We provide an assessment of the current and future states of Arctic sea ice simulated by the Community Earth System Model version 2 (CESM2). The CESM2 is the version of the CESM contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6). We analyze changes in Arctic sea ice cover in two CESM2 configurations with differing atmospheric components: the CESM2(CAM6) and the CESM2(WACCM6). Over the historical period, the CESM2(CAM6) winter ice thickness distribution is biased thin, which leads to lower summer ice area compared to CESM2(WACCM6) and observations. In both CESM2 configurations, the timing of first ice‐free conditions is insensitive to the choice of CMIP6 future emissions scenario. In fact, the probability of an ice‐free Arctic summer remains low only if global warming stays below 1.5°C, which none of the CMIP6 scenarios achieve. By the end of the 21st century, the CESM2 simulates less ocean heat loss during the fall months compared to its previous version, delaying sea ice formation and leading to ice‐free conditions for up to 8 months under the high emissions scenario. As a result, both CESM2 configurations exhibit an accelerated decline in winter and spring ice area, a behavior that had not been previously seen in CESM simulations. Differences in climate sensitivity and higher levels of atmospheric CO₂ by 2100 in the CMIP6 high emissions scenario compared to its CMIP5 analog could explain why this winter ice loss was not previously simulated by the CESM.

中文翻译：

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在20世纪和21世纪，两种形式的CESM2北极海冰

我们通过社区地球系统模型版本2（CESM2）对北极海冰的当前和未来状态进行了评估。CESM2是CESM的版本，是对耦合模型比较项目（CMIP6）第六阶段的贡献。我们分析了两种具有不同大气成分的CESM2配置中北极海冰覆盖的变化：CESM2（CAM6）和CESM2（WACCM6）。在历史时期内，CESM2（CAM6）的冬季冰厚分布偏薄，这导致夏季冰面积比CESM2（WACCM6）和观测值低。在两种CESM2配置中，首次无冰条件的时间对CMIP6未来排放情景的选择都不敏感。实际上，只有当全球变暖保持在1.5°C以下时，北极夏季无冰的可能性仍然很低，CMIP6方案都无法实现。到21世纪末，与以前的版本相比，CESM2在秋季月份模拟的海洋热量损失更少，在高排放情景下，延迟了海冰的形成并导致长达8个月的无冰条件。结果，两种CESM2配置在冬季和春季冰面积上都呈现出加速下降的趋势，这是以前在CESM模拟中未发现的现象。气候敏感性差异和大气CO含量较高 两种CESM2配置在冬季和春季的冰面积上都有加速下降的趋势，这是以前在CESM模拟中未发现的。气候敏感性差异和大气CO含量较高 两种CESM2配置在冬季和春季的冰面积上都有加速下降的趋势，这是以前在CESM模拟中未发现的。气候敏感性差异和大气CO含量较高与CMIP5类似物相比，在CMIP6高排放情景中，到2100年₂可以解释为什么CESM先前没有模拟这种冬季冰损失。