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.