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Big coastal cities sink faster than seas rise

The ruins of a Civil War-era structure, Fort Beauregard, lie partially submerged east of New Orleans. Researchers say many large coastal cities around the world sink faster than sea levels rise. Credit: Frank McMains.

While the threat of rising seas is well established, a phenomenon that is, in a sense, its opposite receives far fewer headlines: large coastal cities sinking faster than oceans can rise.

That is the conclusion of a review article published by a team of scientists who recently assembled in New Orleans, La., and in Venice, Italy, to examine the problem. Extraction of groundwater or fossil fuels, and sometimes simply generations of farming, are causing large metropolitan areas in coastal zones around the world to subside surprisingly quickly—making the relative rise of adjacent seas an even greater potential hazard.

“Sea level rise is a problem, and subsidence is a huge problem, too,” said Cathleen Jones, a researcher at NASA’s Jet Propulsion Laboratory in Pasadena who specializes in such hazards and is a member of the New Orleans team.

“There are areas where it is happening more rapidly than sea level rise,” she said. “I think it is not fully appreciated how much greater subsidence is in some of these areas, and how much it contributes to the loss of wetlands.”

Rapid subsidence makes many of these areas more vulnerable to the effects of climate change, especially those at elevations less than 10 meters (33 feet) above sea level. Several of these qualify as “megacities,” with populations greater than 10 million.

The most threatened: large river deltas, home to an estimated 500 million people. The fastest subsidence rate, 250 millimeters or nearly 10 inches per year, was seen in China’s Huanghe Delta. Southeast Asia saw 30 to 60 millimeters (1.2 to 2.4 inches) per year, while Katrina-ravaged portions of New Orleans saw rates as high as 35 millimeters (1.4 inches) per year.

The article, published in July 2016 in Earth and Space Science News, reports the group’s findings from New Orleans, where they held the first of two workshops. The city is part of the subsiding Mississippi Delta, a cause of wetland loss and the degradation of barrier islands, which, in turn, poses a threat to coastal ecosystems—as well as risks to human populations and infrastructure. Groundwater extraction and drainage of peat-rich soils, the article says, are to blame.

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