Diffuse radiation and climate feedback effects on the land carbon sink

Clouds and aerosols influence land carbon uptake by modifying the quantity and quality of light (direct vs diffuse). Diffuse light tends to enhance canopy photosynthesis and vegetation carbon uptake via the diffuse radiation fertilisation effect, but the net response depends on a balance with the concurrent reduction in total radiation. Global and regional effects remain poorly understood, with modelling studies disagreeing on the magnitude, sign, and spatial variability of impacts. One key source of uncertainty lies in ecosystem-climate feedbacks, which are triggered by the initial photosynthesis response to diffuse radiation and can initiate a cascade of effects that further modulate carbon fluxes and light conditions. Despite this potential importance, very few studies have implemented fully coupled simulations.

Here, we use the UK Earth System Model (UKESM), implemented with a coupled interactive diffuse radiation scheme, to investigate global and regional impacts of diffuse radiation on gross primary productivity (GPP). Between 1984-2008, diffuse radiation enhances global GPP by 557.6 PgC. The diffuse radiation effect of the 1991 Mount Pinatubo eruption resulted in substantial productivity effects compared to simulations driven by climatological mean volcanic aerosol. The interactive scheme resulted in similar global GPP compared to the standard UKESM configuration with a fixed diffuse fraction of 0.4, but with important regional differences including greater diffuse radiation fraction and GPP in boreal regions but reductions in the tropics. Climate feedbacks also show considerable regional variation, acting to either enhance or suppress the initial photosynthesis response to diffuse radiation. For example, in the tropics, reduced diffuse fraction leads to lower ET resulting in warming and drying trends that amplify reductions in GPP.

These results highlight how diffuse radiation and resulting climate feedbacks can significantly influence land carbon uptake. These effects may become even more important under potential solar radiation modification (SRM) scenarios. While our results imply that diffuse radiation effects of SRM may increase global GPP, the varying regional impacts indicate that the extent of this influence is likely to be sensitive to how and where additional aerosol forcing is applied. We therefore recommend that fully coupled simulations which include representation of diffuse radiation processes are needed to better evaluate potential ecosystem impacts of SRM, and to improve understanding of the complex relationship between clouds and aerosols and the global carbon cycle.