Scientists have been uncertain about the impacts of global warming on the world's major rivers. Studies with computer models show that many of the rivers outside the Arctic could lose water because of decreased precipitation in the mid- and lower latitudes, and an increase in evaporation caused by higher temperatures.

Earlier, less comprehensive analyses of major rivers had indicated, however, that global stream flow was increasing.

Dai and his co-authors analyzed the flows of 925 of the planet's largest rivers, combining actual measurements with computer-based stream flow models to fill in data gaps.

The rivers in the study drain water from every major landmass except Antarctica and Greenland and account for 73 percent of the world's total stream flow.

Overall, the study found that from 1948 to 2004, annual freshwater discharge into the Pacific Ocean fell by about 6 percent, or 526 cubic kilometers -- approximately the same volume of water that flows out of the Mississippi River each year.

The annual flow into the Indian Ocean dropped by about 3 percent, or 140 cubic kilometers. In contrast, annual river discharge into the Arctic Ocean rose about 10 percent, or 460 cubic kilometers.

In the United States, the Columbia River's flow declined by about 14 percent during the 1948-2004 study period, largely because of reduced precipitation and higher water usage in the West.

The Mississippi River, however, has increased by 22 percent over the same period because of greater precipitation across the Midwest since 1948.

Some rivers, such as the Brahmaputra in South Asia and the Yangtze in China, have shown stable or increasing flows. But they could lose volume in future decades with the gradual disappearance of the Himalayan glaciers feeding them, the scientists say.

"As climate change inevitably continues in coming decades, we are likely to see greater impacts on many rivers and the water resources that society has come to rely on," says NCAR scientist Kevin Trenberth, a co-author of the paper.

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Editor:Yang Jie