Observational constraints from global ice-phase fraction indicate moderate climate sensitivity. | CU Experts

Cloud feedback remains the largest source of uncertainty in estimates of climate sensitivity. Among its components, the cloud optical depth (τ) feedback acts as a negative feedback that suppresses climate warming, yet it remains poorly constrained by observations. In this study, we constrain the τ feedback in CMIP6 models using global satellite observations of ice-phase fraction from active sensors (CloudSat), passive sensors (MODIS), and a combined active-passive dataset (DARDAR-MODIS). Using the DARDAR-MODIS constraint, the shortwave (SW) τ feedback is updated from the CMIP6 multimodel mean of -0.18 ± 0.14 to -0.43 ± 0.12 watts per square meter per kelvin. This estimate is comparable to, but slightly weaker than, constraints derived independently from MODIS (-0.58 ± 0.17 watts per square meter per kelvin) and CloudSat (-0.46 ± 0.16 watts per square meter per kelvin). Applying Bayesian estimation based on the DARDAR-MODIS-constrained τ feedback, we update the likely range of equilibrium climate sensitivity from [2.6 to 4.0] kelvin to [2.5 to 3.7] kelvin, with the median value decreasing from 3.2 to 3.0 kelvin. The consistency of constraints across multiple satellite datasets suggests a moderate climate sensitivity.

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