The discovery answers a long-standing puzzle about the greenhouse effect, they report in Thursday's issue of Nature.

Over the last century, the flow from the world's big continental rivers has increased by about four percent, coinciding with an increase in the average global temperature of some 1 C (1.8 F).

This is something of a paradox, as more and more water is being extracted from rivers for irrigation, industry and cities to meet the demands of the world's surging population, and yet the amount of rainfall has changed little.

The new study believes it has found the answer to this.

Higher levels of CO2 are changing the amount of evaporated water that vegetation "breathes out" in its leaves, it believes.

Plant respiration is carried out through tiny leaf pores called stomata.

Previous research has already established that less CO2 in the atmosphere causes plant pores to widen and more water to be sucked out of the soil through the roots. More water thus passes into the atmosphere through transpiration.

Conversely, higher levels of CO2 cause plants to be more efficient in their use of soil moisture. They tighten up their stomata or do not open them for so long, and less water is evaporated this way.

As a result, more water stays in the soil instead of getting passed into the atmosphere through the plant -- and thus more of it runs off into rivers.

Crunching through a simulation of the world's big continental rivers and land use, the computer model concludes that less-thirsty vegetation accounts for the lion's share of the extra runoff.

"This answers a key question about what is driving the changes in the global water cycle," said the study's lead author Nicola Gedney, who is a climate impacts experts at Britain's Joint Centre for Hydro-Meteorological Research.

Scientists have long warned that global warming will have a big effect on freshwater.

Changed patterns of rainfall and snowfall and melting glaciers will cause an increase in flooding in some areas and an increase in the frequency of droughts in others.

The new findings add an important factor into this equation.

Increased water runoff into rivers, as CO2 levels continue to rise, will partially offset the feared decrease in water in areas where rainfall drops.

On the other hand, the extra runoff will magnify the risk of flooding in some areas.

Increased runoff from rivers also has an effect on ocean currents.

A study published last November found that a rush of freshwater into the North Atlantic, from melting glaciers in Greenland, melting sea ice and higher flows from Siberian rivers, was slowing the warm Atlantic current that gives western Europe its mild climate.

On current trends, Britain, Ireland and parts of the European continent could be plunged into bitter winters a decade or so from now, it warned.

Global warming has natural sources, but over the past decade or so a mountain of evidence, now only contested by a small number of diehards, has emerged about Man's contribution to the effect.

It comes overwhelming from fossil fuels that, when burned, disgorge into the atmosphere carbon gases that have been stored underground for millions of years, as oil, gas and coal.

These invisible gases trap the Sun's heat instead of letting it radiate out into space, eventually warming the atmosphere, sea and land and -- as this study has shown -- affecting plant life, too.

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