Decomposing Cloud Radiative Feedbacks by Cloud-Top Phase

Abstract Changes in cloud scattering properties and emissivity that arise from atmospheric warming cause substantial radiative feedbacks in model projections of anthropogenic climate change, and the relative importance of the underlying mechanisms is poorly understood. One leading hypothesis is that ice-to-liquid conversions cause clouds to optically thicken, producing a major negative feedback. We test this hypothesis by developing a method to decompose cloud radiative feedbacks by cloud-top phase. The method is applied to an ensemble of six state-of-the-art global climate models run with prescribed sea surface temperature. In these simulations, the global mean of the net cloud scattering and emissivity feedback from cloud-phase conversions ranges from −0.17 to −0.01 W m −2 K −1 , while the overall net cloud feedback ranges from 0.02 to 0.91 W m −2 K −1 . The multimodel mean of the cloud scattering and emissivity feedback from cloud-phase conversions is approximately 19% of the magnitude of the multimodel mean of the overall cloud feedback (−0.10 vs 0.52 W m −2 K −1 ). These results indicate that cloud-phase conversions cause a robust negative feedback by changing cloud scattering and emissivity, but this mechanism makes a modest contribution to the overall cloud feedback at the global scale. Significance Statement Climate warming changes Earth’s cloud properties, which then change the temperature further. One cloud-climate feedback mechanism involves the conversion of cloud ice particles to liquid droplets with warming. This makes clouds more opaque, causing them to reflect more solar radiation back to space. It is widely accepted that this mechanism dampens climate warming, but its importance relative to other feedback mechanisms has been unclear. This study develops a method to estimate the cloud opacity feedback from ice-to-liquid conversions in climate-model simulations. On average, cloud-opacity changes from phase conversions explain approximately 19% of the overall cloud–climate feedback in an ensemble of six climate models. This finding clarifies the importance of cloud-phase conversions in projections of anthropogenic climate change.