Sea Level Increase Is Kept Down by Reservoirs, Masking Ice Melt

By Alex Morales

March 13 (Bloomberg) -- Man-made reservoirs have cut sea- level gains by 30 millimeters (1.2 inches), masking the true extent of the contribution from melting ice, scientists said.

The two main contributors to rising sea levels are the melting of ice caps and glaciers and thermal expansion, when water takes up more space as it warms. Adding the effects of those two, the United Nations Intergovernmental Panel on Climate Change last year could only account for 1.1 millimeters of the 1.8-millimeter annual gain in sea levels observed since 1961.

Factoring in the findings on reservoirs takes the yearly increase to 2.4 millimeters, the researchers at the National Central University in Chung-Li, Taiwan, reported in the journal Science. That means it's probable that scientists have underestimated both melted ice and thermal expansion, Benjamin Fong Chao, lead author of the study, said yesterday.

``It's likely that both are higher, but that's the million- dollar question in the global warming debate,'' Chao said in a telephone interview from Chung-Li. ``We need to explain the sea- level rise budget, but we're not quite there yet.''

The UN panel last year said global warming will cause oceans to swell 18 to 59 centimeters (7 to 23 inches) by 2100, though they weren't certain how much melting ice sheets would contribute.

Of the observed annual sea level rise of 1.8 millimeters from 1961 to 2003, the panel attributed 0.4 millimeter to thermal expansion and 0.7 millimeter to ice-melt. The remaining 0.7 millimeter wasn't accounted for, they said.

Releasing all the water currently contained in reservoirs would add 30 millimeters to sea levels, according to the study.

Dams

The global average sea level rose about 17 centimeters last century, according to the IPCC. Including the water retained in reservoirs would bring the gain up to 20 centimeters.

Chao's team used the International Commission on Large Dams' World Register of Dams to calculate the volume of water that has been contained over land in more than 29,000 reservoirs globally. Including water that seeps into the soil surrounding reservoirs, about 10,800 cubic kilometers (2,600 cubic miles) of water is retained, they found.

``That's equivalent to about 3 centimeters of global sea level: this is surprisingly large,'' Chao said. ``The reservoirs keep water on land from flowing into the oceans. That actually reduces the global sea level.''

The team then used data on when dams were built, to calculate the year-by-year sea-level gain if all the water retained in dams had instead flowed into the oceans. They found that levels climbed at an ``essentially constant'' rate of 2.46 millimeters a year over the past eight decades, compared with the annual variations actually observed.

``Whether this means that the global changes causing the rise have been in operation in a rather steady fashion, or these changes fortuitously compensated one another over at least the past 80 years, remains to be examined,'' the researchers wrote.