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.
The primary cause of dramatic land subsidence is over-extracting groundwater from it, which makes the terrain collapse like an empty water bottle. In San Jose, California, this has lowered the elevation by as much as 12 feet. The combination of sea-level rise and subsidence could inundate up to 165 square miles of Bay Area coastline by 2100, according to Shirzaei’s previous research. Parts of Jakarta are sinking 10 inches a year, forcing Indonesia to move its capital elsewhere. Extracting oil also causes subsidence, a particularly acute problem in the Houston-Galveston area. And landfill or sediments along coastlines can also settle over time.
While scientists have been aware that US coastlines are sinking, they haven’t had much data to show local differences in rates. Subsidence varies significantly even over short distances, given variations in the underlying geology and nearby human activity. For this new paper, Shirzaei and lead author Leonard Ohenhen, also an environmental security expert at Virginia Tech, used data from a highly sensitive satellite that fired radar signals at the Earth, then analyzed what bounced back to determine coastal deformation. They did this for the years between 2007 and 2020, along 3,500 kilometers (2,200 miles) of the Atlantic coast.
The researchers found particularly intense subsidence in agricultural areas, where groundwater is extracted to feed crops—which in turn will be more vulnerable to flooding as the elevation drops. They also found that most Atlantic coastal cities are seeing over 3 millimeters of subsidence a year, including Boston and New York City. As the elevation falls, it destabilizes above-ground infrastructure like buildings and roads, as well as buried pipes and cables.