An observational radiative constraint on hydrologic cycle intensification

The magnitude of global precipitation increase predicted by climate models has a large uncertainty that has been difficult to constrain, but much of the range in predictions is now shown to arise from shortcomings in the modelling of atmospheric absorption of shortwave radiation; if the radiative transfer algorithms controlling the absorption were more accurate, the model spread would narrow and the mean estimate could be about 40% lower. Model answer for future precipitation Increased precipitation is a central feature of climate model projections. But the magnitude of the increase has a large uncertainty that has been difficult to reduce or even understand. Anthony DeAngelis et al . now show that much of the range in predictions arises from a seemingly basic process: atmospheric absorption of shortwave radiation. A moistening atmosphere, as is predicted for the future, should increase precipitation due to higher humidity, however, the higher humidity should then increase shortwave absorption. The present analysis shows that, when compared to observational constraints, models simulate a too-small increase in shortwave absorption, and thus a too-large increase in precipitation. If the radiative transfer algorithms controlling the absorption could be better constrained and more standardized, the uncertainty would drop and the mean estimate would probably be about 40% lower. Intensification of the hydrologic cycle is a key dimension of climate change, with substantial impacts on human and natural systems 1 , 2 . A basic measure of hydrologic cycle intensification is the increase in global-mean precipitation per unit surface warming, which varies by a factor of three in current-generation climate models (about 1–3 per cent per kelvin) 3 , 4 , 5 . Part of the uncertainty may originate from atmosphere–radiation interactions. As the climate warms, increases in shortwave absorption from atmospheric moistening will suppress the precipitation increase. This occurs through a reduction of the latent heating increase required to maintain a balanced atmospheric energy budget 6 , 7 . Using an ensemble of climate models, here we show that such models tend to underestimate the sensitivity of solar absorption to variations in atmospheric water vapour, leading to an underestimation in the shortwave absorption increase and an overestimation in the precipitation increase. This sensitivity also varies considerably among models due to differences in radiative transfer parameter