The early Eocene greenhouse climate maintained by high atmospheric CO₂ concentrations serves as a testbed for future climate changes dominated by increasing CO₂ forcing. In particular, the early Eocene Arctic region is important in the context of future CO₂ driven climate warming in the northern polar region and associated shrinking Arctic sea ice. Here, we present early Eocene Arctic sea ice simulations carried out by six coupled climate models within the framework of the Deep-Time Model Intercomparison Project (DeepMIP). We find differences in sea ice responses to CO₂ changes across the ensemble and compare the results with available proxy-based sea ice reconstructions from the Arctic Ocean. Most of the models simulate seasonal sea ice presence at high CO₂ levels (≥ 840 ppmv = 3× pre-industrial (PI) level of 280 ppmv). However, the threshold when sea ice permanently disappears from the ocean varies considerably between the models (from <840 ppmv to >1680 ppmv). Based on a one-dimensional energy balance model analysis we find that the greenhouse effect likely caused by increased atmospheric water vapor concentration plays an important role in the inter-model spread in Arctic winter surface temperature changes in response to a CO₂ rise from 1× to 3× the PI level. Furthermore, differences in simulated surface salinity in the Arctic Ocean play an important role in the control of local sea ice formation. These differences result from different implementations of river run-off between the models, but also from differences in the exchange of waters between a brackish Arctic and a more saline North Atlantic Ocean that are controlled by the width of the gateway between both basins. As there is no geological evidence for Arctic sea ice in the early Eocene, its presence in most of the simulations with 3× PI CO₂ level indicates either a higher CO₂ level and/or an overly weak polar sensitivity in these models.

_{由高大气 CO 2}浓度维持的始新世早期温室气候可作为未来以 CO ₂强迫增加为主的气候变化的试验台。特别是，在未来 CO ₂驱动的北极地区气候变暖和相关的北极海冰缩小的背景下，早期始新世北极地区非常重要。在这里，我们展示了在深度模型比对项目 (DeepMIP) 框架内由六个耦合气候模型进行的早期始新世北极海冰模拟。我们发现海冰对 CO _{2的反应存在差异}整个集合的变化，并将结果与​​北冰洋可用的基于代理的海冰重建进行比较。_{大多数模型模拟了高 CO 2}水平下的季节性海冰存在（≥ 840 ppmv = 3× 280 ppmv 的工业前 (PI) 水平）。然而，海冰从海洋中永久消失的阈值在模型之间差异很大（从 <840 ppmv 到 >1680 ppmv）。基于一维能量平衡模型分析，我们发现可能由大气水蒸气浓度增加引起的温室效应在北极冬季地表温度响应 CO _{2变化的模型间扩散中起重要作用。}PI 水平从 1 倍上升到 3 倍。此外，北冰洋模拟地表盐度的差异对控制当地海冰的形成起着重要作用。这些差异是由于模型之间河流径流的不同实施方式造成的，而且也是由于受两个流域之间通道宽度控制的咸北极和更咸的北大西洋之间的水交换差异造成的。由于在始新世早期没有北极海冰的地质证据，它在大多数具有 3× PI CO ₂水平的模拟中的存在表明这些模型中的 CO ₂水平较高和/或极地敏感性过弱。