It says efforts to calculate flooding risk from climate change do not take into account the effect that carbon dioxide (CO2) -- the principal greenhouse gas -- has on vegetation.

Plants suck water out of the ground and "breathe" out the excess through tiny pores, called stomata, in their leaves.

Stomata are highly sensitive to CO2. The higher the level of atmospheric CO2, the more the pores tighten up or open for shorter periods.

As a result, less water passes through the plant and into the air in the form of evaporation. And, in turn, this means that more water stays on the land, eventually running off into rivers when the soil becomes saturated.

In a paper published in February 2006, British scientists said the CO2-stomata link explained a long-standing anomaly.

Over the last 100 years the flow of the world's big continental rivers has increased by around four percent, even though global temperatures rose by some 0.8 degrees Celsius (1.35 degrees Fahrenheit) during this period.

Today, as a result of the unbridled burning of oil, gas and coal, levels of CO2 in the atmosphere are around a third more than in pre-industrial times in the middle of the 18th century.

The new study takes the 2006 discovery an important step further by projecting what could happen to water runoff in the future.

If CO2 levels double compared with pre-industrial concentrations -- a common scenario in climate simulations -- the effect on plants alone would lead to an increase of six percent in global runoff, it says.

Until now, scientists have generally estimated an increase in runoff of between five and 17 percent compared with the pre-industrial era.

But this is based only on one yardstick, called radiative forcing. In other words, it only measures the warming effect that greenhouse gases have on the water cycle and not the indirect impact that CO2, the biggest culprit, has on vegetation.

The "radiative forcing" yardstick also predicts that higher temperatures will increase evaporation, causing greater water stress and longer droughts.

Both forecasts are offbeam, says the new paper.

By widening the picture to include the CO2-stomata factor, the likelihood is that the risk of flooding will be worse than thought, but the risk of drought rather less so.

"The risks of rain and river flooding may increase more than has been previously anticipated, because intense precipitation events would be more likely to occur over saturated ground," it says.

"In contrast, the risks of hydrological drought may not increase as much as expected on the basis of meteorological changes alone."

Flooding is a major problem, especially in poor countries that do not have the money to upgrade drainage systems to cope with runoff from saturated soils.

Since June, nearly 3,200 people in South Asia have been killed by heavy monsoon rains and snow melt. More than 20 million people have been affected in the eastern Indian state of Bihar alone.

The authors of the Nature paper are led by Richard Betts of the Hadley Centre for Climate Prediction and Research, part of Britain's Met Office.

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