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Impact of changes in diffuse radiation on the global land carbon sink


Plant photosynthesis tends to increase with irradiance. However, recent theoretical and observational studies have demonstrated that photosynthesis is also more efficient under diffuse light conditions1,2,3,4,5. 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’ period6,7,8, 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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Figure 1: JULES model evaluation against observations.
Figure 2: Net ecosystem exchange (NEE) and net primary productivity (NPP).
Figure 3: Impact of changes in diffuse fraction on the land carbon sink during the twentieth century.
Figure 4: Historical and twenty-first-century projections (according to the ENSEMBLES A1B-450 stabilization scenario).

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We thank A. Knohl and C. Rebmann for supplying the eddy flux data for model evaluation, D. van Vuuren and the group that developed the IMAGE model for providing scenario data for the simulation of the twenty-first century. We also thank C. D. Jones for advice on the experimental design, C. M. Taylor for discussions on early results, R. Ellis and P. Harris for both scientific and technical support, A. Everitt for computer support and G. Weedon for discussions. The authors acknowledge funding from the UK Natural Environment Research Council CLASSIC programme (L.M.M., C.H. and P.M.C.) and the UK Department for Environment, Food and Rural Affairs (Defra) and the UK Ministry of Defence (MoD) (N.B., O.B. and S.S.) under GA01101 (Defra) and CBC/2B/0417_Annex C5 (MoD) and from the Swiss NCCR Climate (M.W.).

Author Contributions L.M.M. and P.M.C. developed the modification of the JULES model to include sunfleck penetration through the canopy. L.M.M. validated the model at site level, analysed and performed the global simulations and wrote the initial version of the manuscript. O.B. and N.B. developed the framework for producing the shortwave and PAR fields. N.B. developed the look-up tables to reconstruct shortwave and PAR fields under clear- and cloudy-sky conditions and validated the output against ground-based observations. O.B. provided the sulphate aerosol burden for the simulation of the twenty-first century. P.M.C. contributed to the entire study and S.S. contributed to the analysis of results. C.H. developed the IMOGEN software that enabled the global simulations to be carried out. M.W. provided ground-based observations of shortwave- and diffuse-radiation time series and also advised on model validation. All authors discussed the results and the structure of the paper and developed and improved the manuscript.

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Correspondence to Lina M. Mercado.

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Mercado, L., Bellouin, N., Sitch, S. et al. Impact of changes in diffuse radiation on the global land carbon sink. Nature 458, 1014–1017 (2009).

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