Diffuse radiation forcing constraints on gross primary productivity and global terrestrial evapotranspiration

The diffuse radiation fertilization effect-the increase in plant productivity in the presence of higher diffuse radiation (K-down arrow,K-d)-is an important yet understudied aspect of atmosphere-biosphere interactions and can modify the terrestrial carbon, energy, and water budgets. The K-down arrow,K-d fertilization effect links the carbon cycle with clouds and aerosols, all of which are large sources of uncertainties for our current understanding of the Earth system and for future climate projections. Here we establish to what extent observational and modeling uncertainty in sunlight's diffuse fraction (k(d)) affects simulated gross primary productivity (GPP) and terrestrial evapotranspiration (lambda E). We find only 48 eddy covariance sites with simultaneous sufficient measurements of K-down arrow,K-d with none in the tropical climate zone, making it difficult to constrain this mechanism globally using observations. Using a land modeling framework based on the latest version of the Community Land Model, we find that global GPP ranges from 114 Pg C year(-1) when using k(d) forcing from the Modern-Era Retrospective analysis for Research and Applications, version 2 reanalysis to a similar to 7% higher value of 122 Pg C year(-1) when using the Clouds and the Earth's Radiant Energy System satellite product, with especially strong differences apparent over the tropical region (mean increase similar to 9%). The differences in lambda E, although smaller (-0.4%) due to competing changes in shaded and sunlit leaf transpiration, can be greater than regional impacts of individual forcing agents like aerosols. Our results demonstrate the importance of comprehensively and systematically validating the simulated k(d) by atmosphere modules as well as the response to differences in k(d) within land modules across Earth System Models.