CLIMATE SCIENTISTS ALREADY know that the East Coast of the United States could see around a foot of sea-level rise by 2050, which will be catastrophic on its own. But they are just beginning to thoroughly measure a “hidden vulnerability” that will make matters far worse: The coastline is also sinking. It’s a phenomenon known as subsidence, and it’s poised to make the rising ocean all the more dangerous, both for people and coastal ecosystems.

New research published in the journal Nature Communications finds that the Atlantic Coast—home to more than a third of the US population—is dropping by several millimeters per year. In Charleston, South Carolina, and the Chesapeake Bay, it’s up to 5 millimeters (a fifth of an inch). In some areas of Delaware, it’s as much as twice that.

Five millimeters of annual sea-level rise along a stretch of coastline, plus 5 millimeters of subsidence there, is effectively 10 millimeters of relative sea-level rise. Atlantic coastal cities are already suffering from persistent flooding, and the deluge will only get worse as they sink while seas rise. Yet high-resolution subsidence data like this isn’t yet taken into account for coastal hazard assessments. “What we want to do here is to really bring awareness about this missing component, that based on our analysis actually makes the near-future vulnerability a lot worse than what you would expect from sea-level rise alone,” says Manoochehr Shirzaei, an environmental security expert at Virginia Tech and coauthor of the new paper.

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