Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection

Abstract

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 2(0.96 6 0.22)% K 21 (90% confidence level) for deseasonalized variations. Additionally, the temporal covariance of low-cloud reflection with temperature in historical simulations with current climate models correlates strongly (r 5 20.67) 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 2.3 K becomes very unlikely (90% confidence).

On the Dependence of Cloud Feedbacks on Physical Parameterizations in WRF Aquaplanet Simulations

“…Six of the eight WRF simulations predict a clear decrease of low‐level clouds (Figure d) as opposed to an increase in all CMIP5 models but IPSL (Figure S6). These increases or decreases of the low‐level CF are similar with the two behaviors described by Brient et al () using CMIP5 models sorted as a function of their climate sensitivity. They argue that (i) for models with shallow low cloud in present‐day climate, the convective drying in a warmer climate favors the deepening of the clouds and thus a decrease of the CF as in most WRF simulations and the IPSL model and (ii) for models with more extended low clouds in present‐day climate, the turbulent moistening of the boundary layer favors a shallowing of the clouds and likely an increase of the CF as in the COtie simulation and all CMIP5 models but IPSL.…”

Cloud feedback mechanisms and their representation in global climate models

“…These studies show that low clouds in both models and observations are mostly sensitive to changes in SST and inversion strength. Although these two effects would tend to cancel each other, observations and GCM simulations constrained by observations suggest that SST‐mediated low cloud reduction with warming dominates, increasing the likelihood of a positive low cloud feedback and high climate sensitivity . Nevertheless, recent ground‐based observations of covariations of ShCu with meteorological conditions suggest that a majority of GCMs are unlikely to represent the temporal dynamics of the cloudy boundary layer .…”

Observational Constraints on Cloud Feedbacks: The Role of Active Satellite Sensors

“…A number of other recent studies use one or both of these short-term variability approaches (Brient and Schneider 2016;Myers and Norris 2016;McCoy et al 2017). Uncertainties in feedbacks estimated from these studies are still relatively large, but a meta-analysis by Klein et al (2017, this issue) is able to derive a useful constraint on global models which indicates negative and near-zero tropical cloud feedbacks are unlikely.…”