Rising levels of carbon dioxide pollution, caused by the unbridled burning of oil, coal and gas, will warm the troposphere, the lowest layer of the world's atmosphere, in addition to the land and seas, it says.

Warmer air temperatures will affect water vapour, cloud cover, solar radiation and ozone, which in turn will have an impact on evaporation and rainfall.

In a computer model that factors in these changes, Princeton University researchers found that precipitation over the next three centuries will increase, boosting the discharge of fresh water around the world by nearly 15 percent.

But the regions that will be benefit are those that are already abundant in water or are sparsely populated, such as the tropics, the far north of Canada and northern Russia.

By contrast, there will be lower flows in many mid-latitude rivers which run through heavily populated regions.

"Those that will start to decline include the Mississippi, Mekong and especially the Nile, one of the world's most heavily used and politically contested rivers, where [the] model predicts an 18 percent fall in flow," the report says.

The scenario, which is published in full in the journal Climate Change, is based on the likely effect from a quadrupling of levels of atmospheric carbon dioxide from levels of this greenhouse gas before the Industrial Revolution.

The predictions are based on what would happen over the next 300 years.

However, research published by experts at France's National Centre for Scientific Research (CNRS) suggests that rivers have already started to be affected by climate change.

They simulated runoff from 200 of the world's largest rivers since 1875 and found that in the past few decades alone, rivers in North and South America and Asia saw an increase in volume, while runoff in Europe was stable, and the flow of water from African rivers had fallen.

The time scale is significant, because records say that global warming moved up a gear in the 1970s. It was at that time that surface temperatures rose quickly in response to the greenhouse effect, New Scientist says.

TERRA.WIRE