Water in Dams, Reservoirs Preventing Sea-Level Rise

Mason Inman
for National Geographic News
March 13, 2008
Dams and reservoirs have stored so much water over the past several decades that they have masked surging sea levels, a new study says.

But dam building has slowed, meaning sea levels could rise more quickly than researchers predicted in a 2007 Intergovernmental Panel on Climate Change (IPCC) report.

Sea levels have been rising for decades, due mostly to global warming caused by greenhouse gases.

The oceans are on average about 6.3 inches (16 centimeters) higher now than in 1930, when they started a noticeable upward climb. Melting glaciers and ice caps, along with ocean warming—water expands as it heats up—are the main culprits behind the increase.

But the new study shows that reservoirs are also an important factor. Rather than adding to sea-level rise, however, they have counteracted it by storing more water on land.

Since 1930 the storage of water has prevented a total of about 1.2 inches (3 centimeters) of sea-level rise.

Without dams, sea levels would have risen 30 percent more than they already have, according to research led by Benjamin Chao of National Central University in Taiwan.

Chao and colleagues report their findings in tomorrow's issue of the journal Science.

Better Estimate

The latest IPCC report on climate change did not factor in the water stored in reservoirs when examining the causes of sea-level rise, Chao said.

"The reason [this was left out] is the big uncertainty and incompleteness of earlier estimates," he said. So Chao and colleagues set out to make a better estimate.

The researchers tallied up the water stored behind nearly 30,000 dams built worldwide since 1900.

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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