Anvil cloud thinning in high-resolution models implies greater climate sensitivity

Anvil clouds produced by tropical convection are expected to shrink in area as the climate warms, and the associated radiative feedback has long been the subject of controversy. In the World Climate Research Programme’s (WCRP) recent assessment of equilibrium climate sensitivity (ECS), the anvil area feedback was the least certain of any individual feedback process but was nevertheless estimated to be significantly negative. Here we show that such a negative feedback is not supported by an ensemble of high-resolution atmospheric models. On the contrary, the models suggest that changes in high cloud area and opacity act as a modest positive feedback. The positive opacity component arises from the disproportionate reduction in the area of thick, climate-cooling anvils relative to thin, climate-warming clouds. This suggests that thick cloud area is tightly coupled to the rate of convective overturning—which is expected to slow with warming—whereas thin cloud area is influenced by other processes. The cloud response is examined from a novel perspective that treats high ice clouds as part of an optical continuum as opposed to entities with fixed opacity. The positive feedback differs significantly from previous estimates and leads to a 0.3 °C increase in the WCRP estimate of ECS and a 10% widening of the likely range. We find that constraining the response of thin, high clouds in the Tropics to warming is critical for improved estimates of cloud feedback and global change.