We examine the response of the Community Earth System Model Versions 1 and 2 (CESM1 and CESM2) to abrupt quadrupling of atmospheric CO₂ concentrations (4xCO2) and to 1% annually increasing CO₂ concentrations (1%CO2). Different estimates of equilibrium climate sensitivity (ECS) for CESM1 and CESM2 are presented. All estimates show that the sensitivity of CESM2 has increased by 1.5 K or more over that of CESM1. At the same time the transient climate response (TCR) of CESM1 and CESM2 derived from 1%CO2 experiments has not changed significantly—2.1 K in CESM1 and 2.0 K in CESM2. Increased initial forcing as well as stronger shortwave radiation feedbacks are responsible for the increase in ECS seen in CESM2. A decomposition of regional radiation feedbacks and their contribution to global feedbacks shows that the Southern Ocean plays a key role in the overall behavior of 4xCO2 experiments, accounting for about 50% of the total shortwave feedback in both CESM1 and CESM2. The Southern Ocean is also responsible for around half of the increase in shortwave feedback between CESM1 and CESM2, with a comparable contribution arising over tropical ocean. Experiments using a thermodynamic slab‐ocean model (SOM) yield estimates of ECS that are in remarkable agreement with those from fully coupled Earth system model (ESM) experiments for the same level of CO₂ increase. Finally, we show that the similarity of TCR in CESM1 and CESM2 masks significant regional differences in warming that occur in the 1%CO2 experiments for each model.

中文翻译：

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使用CESM的二氧化碳增加实验：与气候敏感性的关系以及CESM1与CESM2的比较

我们研究了社区地球系统模型版本1和2（CESM1和CESM2）对大气CO ₂浓度突然翻两番（4xCO2）和每年增加1％CO _2的响应浓度（1％CO2）。提出了关于CESM1和CESM2的平衡气候敏感性（ECS）的不同估计。所有估计都表明，CESM2的灵敏度比CESM1提高了1.5 K或更多。同时，源自1％CO2实验的CESM1和CESM2的瞬态气候响应（TCR）并没有显着变化-CESM1中为2.1 K，CESM2中为2.0K。初始强迫的增加以及短波辐射反馈的增强是导致CESM2中ECS增加的原因。区域辐射反馈的分解及其对全球反馈的贡献表明，南洋在4xCO2实验的整体行为中起着关键作用，在CESM1和CESM2中约占总短波反馈的50％。CESM1和CESM2之间的短波反馈增加中，约一半来自南洋，而热带海洋的贡献也相当大。使用热力学平板海洋模型（SOM）对ECS进行的产量估算与在相同CO水平下的完全耦合地球系统模型（ESM）实验所得的估算显着一致₂增加。最后，我们表明，在每个模型的1％CO2实验中，CESM1和CESM2中TCR的相似性掩盖了升温中的显着区域差异。