Boulder (CO) - Some of the world's greatest rivers - including the Ganges, the Niger, the Yellow River and the Colorado River - are drying up. The reduced flows are in many cases associated with climate change, say researchers, and potentially threaten future supplies of food and water.
There has been much debate in the past about the impact of global warming on the world's major rivers. Computer models have tended to show that many rivers outside the Arctic could lose water because of a decline in precipitation in the mid- and lower latitudes and a rise in evaporation caused by higher temperatures. However, in the past some smaller analyses of major rivers have indicated that global stream flow was actually increasing.
For the new study, scientists at the National Center for Atmospheric Research (NCAR) examined 925 of the planet's largest rivers, accounting for 73 percent of the world's total stream flow, from 1948 to 2004. They found significant changes in about one-third of the world's largest rivers, of which about 70 percent showed a decreased flow.
The NCAR findings have important implications for human health. Several of the affected rivers serve large populations, including the Yellow River in northern China, the Ganges in India, the Niger in West Africa and the Colorado in the southwestern United States. The rivers showing increased flow don't do much to help, as they tend to be found in sparsely populated areas near the Arctic Ocean, fuelled by rapidly-melting snow and ice.
"Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases," said NCAR scientist Aiguo Dai, the lead author. "Freshwater being a vital resource, the downward trends are a great concern."
Obviously, there are plenty of factors that can affect river discharge, including dams and the diversion of water for agriculture and industry. The researchers found, however, that the reduced flows in many cases appeared to be related to global climate change, which is altering precipitation patterns and increasing the rate of evaporation. The results are consistent with previous research by Dai and others showing widespread drying and increased drought over many land areas.
Overall, the study found that, from 1948 to 2004, annual freshwater discharge into the Pacific Ocean fell by about six 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 three percent, or 140 cubic kilometers. In contrast, annual river discharge into the Arctic Ocean rose about ten 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, showed stable or increasing flows. But, the authors warn, they too could lose volume in future decades with the gradual disappearance of the Himalayan glaciers which feed them.
"As climate change inevitably continues in coming decades, we are likely to see greater impacts on many rivers and water resources that society has come to rely on," says NCAR scientist Kevin Trenberth, a co-author of the study.
The results will be published May 15 in the American Meteorological Society's Journal of Climate. The research was supported by the National Science Foundation, NCAR's sponsor.