The role of sunshine absorption on rainfall increase

Changes in atmospheric cooling due to enhanced divergence of radiative fluxes are well accepted to explain the long-term changes in global precipitation coming along with global warming. Recent studies have shown that the rate of change of global annual mean precipitation in 21st century climate scenarios are well correlated with changes in clear-sky radiative divergence for both CMIP3 and CMIP5 multi-model ensembles. The shortwave component of the clear-sky radiative cooling has been found to explain both the rates of change in rainfall and the wide diversity between models in most future climate scenarios. But there is low agreement on the reasons for the inter model spread in clear-sky shortwave radiative divergence. In particular, it has not been shown if this inter-model range primarily results from differences in the characteristics of water vapor or in the parameterization of shortwave absorption in radiative transfer codes. In the present study we examine the reasons for the large diversity in clear-sky shortwave radiative divergence and their implications for global-mean precipitation, using the models assembled for the Coupled Model Intercomparison Project and assessed by the Intergovernmental Panel on Climate Change. We quantify the contributions of the parameterization of shortwave absorption, of changes in the concentration and spatial distribution of water vapor and of the short- and long-lived anthropogenic forcing agents to the inter-model range in shortwave radiative cooling. We conclude by discussing the need to reduce the spread in radiative physics, anthropogenic shortwave diabatic heating, and background state, especially water vapor, across the next generation of multi-model ensembles used to project future precipitation.