Closed pores mean more fresh water.

Global temperatures may be on the rise, but plants are drinking and sweating less water. This plant-tissue response to increased carbon dioxide in the atmosphere is having a significant trickle-down effect, a new study finds.

Plants control carbon dioxide intake by opening and closing tiny pores, called stomata, in their leaves. During photosynthesis, they open the stomata to take in carbon dioxide and, inevitably, release some water vapor in the process. How much water is lost when plants sweat, or transpire, in this way affects how much water the plants pull out of the soil.

With more carbon dioxide in Earth's atmosphere, "plants are becoming more efficient" and opening their stomata less, says climate scientist Peter Stott of the Hadley (England) Centre for Climate Prediction and Research. However, the carbon dioxide effect on transpiration, well-known in the laboratory, has been overlooked in models that parcel fresh water among the atmosphere, rivers, and oceans.

To gauge the relative importance of the transpiration change to global freshwater flow, Stott and his colleagues compared actual river-runoff data from the past century with runoff calculated in models that take account of climate change, solar radiation, deforestation, and carbon dioxide--driven changes in transpiration.

The calculations show that reduced plant transpiration played a significant role in the past century's observed increases in river runoff, the team reports in the Feb. 16 Nature.

"It's a good study," says climate scientist Damon Matthews of the University of Calgary in Canada. "To be able to say how the biosphere is changing as a result of elevated carbon dioxide--and to detect that in runoff records--is surprising."