至49.1 Ma）。使用六种不同的方法，我们发现，最新期间的平均GMST估计值（66％置信度）古新世，PETM和EECO为26.3  ^∘ C（22.3至28.3  ^∘ C），31.6  ^∘ C（27.2至34.5  ^∘ C），和27.0  ^∘ C（23.2至29.7  ^∘ C），分别。GMST从EECO估计是〜10至16  ^∘ ç温度超过工业化前，不是由政府间气候变化专门委员会（IPCC）第五次评估报告（9至14给出的预期高出 ^∘C高于工业化前）。利用与这些极端温暖气候相关的巨大“信号”，我们结合了最新古新世，PETM和EECO的GMST和CO ₂估算值，以计算出古近纪早期和今天之间平均气候敏感性的总体估算值。我们表明，“大量”平衡气候敏感性（ECS; 66％的置信度）的最新期间古，PETM和EECO是4.5  ^∘ C（2.4至6.8  ^∘ C），3.6  ^∘ C（2.3至4.7  ^∘ C），和3.1  ^∘ C（1.8至4.4  ^∘ C）每加倍CO ₂。这些值通常与IPCC评估的值相似（1.5至 ^4.5∘C每增加一倍的 CO ₂），但与低ECS值不兼容（<1.5每增加一倍的 CO ₂）。