Dam building took off in the 1950s and reached a peak in the 1970s. Today few dams are being built and the amount of water being stored in reservoirs is leveling off.
However some megadams, such as China's Three Gorges Dam and Malaysia's Bakun Dam, have been recently built.
(Related news: "China's Three Gorges Dam, by the Numbers" [June 9, 2006].)
The study estimates that human-made reservoirs worldwide store about 2,600 cubic miles (10,800 cubic kilometers) of water—nearly as much as is found in Lake Superior, one of the world's largest lakes.
Water stored in a multitude of smaller reservoirs also adds up, the researchers said. Lots of water also soaks into the ground underneath reservoirs, adding to the amount of water locked up on land.
Vivien Gornitz, of NASA's Goddard Institute for Space Studies in New York City, was not involved in the research.
"The study provides the latest, more accurate estimate of water impoundment by reservoirs," she said.
Dork Sahagian, of Lehigh University in Bethlehem, Pennsylvania, said "we've fooled our own measurements."
A spate of dam building began at about the same time scientists began accurately measuring sea-level rise, he pointed out.
Sahagian also said that the new study may underestimate the effect of reservoirs. It's hard to gauge the amount of water stored in and underneath innumerable small reservoirs, but these figures could be larger than the estimates used in the new study, he said.
"It looks like we will not continue building dams at the same rate, so we will not mask [sea-level rise] anymore," Sahagian said.
In that case "the rate of sea-level rise could double just on account of our stopping building dams."
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