Abstract:

Earth’s tropical regions are blanketed by high clouds produced by tropical thunderstorms (“anvil” clouds). After 25 years of active research, we still lack a complete theory for the total anvil cloud coverage in the Tropics. This lack of understanding makes anvil clouds one of the greatest sources of uncertainty in estimates of Earth’s climate sensitivity—the change in global mean surface temperature resulting from increased atmospheric CO2. While anvil cloud area is broadly expected to shrink as climate warms, whether this would constitute a positive or negative feedback on climate change has long been the subject of controversy. Despite conflicting evidence, a recent community assessment of climate sensitivity assumed that anvil clouds provide a large, negative feedback on global warming. We show that this negative feedback is not supported by an ensemble of high-resolution atmospheric models. On the contrary, the models produce a weak but positive feedback, implying that climate sensitivity is greater than the previous community estimate. The positive feedback arises from a disproportionate reduction in the area of thick, climate-cooling anvils relative to thin, climate-warming anvils. We argue that the mechanisms behind this feedback are consistent with our observational understanding of anvil evolution and with known thermodynamic constraints on the tropical circulation. With concurrent observational analyses supporting our results, this 25-year debate is finally converging on an answer.