Mason Inman, National Geographic News 13 Mar 08;
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.
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.
Fooled?
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."
Dams Lower Global Sea Level
Andrea Thompson, LiveScience Staff Yahoo News 13 Mar 08;
Global sea levels would be higher and rising faster, if not for reservoir water trapped behind dams around the world, a new study suggests.
But the conclusion does not fully account for other human-caused changes to the water cycle, another researcher cautions.
Sea level rise caused by global warming has the potential to severely impact coastal and island communities by encroaching on populations there and increasing storm damage.
Most of the sea level rise in recent decades has been attributed to the thermal expansion of the oceans (water expands as it heats up) and ice melt from the Antarctic and Greenland ice sheets.
Scientists have known for some time that changes in land water storage, including the amount of water locked behind dams, was an important element in the sea level rise equation, but just how important was uncertain, said study leader Ben Chao of the National Central University in Taiwan.
Chao and his colleagues investigated this question by doing a comprehensive tally of all the world's dams constructed since 1900 (about 29,484) and estimating the amount of water they hold. The data was taken from the International Commission on Large Dams' World Register of Dams.
Chao and his team found that altogether these dams hold about 2,600 cubic miles (10,800 cubic kilometers) of water. This corresponds to a drop in global sea levels of about 1.2 inches (30 millimeters). Meaning ocean levels today would be that much higher if some river water wasn't trapped behind dams and prevented from flowing back into the ocean.
"If you look in just the past half century, the observed sea level rise is about 10 centimeters, and the negative effect of the reservoirs in total has been as much 3 centimeters, so in other words, the sea level could have risen 13 centimeters," Chao told LiveScience.
The study's findings are detailed in the March 14 issue of the journal Science.
Accounting for the drop from dams means that the average sea level rise over the 20th century estimated by the Intergovernmental Panel on Climate Change (IPCC) (about 1.7 millimeters per year over the last century) would actually be higher than thought, Chao said.
But other scientists caution against making the leap from the amount of sea level rise to the rate at which it has risen. "That may be one step too far," said Vivien Gornitz of Columbia University's Center for Climate Systems Research, who was not involved with Chao's study.
All of the ways in which humans manipulate water on land must be considered, Gornitz says, from groundwater pumping to the increased runoff from cities covered in concrete, which isn't as permeable as soil.
"You have to consider all of these aspects together," Gornitz said, and some scientists think these effects may cancel each other out in terms of their impact on sea level. This view was noted in the last IPCC report.
