A new study from Virginia Tech and the U.S. Geological Survey uses satellite data to highlight the growing threat to coastal communities: About 40 percent of Americans live on the coast, where aging buildings, roads and railroads face structural damage from flooding.
“The danger is getting worse every day because of rising sea levels,” said lead author Leonard Auchenhen, a doctoral student at Virginia Tech.
According to NASA satellite data, the global average rate of sea level rise has been about 3.3 millimeters per year since the early 1990s, and long-term tide gauge records suggest the rate of rise is accelerating, said Kenneth Miller, a professor of earth and planetary sciences at Rutgers University. Local subsidence, or sinking land, means the threat of sea level rise varies greatly from place to place.
Land subsidence driving sea level rise in some areas
Source: Dangendorf (2019) and National Oceanographic Centre.
Note: Global mean sea level rise data is taken from tide gauge data for values prior to 1993 and from satellite data for values after 1993.
The New York Times
Researchers say the main cause of subsidence is groundwater depletion. Layers of overlapping aquifers stretch along the coast from New Jersey to Florida, providing a steady source of water for drinking, irrigation, and industry. The region receives regular rainfall, but deeper aquifers, which lie beneath clay and bedrock, take hundreds or thousands of years to replenish after they are pumped out. Surface aquifers are vulnerable to contamination and saltwater intrusion.
When the water is removed, the soil compacts and collapses, causing the land surface to sink. Cities built on drained wetlands or reclaimed land are especially vulnerable to compaction.
Other forces also influence the vertical movement of land: Sediment is deposited in river deltas and naturally compressed by its own weight, and the weight of huge glaciers that once extended into New Jersey was removed after the last ice age, causing a seesaw effect that is causing bedrock in the mid- and southern Atlantic to sink by about a millimeter per year.
Boston should be on the rising side of the seesaw, but many areas of the city were built on soft soil that is prone to compression.
Parts of Boston were built on marshes that were reclaimed from the sea for buildings, docks, and other structures.
A map of Boston shows increasing rates of subsidence in dark orange, with almost the entire city painted in that shade of orange.
This is the location of the coastline in the 17th century. Areas that were added to the original coastline show greater subsidence.
The same map of Boston, but showing the amount of subsidence only in areas added to the coastline since 1630. That’s a lot.
Sources: Boston City Archives, Boston Public Library, Boston Groundwater Trust, Ohenhen, Shirzaei, Barnard, PNAS Nexus (2024).
The New York Times
To stabilize early buildings in Boston, wooden piles were driven into the waterlogged ground where groundwater protected the buildings. As the aquifer depleted, the wood was exposed to air, and in some cases the foundations rotted. Some homeowners had to spend hundreds of thousands of dollars to repair the piles to prevent their houses from collapsing.
Although a few millimeters of subsidence per year may seem gradual, the effects can be devastating. A sudden storm surge could wash away soil under roads, or floodwaters could fill basements and cut off emergency routes. Every inch of land sinking toward the water table can make flooding significantly, and potentially catastrophically, worse.
“Sea level rise [rise]”It happens slowly, insidiously, and continuously,” says Dr. Miller of Rutgers University. That is, until the next big storm hits. “We’ve been fortunate in the last 11 years to see that. [Hurricane] Sandy said he has never seen anything like this happen, and clearly there is a very good chance we will see something similar happen in the next decade or so.”
Areas of land that sink or rise faster than adjacent areas can also cause significant damage over time, such as cracking foundations and destabilizing structures. The study authors found hotspots of such distortion near Cape Canaveral, in and around Boston, and on the Delmarva Peninsula in Delaware and Maryland.
More than 40% of the nation’s roads are in poor or fair condition, according to a 2021 assessment by the American Society of Civil Engineers. The same goes for bridges, airports and water infrastructure. The Army Corps of Engineers, which maintains aging levees and plans and builds new coastal defenses, says the increase in disasters in recent years is a direct result of the rise in resilience projects.
“Over the past decade, we’ve seen a steady increase in the number of flood-related subsidence incidents,” said Sandy Hartz, director of the Maryland Department of Transportation’s Office of Climate Change Resilience and Adaptation. The state projects an additional 2.5 feet of relative sea level rise by 2100, according to a study published last year.
Maryland will receive $107 million to improve the climate resilience of its transportation system, out of a total of $7.3 billion allocated to the state through the Bipartisan Infrastructure Act of 2021. Coastal regions will also receive $2.6 billion over five years from the Inflation Reduction Act of 2022 to prepare for and respond to climate hazards.
In recent years, multiple federal agencies have partnered to expand efforts to address climate risks in these regions. This detailed look at land change along the Atlantic coast is part of a broader USGS study of a range of coastal hazards. It adds to a wealth of new data that can inform decisions about coastal risk, including where people live and rely on critical infrastructure, and what water levels are today versus 10, 20, 50 years from now or beyond.
“Water does not follow geographic boundaries, and neither does subsidence,” Hartz said. “We need a shared approach to resilience to protect not only emergency evacuation routes, but also critical infrastructure and destinations across the East Coast.”
Correction made on Feb. 14, 2024: An earlier version of this article misstated Miami’s rate of subsidence. Some areas of the city are sinking up to 1.5 centimeters per decade, instead of 1.5 centimeters per year.
Correction made on Feb. 23, 2024: A map in an earlier version of this article placed Fort Myers, Florida, in the wrong location. The city is located south of Gasparilla Sound-Charlotte Harbor, not north.