Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection
Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection
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DOI：
10.1175/jcli-d-15-0897.1
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发表时间：
2016-08-01
影响因子：
4.9
通讯作者：
Schneider, Tapio
中科院分区：
文献类型：
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作者：
Brient, Florent;Schneider, Tapio
全球变暖预测中的物理不确定性主要是关于云层反射的入射短波辐射的比例将如何随着温室气体浓度上升而变化的不确定性。仅热带海洋上空低云的短波反射差异就占气候模型之间平衡气候敏感度变化的一半以上，范围从2.1至4.7K。现在，天基测量提供了一个机会，以评估模型在季节至年际时间尺度上再现这种短波反射的时间变化的情况。在这里，这种基于空间的测量被用来表明，当下垫面变暖时，热带海洋上空低云的短波反射显著减少，例如，对于反季节性变化，下降了-(0.96+/-0.22)%K-1(90%置信度水平)。此外，在历史模拟中，当前气候模式的低云反射与温度的时间协方差与模式的ECS有很强的相关性(r=-0.67)。因此，对低云时间变化的测量可以用来约束基于气候模型的ECS估计。对气候模型进行信息论加权，根据它们再现测量的短波云反射与温度的反季节协方差的程度，得出最有可能的ECS估计值在4.0K左右；ECS低于2.3K的可能性变得非常小(90%置信度)。
Physical uncertainties in global-warming projections are dominated by uncertainties about how the fraction of incoming shortwave radiation that clouds reflect will change as greenhouse gas concentrations rise. Differences in the shortwave reflection by low clouds over tropical oceans alone account for more than half of the variance of the equilibrium climate sensitivity (ECS) among climate models, which ranges from 2.1 to 4.7 K. Space-based measurements now provide an opportunity to assess how well models reproduce temporal variations of this shortwave reflection on seasonal to interannual time scales. Here such space-based measurements are used to show that shortwave reflection by low clouds over tropical oceans decreases robustly when the underlying surface warms, for example, by -(0.96 +/- 0.22)% K-1 (90% confidence level) for de-seasonalized variations. Additionally, the temporal covariance of low-cloud reflection with temperature in historical simulations with current climate models correlates strongly (r = -0.67) with the models' ECS. Therefore, measurements of temporal low-cloud variations can be used to constrain ECS estimates based on climate models. An information-theoretic weighting of climate models by how well they reproduce the measured deseasonalized covariance of shortwave cloud reflection with temperature yields a most likely ECS estimate around 4.0 K; an ECS below 2.3K becomes very unlikely (90% confidence).