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

Primary Topic: 
Vulnerability & Adaptation
Water Resources
Document Type: 
Document Author: 
Document Author (other): 
C. Huntingford & P. A. Stott, N. Gedney, O. Boucher, P. M. Cox, R. A. Betts
Publication Date: 
2006 February 6

Continental runoff has increased through the twentieth century 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 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 fresh water availability, and also represents the detection of a direct CO2 effect on the functioning of the terrestrial biosphere.