1. Centre for Ecology and Hydrology, Wallingford OX10 8BB, UK
2. Met Office Hadley Centre, Exeter EX1 3PB, UK
3. ETH Zurich, Institute for Atmospheric and Climate Science, CH 8092 Zurich, Switzerland
4. School of Engineering, Computer Science and Mathematics, University of Exeter, Exeter EX4 4QF, UK

Correspondence to: Lina M. Mercado¹ Correspondence and requests for materials should be addressed to L.M.M. (Email: lmme@ceh.ac.uk).

Plant photosynthesis tends to increase with irradiance. However, recent theoretical and observational studies have demonstrated that photosynthesis is also more efficient under diffuse light conditions^{1, 2, 3, 4, 5}. Changes in cloud cover or atmospheric aerosol loadings, arising from either volcanic or anthropogenic emissions, alter both the total photosynthetically active radiation reaching the surface and the fraction of this radiation that is diffuse, with uncertain overall effects on global plant productivity and the land carbon sink. Here we estimate the impact of variations in diffuse fraction on the land carbon sink using a global model modified to account for the effects of variations in both direct and diffuse radiation on canopy photosynthesis. We estimate that variations in diffuse fraction, associated largely with the 'global dimming' period^{6, 7, 8}, enhanced the land carbon sink by approximately one-quarter between 1960 and 1999. However, under a climate mitigation scenario for the twenty-first century in which sulphate aerosols decline before atmospheric CO₂ is stabilized, this 'diffuse-radiation' fertilization effect declines rapidly to near zero by the end of the twenty-first century.

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