If the greenhouse gases that fuel global warming continue their unbraked rise, rivers in many regions of the world will be greatly affected by the end of the 21st century, said the Hadley Centre, part of the UK Met Office (meteorological service).

"There are large predicted reductions in river flow across much of Europe, North Africa, Midwest America and northern South America, whilst large increases in river flow occur in West Africa, northern China and the boreal regions," they said in a report issued on the sidelines of the UN climate conference here.

"Important changes in the seasonality of river flow could also occur."

Scientist Vicky Pope said farmers and cities which depended on rivers for irrigation, transport and water supply "could be seriously affected," although further work was needed to predict the impact more accurately.

The findings come from a new computer model, HadGEM1, put together by the Centre, located in the southwestern English city of Exeter.

The model is crunched by one of the world's most powerful super-computers, capable of two million million calculations per second.

Such power is essential for figuring out the impact of a wide range of variables, such as winds, rainfall patterns, ocean temperatures, soil and vegetation.

Under the so-called business-as-usual scenario, emissions of carbon dioxide, the principal greenhouse gas, would continue to rise, causing global temperatures by the end of the century to rise by 3.4 C (6.1 F).

This will have an impact not only on rainfall and other weather patterns, but also on vegetation, the researchers said.

More CO2 causes the stomata, the leaf pores through which plants and trees breathe, to close up.

As a result, the plants transpire less and give up less moisture, which they take from the ground, to the atmosphere. As a result, more water goes into the soil and ultimately ends up in rivers, increasing the flow.

The Hadley researchers also concluded that there has already been a significant increase in flows into the Arctic Ocean from six big Siberian rivers -- the Yenisey, Lena, Ob, Kolyma, Pechora and Dvina.

That finding backs a study by University of Southampton oceanographers, published last week, which reported a slowdown in the warm current that bathes Northwestern Europe, giving the region a relatively balmy climate for its northerly latitude.

The trend could lead to a plunge in regional temperatures of between 1-2 C (1.8-3.6 F) and as much as 4 C (7.2 F) a decade from now, they said.

The "brakes" acting on this current are an inrush of freshwater from the Arctic Ocean, that study suggested.

Separately, a study sponsored by the environmental group Greenpeace found that global warming was badly affecting the Yellow River, dubbed China's "mother river" for its importance.

"In recent years, due to climate change, the glaciers, permafrost, lakes and wetlands, and the hydrological and ecological environments in this region have undergone dramatic changes that have far-reaching implications," it said.

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