Global Climate Change Impacts in the United States 2009 Report Legacy site

Continental runoff has increased through the twentieth century(1,2) despite more intensive human water consumption(3). Possible reasons for the increase include: climate change and variability, deforestation, solar dimming(4), and direct atmospheric carbon dioxide (CO2) effects on plant transpiration(5). 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(6) and optimal fingerprinting statistical techniques(7) to attribute observational runoff changes(1) 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.