Causes of Higher Climate Sensitivity in CMIP6 Models
Causes of Higher Climate Sensitivity in CMIP6 Models
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DOI：
10.1029/2019gl085782
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发表时间：
2020-01-16
影响因子：
5.2
通讯作者：
Taylor, Karl E.
中科院分区：
文献类型：
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作者：
Zelinka, Mark D.;Myers, Timothy A.;Taylor, Karl E.
平衡气候敏感度，即全球地表温度对二氧化碳倍增的反应，一直是不确定的。最近的共识认为，这可能在1.5-4.5K之间。全球气候模型(GCM)试图代表所有相关的物理过程，它提供了通过二氧化碳翻两番实验来估计气候敏感性的最直接手段。结果表明，在耦合模式对比项目第六阶段(CMIP6)中，与之密切相关的有效气候敏感性显著增加，27个模式的有效气候敏感度在1.8-5.6K之间，其中10个模式的有效气候敏感度超过4.5K。这一(统计上微不足道的)增长主要是由于温带外低云层覆盖率和反照率减少带来的更强的正向云反馈。这两者都与云的物理表示有关，在CMIP6模型中，云的物理表示导致温带外低云量和水含量对表面温度的非强制变化的反应较弱。鉴于它们隐含的社会后果，建立这些更高敏感性模型的合理性势在必行。
Equilibrium climate sensitivity, the global surface temperature response to CO2 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5-4.5 K. Global climate models (GCMs), which attempt to represent all relevant physical processes, provide the most direct means of estimating climate sensitivity via CO2 quadrupling experiments. Here we show that the closely related effective climate sensitivity has increased substantially in Coupled Model Intercomparison Project phase 6 (CMIP6), with values spanning 1.8-5.6 K across 27 GCMs and exceeding 4.5 K in 10 of them. This (statistically insignificant) increase is primarily due to stronger positive cloud feedbacks from decreasing extratropical low cloud coverage and albedo. Both of these are tied to the physical representation of clouds which in CMIP6 models lead to weaker responses of extratropical low cloud cover and water content to unforced variations in surface temperature. Establishing the plausibility of these higher sensitivity models is imperative given their implied societal ramifications.