Abstract
The evolution of the Earthâs climate over the twenty-first century depends on the rate at which anthropogenic carbon dioxide emissions are removed from the atmosphere by the ocean and land carbon cycles1. Coupled climateâcarbon cycle models suggest that global warming will act to limit the land-carbon sink2, but these first generation models neglected the impacts of changing atmospheric chemistry. Emissions associated with fossil fuel and biomass burning have acted to approximately double the global mean tropospheric ozone concentration3, and further increases are expected over the twenty-first century4. Tropospheric ozone is known to damage plants, reducing plant primary productivity and crop yields5, yet increasing atmospheric carbon dioxide concentrations are thought to stimulate plant primary productivity6. Increased carbon dioxide and ozone levels can both lead to stomatal closure, which reduces the uptake of either gas, and in turn limits the damaging effect of ozone and the carbon dioxide fertilization of photosynthesis6. Here we estimate the impact of projected changes in ozone levels on the land-carbon sink, using a global land carbon cycle model modified to include the effect of ozone deposition on photosynthesis and to account for interactions between ozone and carbon dioxide through stomatal closure7. For a range of sensitivity parameters based on manipulative field experiments, we find a significant suppression of the global land-carbon sink as increases in ozone concentrations affect plant productivity. In consequence, more carbon dioxide accumulates in the atmosphere. We suggest that the resulting indirect radiative forcing by ozone effects on plants could contribute more to global warming than the direct radiative forcing due to tropospheric ozone increases.
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Acknowledgements
We thank N. Gedney for technical support, and M. Sanderson for information on the STOCHEM fields used in this study; we acknowledge discussions with the aforementioned and with M. Ashmore, R. Betts, D. Hemming, O. Boucher and L. Mercado. We also thank A. Everitt for computer support. S.S. was supported by the UK Department for Environment, Food and Rural Affairs (DEFRA) Climate Prediction Programme. W.J.C. was supported by the MoD, and by DEFRA Air and Environment Quality Division, and C.H. by the UK Natural Environment Research Council.
Author Contributions P.M.C. developed the modification to MOSES to include ozone effects on photosynthesis and stomatal conductance; W.J.C. provided the projections of future changes in tropospheric ozone; C.H. developed the IMOGEN software that enabled the global simulations to be carried out; and S.S. calibrated the ozone effects model against data from manipulative field experiments, and carried out and analysed the global simulations. All four authors were involved in the drafting of the paper, although SS took the lead role.
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Sitch, S., Cox, P., Collins, W. et al. Indirect radiative forcing of climate change through ozone effects on the land-carbon sink. Nature 448, 791â794 (2007). https://doi.org/10.1038/nature06059
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DOI: https://doi.org/10.1038/nature06059