Scientists have known for years that water levels are rising faster in the Chesapeake Bay region than just about anywhere else in the world — because water is only part of the problem here. The land is also sinking.
That phenomenon, known as “land subsidence,” accounts for more than half of the sea level rise in many places around the Bay. But if researchers and land managers look up subsidence rates in the scientific literature for some important piece of real estate, they encounter a barrage of different estimates.
Credit: D. Sarah Stamps
Some clarity has arrived. Scientists undertaking the largest research effort of its kind in the Chesapeake region have officially clocked the average speed of the Bay region’s sinking topography at a rate of 1.4 millimeters per year.
That amount — just under the width of a penny — is roughly consistent with what smaller, more localized studies have suggested.
But the authors of this study, led by Virginia Tech and the U.S. Geological Survey, say their effort is unique for its size and standardized approach. They measured vertical land motion at dozens of previously unstudied sites around the Bay and combined their findings with information gathered at 120 existing stations.
The result, they say, is the first Baywide clearinghouse for subsidence data.
“We were able to get a really comprehensive map that covered the entire Chesapeake Bay area,” said Karen Williams, the paper’s lead author. She was a doctoral student at Virginia Tech at the time of the study and is now doing postdoctoral research at the University of Alaska Fairbanks.
The study showed sinkage at slower rates in the northern part of the Bay region and inland portions west of Interstate 95. Researchers observed the fastest rates in the southern half of the study area, especially around Virginia’s Hampton Roads, Southern Maryland and Maryland’s lower Eastern Shore.
Some of that sinkage is natural. When areas to the north more than 10,000 years ago were covered by massive ice sheets, experts say, that weight squeezed material beneath it outward, causing the land to bulge at its edges. When the ice melted, that bulked-up land (including around the Bay region) began a slow descent.
“We’re like a teeter-totter,” said Michael Scott, a geographer at Salisbury University in Maryland who wasn’t involved in the study.
How to explain the variations in subsidence rates in the region? That remains an “open question,” said Virginia Tech’s D. Sarah Stamps, a geophysicist who monitors the Earth’s size and shape and was one of the paper’s main authors. One possible answer, suggested by previous studies, is that areas with faster sinkage are those that have higher rates of groundwater withdrawals for drinking water and irrigation.
Extracting water from underground reservoirs at high rates is known to compress the layers of sediment that remain behind, causing the surface of the land to sink.
Stamps, Williams and their colleagues compared their subsidence rates with those recorded during a similar study conducted 50 years earlier. They found signs that the pace of sinkage is accelerating for about one-quarter of the studied communities. Most of those sites were concentrated along the Delmarva Peninsula.
The middle Delmarva Peninsula — around Cambridge and Salisbury in Maryland and across central Delaware — has at least pulled even with Hampton Roads for having the region’s fastest rate of subsidence, the study suggests. Williams said it wasn’t clear what is causing Delmarva’s sinkage to speed up.
In the Hampton Roads area, subsidence rates have actually decreased across the board, with Norfolk, Portsmouth and Smithfield notching the most notable improvements, according to the study. The paper suggests the slowdown may be connected to steps taken in recent decades to regulate and reduce groundwater withdrawals in the region.
A millimeter or two of land-sinking may not sound like much, Scott noted, but “we’re dealing with timespans that are so long that those millimeters really start to add up.”
The waters along Maryland’s shores are expected to rise anywhere from 2.5 to 4.5 feet by 2100 due to the combined effects of sea level rise and subsidence, according to a 2023 report by the University of Maryland Center for Environmental Science. That forecast relied on subsidence estimates predating the new Virginia-led study.
Climate researchers in the Bay region have often struggled to account for land subsidence in their estimates of rising water, the study’s authors wrote. Some earlier subsidence studies only looked at certain portions of the region, while others were conducted over different time periods or used varying scientific tools to measure the phenomenon.
Stamps said the team obtained their hyper-precise measurements using GPS, the same technology found in smartphones. The study compiled data collected from 2019 to 2023.
“It was a pretty amazing effort,” said Stamps, adding that the team included members from four universities in the Bay region, five federal agencies and two state agencies.
Previous radar-based studies offer a broader look at the Bay region’s geography. But their subsidence measurements are known to be less reliable, owing to atmospheric interference and other potential confounding factors, Williams said. GPS studies are more dependable, but they require a phalanx of ground-based stations to measure subsidence over a wide area.
So, every fall for five years, the researchers in the study led by USGS and Virginia Tech fanned out across 60 new sites to measure subsidence.
The study was published in December in the journal Scientific Reports. Stamps said she hopes that by including data on more locations the research will lead to better land-management decisions at the local level.
The researchers expect to continue taking measurements every year for as long as their funding holds out. They want to keep tabs on changes in subsidence rates across the region over time.