A spectroscopic theory for how mean rainfall changes with surface temperature

Surface warming is projected to increase global mean rainfall primarily by increasing the radiative cooling of the atmosphere. However, the radiative mechanisms which cause cooling to increase are not well understood. Here, we show that changes in cooling are driven primarily by changes in atmospheric opacity, particularly within the water vapor window. This suggests that changes in mean rainfall are primarily controlled by the thermodynamic and spectroscopic properties of Earth's main greenhouse gases: water vapor and carbon dioxide. Consistent with comprehensive general circulation models, our results explain why mean rainfall increases with surface warming at about 2% per kelvin, why this rate is largely unchanged over numerous doublings of atmospheric carbon dioxide, and why mean rainfall decreases in hothouse climates.