PNNL

Abstract

The diffuse radiation fertilization effect – the increase in plant productivity due to higher diffuse radiation (K?,d) – is an important yet understudied aspect of atmosphere-biosphere interactions and can modify the terrestrial carbon, energy, and water budgets. The K?,d fertilization effect links the carbon cycle with clouds and aerosols, all of which are large sources of uncertainties for future climate projections. Here we examine how a realistic range of diffuse fraction (kd) of sunlight would impact simulated gross primary productivity (GPP) and terrestrial evapotranspiration (?E) using monthly-climatology-adjusted kd from current-generation global gridded products and a land modeling framework. Within this framework, global GPP varies from 114.1 Pg C year-1 when using kd forcing from the MERRA-2 reanalysis to a 6.6% higher value of 121.7 Pg C year-1 when using the CERES product, with stronger perturbations seen over the tropics. The changes in ?E are smaller (- 0.4%) but can be larger than regional impacts of individual forcing agents. With none of the current Model Intercomparison Projects focusing on K?,d or its impacts, our results demonstrate the importance of comprehensively and systematically examining the simulated kd by atmosphere modules and response to the same in land modules across earth system models.