We present results from an ensemble of eight climate models, each of which has carried out simulations of the early Eocene climate optimum (EECO, ∼ 50 million years ago). These simulations have been carried out in the framework of the Deep-Time Model Intercomparison Project (DeepMIP; http://www.deepmip.org, last access: 10 January 2021); thus, all models have been configured with the same paleogeographic and vegetation boundary conditions. The results indicate that these non-CO₂ boundary conditions contribute between 3 and 5 ^∘C to Eocene warmth. Compared with results from previous studies, the DeepMIP simulations generally show a reduced spread of the global mean surface temperature response across the ensemble for a given atmospheric CO₂ concentration as well as an increased climate sensitivity on average. An energy balance analysis of the model ensemble indicates that global mean warming in the Eocene compared with the preindustrial period mostly arises from decreases in emissivity due to the elevated CO₂ concentration (and associated water vapour and long-wave cloud feedbacks), whereas the reduction in the Eocene in terms of the meridional temperature gradient is primarily due to emissivity and albedo changes owing to the non-CO₂ boundary conditions (i.e. the removal of the Antarctic ice sheet and changes in vegetation). Three of the models (the Community Earth System Model, CESM; the Geophysical Fluid Dynamics Laboratory, GFDL, model; and the Norwegian Earth System Model, NorESM) show results that are consistent with the proxies in terms of the global mean temperature, meridional SST gradient, and CO₂, without prescribing changes to model parameters. In addition, many of the models agree well with the first-order spatial patterns in the SST proxies. However, at a more regional scale, the models lack skill. In particular, the modelled anomalies are substantially lower than those indicated by the proxies in the southwest Pacific; here, modelled continental surface air temperature anomalies are more consistent with surface air temperature proxies, implying a possible inconsistency between marine and terrestrial temperatures in either the proxies or models in this region. Our aim is that the documentation of the large-scale features and model–data comparison presented herein will pave the way to further studies that explore aspects of the model simulations in more detail, for example the ocean circulation, hydrological cycle, and modes of variability, and encourage sensitivity studies to aspects such as paleogeography, orbital configuration, and aerosols.

中文翻译：

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DeepMIP：早期始新世气候最佳（EECO）大规模气候特征的模型比对，并与代理数据进行比较

我们介绍了八个气候模型的合奏结果，每个模型都对早期始新世气候最佳条件（EECO，约 5000万年前）进行了模拟。这些模拟是在“深度模型比较项目”（DeepMIP；http：//www.deepmip.org，最后访问时间：2021年1月10日）的框架内进行的 ；因此，所有模型都配置了相同的古地理和植被边界条件。结果表明，这些非CO ₂边界条件贡献了3到 ^5∘C始新世温暖。与以前的研究结果相比，DeepMIP模拟通常显示在给定的大气CO ₂浓度下，整体平均表面温度响应在整个集合中的分布减小，并且平均而言对气候的敏感性增加。对模型集合的能量平衡分析表明，始新世相比工业化前时期的全球平均变暖主要是由于CO ₂浓度升高（以及相关的水蒸气和长波云反馈）导致的发射率降低，而减少始新世在子午温度梯度方面的变化主要是由于非CO ₂引起的辐射率和反照率变化边界条件（即南极冰原的清除和植被变化）。其中三个模型（社区地球系统模型，CESM；地球物理流体动力学实验室，GFDL模型，以及挪威地球系统模型，NorESM）显示了与全球平均温度，子午SST代理一致的结果。梯度和CO ₂，而不规定更改模型参数。此外，许多模型与SST代理中的一阶空间模式非常吻合。但是，在更大的区域范围内，模型缺乏技巧。特别是，模拟异常明显低于西南太平洋代理人指出的异常。在这里，模拟的大陆地表气温异常与地表气温代理更一致，这意味着该区域的代理或模型中海洋和陆地温度之间可能存在不一致。我们的目标是，本文介绍的大规模特征文档和模型数据比较将为进一步研究铺平道路，这些研究将更详细地探索模型模拟的各个方面，例如海洋环流，水文循环，