Sweat shop.
When CO2 levels are high, plants let less water evaporate through pores called stomata (above) and draw less water from the ground.

Mike Morgan / www.microscopy-uk.org.uk

Global Warming Is No Sweat--for Plants

A rising level of carbon dioxide has made people sweat more by warming the globe, but it's also draining the world's supply of fresh water by making plants sweat less. That's the conclusion of a new study that uses computer simulations to figure out why so much fresh water is running into the salty sea.

Over the past century, more and more fresh river water has been spilling off the continents into the oceans. But mysteriously, no change in overall precipitation can account for this increased flow. The net loss of water is worrying because it increases the risk of drought. Scientists have suspected that human-induced climate change is to blame, but it has proved difficult to finger just where the water budget has sprung a leak.

To crack the case, a team led by Nicola Gedney, a climatologist at the Hadley Centre for Climate Prediction and Research in Wallingford, U.K., and colleagues simulated the past century of weather on Earth. The researchers tweaked four climate change variables: increasing temperatures, changes in global vegetation, the dimming of the sun due to hazier skies, and higher concentrations of carbon dioxide in the atmosphere. They compared how river run-off is affected while introducing each factor separately, and then all four together.

Rising carbon dioxide levels alone appear to have caused the leak. A statistical analysis of the simulations revealed that increasing levels of the greenhouse gas are the main driver of river run-off, but not through global warming. Instead, CO2 is acting as a plant antiperspirant. Plants respond to increased levels of the gas by letting less water evaporate through their pores--known as stomata--and consequently taking up less water from the soil. This leaves extra water in the ground, which is eventually lost to river runoff rather than keeping the air moist--which would keep it circulating as fresh water.

The study is "clear and convincing," says Ian Woodward, a climate scientist at the University of Sheffield, U.K. The effect of CO2 on plant sweating is well known from greenhouse experiments, he says, but detecting the effect on a global scale is "a major result."

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