The middle to late Eocene greenhouse climate modelled using the CESM 1.0.5
The middle to late Eocene greenhouse climate modelled using the CESM 1.0.5
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DOI：
10.5194/cp-16-2573-2020
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发表时间：
2020-12-23
影响因子：
4.3
通讯作者：
Dijkstra, Henk A.
中科院分区：
文献类型：
--
作者：
Baatsen, Michiel;von der Heydt, Anna S.;Dijkstra, Henk A.
早始新世和晚始新世都是许多模拟研究的主题，但很少有人关注始新世中期。后者仍然对气候建模者提出了许多挑战，但也是理解导致始新世-渐新世过渡期南极冰川形成所需条件的事件的关键。在这里，我们提出了CMIP 5的耦合气候模拟使用社区地球系统模式（CESM）版本1的结果。使用一个新的详细的38 Ma的地理重建和更高的模型分辨率相比，大多数以前的建模研究和足够长的平衡时间，这些模拟将有助于进一步了解中晚始新世气候。在大气温室气体的实际水平上，该模型能够显示出与代用记录的总体良好一致性，并捕捉到始新世期间温暖温室气候的重要方面。（即分别为1120 ppm和类似于2700 ppb，或4 x事先知情同意;工业化前的碳），海表温度与现有的中始新世晚期（42-38 Ma;类似于Bartonian）替代数据相当。由于普遍较冷，在2倍PIC强迫下模拟的气候与始新世晚期（38-34 Ma;类似于Priabonian）的气候是一个很好的模拟。陆地温度代理，虽然他们的地理覆盖范围是稀疏的，也表明，这里提出的结果与现有的information.Our模拟的中晚始新世气候相比，减少了赤道到极点的温度梯度和更对称的纬向热分布工业化前的参考。地理、植被和冰的集体效应导致了工业化前和38 Ma始新世边界条件之间的全球平均5-7摄氏度差异，其中云和水蒸气反馈的贡献很大。这有助于解释始新世的温暖，而不需要温室气体水平远高于替代估计（即类似于500-1200 ppm CO2）或低纬度地区变得不合理的温暖。高纬度温暖支持大多数无冰极地地区的想法，即使在2倍PIC，南极洲经历特别温暖的夏天。在模拟的始新世气候中，气候总体湿润，具有强烈的季风特征。与现今气候（0.80 ° C W-1 m（2）; CO2每增加一倍3.17 ° C）相比，平衡气候敏感性降低（0.62 ° C W-1 m（2）; 38 Ma 2 x PIC和4 x PIC之间升温3.21 ° C）。虽然实际的变暖是相似的，但我们主要看到第二次PIC加倍产生的更高的辐射强迫。对能量通量的更详细分析表明，区域辐射平衡主要是维持始新世气候中低纬向温度梯度的原因，以及在更温暖的条件下看到的极地放大作用。这些模型结果可能有助于重新考虑始新世温暖和始新世-渐新世过渡（EOT）的驱动因素，但也可以作为未来代理估计进行更详细比较的基础。
The early and late Eocene have both been the subject of many modelling studies, but few have focused on the middle Eocene. The latter still holds many challenges for climate modellers but is also key to understanding the events leading towards the conditions needed for Antarctic glaciation at the Eocene-Oligocene transition. Here, we present the results of CMIP5-like coupled climate simulations using the Community Earth System Model (CESM) version 1. Using a new detailed 38Ma geography reconstruction and higher model resolution compared to most previous modelling studies and sufficiently long equilibration times, these simulations will help to further understand the middle to late Eocene climate. At realistic levels of atmospheric greenhouse gases, the model is able to show overall good agreement with proxy records and capture the important aspects of a warm greenhouse climate during the Eocene.With a quadrupling of pre-industrial concentrations of both CO2 and CH4 (i.e. 1120 ppm and similar to 2700 ppb, respectively, or 4 x PIC; pre-industrial carbon), sea surface temperatures correspond well to the available late middle Eocene (42-38 Ma; similar to Bartonian) proxies. Being generally cooler, the simulated climate under 2 x PIC forcing is a good analogue for that of the late Eocene (38-34 Ma; similar to Priabonian). Terrestrial temperature proxies, although their geographical coverage is sparse, also indicate that the results presented here are in agreement with the available information.Our simulated middle to late Eocene climate has a reduced Equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. The collective effects of geography, vegetation, and ice account for a global average 5-7 degrees C difference between pre-industrial and 38 Ma Eocene boundary conditions, with important contributions from cloud and water vapour feedbacks. This helps to explain Eocene warmth in general, without the need for greenhouse gas levels much higher than indicated by proxy estimates (i.e. similar to 500-1200 ppm CO2) or low-latitude regions becoming unreasonably warm. Highlatitude warmth supports the idea of mostly ice-free polar regions, even at 2 x PIC, with Antarctica experiencing particularly warm summers. An overall wet climate is seen in the simulated Eocene climate, which has a strongly monsoonal character.Equilibrium climate sensitivity is reduced (0.62 degrees C W-1 m(2); 3.21 degrees C warming between 38 Ma 2 x PIC and 4 x PIC) compared to that of the present-day climate (0.80 degrees C W-1 m(2); 3.17 degrees C per CO2 doubling). While the actual warming is similar, we see mainly a higher radiative forcing from the second PIC doubling. A more detailed analysis of energy fluxes shows that the regional radiative balance is mainly responsible for sustaining a low meridional temperature gradient in the Eocene climate, as well as the polar amplification seen towards even warmer conditions. These model results may be useful to reconsider the drivers of Eocene warmth and the Eocene-Oligocene transition (EOT) but can also be a base for more detailed comparisons to future proxy estimates.