Rising CO2 could lead to rising rivers as plants lose thirst

River flow increases already predicted by climate change models could be underestimating the scale of the problem, as rising levels of CO2 in the atmosphere will make plants less thirsty, say scientists.

Existing models suggest that, even without changes in plant behaviour, climate change will increase river levels on the whole.

But according to joint research by the Met Office, the University of Exeter and the Centre for Ecology & Hydrology, the exact impact is more complex than previously thought.

The team of scientists believe that increased levels of CO2 will cause plants to extract less water from the soil, meaning more water from precipitation will drain into rivers, increasing their flow.

If correct, the predictions mean more flooding, but less droughts are likely.

"It's a double-edged sword," said leader of the team, Dr Richard Betts, a climate impact scientist at the Met Office.

"It means that increases in drought due to climate change could be less severe as plants lose less water. On the other hand, if the land is saturated more often, you might expect that intense rainfall events are more likely to cause flooding."

Last year, members of the research team showed that this effect can already be seen in historical river flow records. This study shows that the effect of plant responses to CO2 could be as important as those of increased rainfall due to man-made climate change.

Dr Betts, a lead author on the recent IPCC report, added that this effect also makes it more difficult to compare CO2 with other greenhouse gases which do not affect plants in the same way.

"We often hear about the CO2 equivalent of other gases such as methane and nitrous oxide, used in calculating carbon footprints," he said.

"But this only accounts for the effect of these gases on global warming. If we want to compare their full impacts on droughts and flooding, we need to consider direct effects on plants too."

The findings of the research were published in science journal Nature on August 30.

David Gibbs



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