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Projected increase in continental runoff due to plant responses to increasing carbon dioxide


In addition to influencing climatic conditions directly through radiative forcing, increasing carbon dioxide concentration influences the climate system through its effects on plant physiology1. Plant stomata generally open less widely under increased carbon dioxide concentration2, which reduces transpiration1,3,4,5,6 and thus leaves more water at the land surface7. This driver of change in the climate system, which we term ‘physiological forcing’, has been detected in observational records of increasing average continental runoff over the twentieth century8. Here we use an ensemble of experiments with a global climate model that includes a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff, in the context of uncertainties in future precipitation. We find that the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 per cent relative to pre-industrial levels; an increase that is comparable to that simulated in response to radiatively forced climate change (11 ± 6 per cent). Assessments of the effect of increasing carbon dioxide concentrations on the hydrological cycle that only consider radiative forcing9,10,11 will therefore tend to underestimate future increases in runoff and overestimate decreases. This suggests that freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought. Moreover, our results highlight that the practice of assessing the climate-forcing potential of all greenhouse gases in terms of their radiative forcing potential relative to carbon dioxide does not accurately reflect the relative effects of different greenhouse gases on freshwater resources.

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Figure 1: Impact of physiological forcing on global mean runoff, precipitation and temperature.
Figure 2: Impact of physiological forcing on relationship between changes in runoff and precipitation on doubling CO2.


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We thank G. Dupre, D. Matthews, A. Nobre, C. Rye, M. Sanderson, S. Sitch and T. Wheeler for comments. This work was supported by the UK Ministry of Defence project “Defence and Security Implications of Climate Change” and the Climate Prediction Programme of the UK Department for Environment, Food and Rural Affairs. P.M.C. and C.H. were supported by the UK Natural Environment Research Council.

Author Contributions R.A.B. proposed the study, performed the dynamic vegetation simulations and led the analysis and writing. D.L.H. performed statistical analysis of the ensemble simulations and contributed expertise on field experiments on plant physiology. P.D.F. analysed the dynamic vegetation simulations. P.M.C. developed the MOSES and TRIFFID models and contributed to the interpretation. C.D.J., N.G., C.H. and O.B. contributed to the analysis and provided further expertise on modelling plant physiology, hydrology and land–atmosphere interactions. M.C., D.M.H.S. and M.J.W. designed and performed the ensemble simulations and advised on their interpretation. All co-authors contributed to the text.

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Correspondence to Richard A. Betts.

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Supplementary Tables

This file contains Supplementary Tables 1-6 showing impacts of physiological forcing on global mean precipitation, runoff and runoff/precipitation ratio for each continent except Antarctica. (PDF 107 kb)

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Betts, R., Boucher, O., Collins, M. et al. Projected increase in continental runoff due to plant responses to increasing carbon dioxide. Nature 448, 1037–1041 (2007).

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