for National Geographic News
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.
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.
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