1. Met Office, Hadley Centre for Climate Prediction and Research (JCHMR), Maclean Building, Wallingford OX10 8BB, UK
2. Centre for Ecology and Hydrology Dorset, Winfrith Technology Centre, Winfrith Newburgh, Dorchester DT2 8ZD, UK
3. Met Office, Hadley Centre for Climate Prediction and Research, Fitzroy Road, Exeter EX1 3PB, UK
4. Centre for Ecology and Hydrology Wallingford, Maclean Building, Wallingford OX10 8BB, UK
5. Met Office, Hadley Centre for Climate Prediction and Research (Reading Unit), Meteorology Building, University of Reading, Earley Gate, Reading RG6 6BB, UK

Correspondence to: N. Gedney¹ Correspondence and requests for materials should be addressed to N.G. (Email: nicola.gedney@metoffice.gov.uk).

Abstract

Continental runoff has increased through the twentieth century^{1, 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.

1. Met Office, Hadley Centre for Climate Prediction and Research (JCHMR), Maclean Building, Wallingford OX10 8BB, UK
2. Centre for Ecology and Hydrology Dorset, Winfrith Technology Centre, Winfrith Newburgh, Dorchester DT2 8ZD, UK
3. Met Office, Hadley Centre for Climate Prediction and Research, Fitzroy Road, Exeter EX1 3PB, UK
4. Centre for Ecology and Hydrology Wallingford, Maclean Building, Wallingford OX10 8BB, UK
5. Met Office, Hadley Centre for Climate Prediction and Research (Reading Unit), Meteorology Building, University of Reading, Earley Gate, Reading RG6 6BB, UK

Correspondence to: N. Gedney¹ Correspondence and requests for materials should be addressed to N.G. (Email: nicola.gedney@metoffice.gov.uk).

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