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Detection of a direct carbon dioxide effect in continental river runoff records


Continental runoff has increased through the twentieth century1,2 despite more intensive human water consumption3. Possible reasons for the increase include: climate change and variability, deforestation, solar dimming4, and direct atmospheric carbon dioxide (CO2) effects on plant transpiration5. All of these mechanisms have the potential to affect precipitation and/or evaporation and thereby modify runoff. Here we use a mechanistic land-surface model6 and optimal fingerprinting statistical techniques7 to attribute observational runoff changes1 into contributions due to these factors. The model successfully captures the climate-driven inter-annual runoff variability, but twentieth-century climate alone is insufficient to explain the runoff trends. Instead we find that the trends are consistent with a suppression of plant transpiration due to CO2-induced stomatal closure. This result will affect projections of freshwater availability, and also represents the detection of a direct CO2 effect on the functioning of the terrestrial biosphere.

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Figure 1: Trends in continental water budgets.
Figure 2: β scale factors obtained from the optimal fingerprinting technique.
Figure 3: Attribution of post-1960 overall runoff trend.


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We thank D. Labat (Laboratoire de Mécanisme de Transferts en Géologie, UMR CNRS, Toulouse, France) for providing the observational runoff data. We thank D. Sexton for statistical advice and A. Jones for discussions. N.G., R.A.B., O.B. and P.A.S. were supported by the UK Department for Environment, Food and Rural Affairs, and P.M.C. and C.H. by the UK Natural Environment Research Council.

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Correspondence to N. Gedney.

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Supplementary information

Supplementary Figure 1

Time series of the annual mean anomalies of continental runoff. The observational data (black) and the best fit from the optimal finger-printing technique (red) are shown. (PDF 33 kb)

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Gedney, N., Cox, P., Betts, R. et al. Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439, 835–838 (2006).

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