Nature doi:10/1038/nature06045 (2007)

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The response of plants to atmospheric carbon dioxide levels could drive increased continental water runoff in the future, finds a new study. Plant stomata open less widely under higher CO2 levels and therefore return less water directly to the atmosphere through the process known as transpiration. Therefore, more water remains at the land surface, contributing to continental runoff to rivers.

Using 224 versions of a climate model, Richard Betts of the UK Met Office Hadley Centre and colleagues simulated the effect of doubled carbon dioxide concentrations on plant transpiration relative to that of pre-industrial levels to assess the impact on runoff. They found that reduced plant transpiration resulting from rising carbon emissions has increased continental water runoff by 6% since 1860, an increase comparable to that resulting from the direct impact of greenhouse gases on the water cycle.

The findings suggest that, because there will be more runoff to rivers, the risks of future flooding may be greater than previously thought. The researchers emphasize that greenhouse gases affect the climate through indirect means, such as this previously unquantified effect, as well as influencing the climate directly. They recommend that indirect effects should be accounted for in assessing future climate change.