The Land is Steadily Sinking Up and Down America's Atlantic Coast

In Jakarta, Indonesia, "the land is sinking nearly a foot a year because of collapsing aquifers," reports Wired. "Accordingly, within the next three decades, 95 percent of North Jakarta could be underwater." "Subsidence" is caused by over-extracting groundwater, or the settling of sediments - and it's not just happening in Indonesia. "In California's agriculturally intensive San Joaquin Valley, elevations have plummeted not by inches, but by dozens of feet."Last year, scientists reported that the US Atlantic Coast is dropping by several millimeters annually, with some areas, like Delaware, notching figures several times that rate. So just as the seas are rising, the land along the eastern seaboard is sinking, greatly compounding the hazard for coastal communities. In a follow-up study just published in the journal PNAS Nexus, the researchers tally up the mounting costs of subsidence - due to settling, groundwater extraction, and other factors - for those communities and their infrastructure... [O]ver 3,700 square kilometers [1,428 square miles] along the Atlantic Coast are sinking more than 5 millimeters annually. That's an even faster change than sea-level rise, currently at 4 millimeters a year... A few millimeters of annual subsidence may not sound like much, but these forces are relentless: Unless coastal areas stop extracting groundwater, the land will keep sinking deeper and deeper... The researchers selected 10 levees on the Atlantic Coast and found that all were impacted by subsidence of at least 1 millimeter a year. That puts at risk something like 46,000 people, 27,000 buildings, and $12 billion worth of property. But they note that the actual population and property at risk of exposure behind the 116 East Coast levees vulnerable to subsidence could be two to three times greater. "Levees are heavy, and when they're set on land that's already subsiding, it can accelerate that subsidence," says independent scientist Natalie Snider, who studies coastal resilience but wasn't involved in the new research. "It definitely can impact the integrity of the protection system and lead to failures that can be catastrophic...." The study finds that subsidence is highly variable along the Atlantic Coast, both regionally and locally, as different stretches have different geology and topography, and different rates of groundwater extraction. It's looking particularly problematic for several communities, like Virginia Beach, where 451,000 people and 177,000 properties are at risk. In Baltimore, Maryland, it's 826,000 people and 335,000 properties, while in NYC - in Queens, Bronx, and Nassau - that leaps to 5 million people and 1.8 million properties. Highways, airports, and even railway tracks could also be affected....

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