An upper bound on global average precipitation in a warming world

The surface energy budget places strong constraints on how aspects of Earth's hydrological cycle—such as global-mean precipitation—respond to warming. In extremely warm, ice-free "hothouse" climates, evaporation is nearly balanced by absorbed shortwave radiation at the surface, limiting precipitation even as temperatures rise. While this behavior has been documented in idealized models, it remains unclear whether more comprehensive climate models, which incorporate additional physical processes, exhibit the same response. In this talk, I will show that the Community Atmosphere Model (CAM) exhibits additional non-monotonic behavior: global-mean precipitation increases with surface temperature up to 330 K, peaking at 5 mm/day, but then declines despite continued warming. This unexpected decrease arises from enhanced atmospheric shortwave absorption, which reduces the energy available for surface evaporation. To further examine this behavior, I will introduce a simple analytical model that predicts the maximum precipitation rate from surface temperature alone. The model estimates the net surface shortwave flux by identifying the temperature at which net upward longwave and sensible heat fluxes vanish, assuming that precipitable water follows Clausius–Clapeyron scaling. It accurately predicts the maximum global-mean precipitation simulated by CAM across a wide range of solar constants and carbon dioxide concentrations. When applied to contemporary climate model output, it shows that precipitation peaks between 300 and 350 K, with corresponding rates of 3 to 6 mm/day. These results suggest that global-mean precipitation may have peaked in past hothouse climates and provide a framework for understanding Earth's hydrological cycle in warm climate states.