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

Gedney N, Cox PM, Betts RA, Boucher O, Huntingford C, Stott PA

Met Office, Hadley Centre for Climate Prediction and Research (JCHMR), Maclean Building, Wallingford OX10 8BB, UK. nicola.gedney@metoffice.gov.uk

Continental runoff has increased through the twentieth century despite more intensive human water consumption. Possible reasons for the increase include: climate change and variability, deforestation, solar dimming, and direct atmospheric carbon dioxide (CO2) effects on plant transpiration. All of these mechanisms have the potential to affect precipitation and/or evaporation and thereby modify runoff. Here we use a mechanistic land-surface model and optimal fingerprinting statistical techniques to attribute observational runoff changes 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.

Published 16 February 2006 in Nature, 439(7078): 835-8.
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