But the bigger problem is “differential subsidence,” in which different areas of land sink at different rates. If the land were to sink uniformly, you might just need to keep raising the elevation of a levee to compensate. This new study warns that levees and other critical infrastructure along the Atlantic Coast are in similar danger. (In the map below, warmer colors represent more subsidence, up to 6 millimeters.) That’s an even faster change than sea level rise, currently at 4 millimeters a year. And over 3,700 square kilometers along the Atlantic Coast are sinking more than 5 millimeters annually. Using satellite measurements, they have found that up to 74,000 square kilometers (29,000 square miles) of the Atlantic Coast are exposed to subsidence of up to 2 millimeters (0.08 inches) a year, affecting up to 14 million people and 6 million properties. 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. So just as the seas are rising, the land along the eastern seaboard is sinking, greatly compounding the hazard for coastal communities. 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.
