Washington, D.C., could sink 16 centimeters in the 21st century, and keep sinking 40 meters or more over 80,000 years—enough to put a fifth of the Washington Monument under the waves of the Atlantic Ocean. That’s according to new research looking at the geologic record of past ups and downs as glaciers came and went over the Northeast United States.
In a kind of slow-motion roller coaster, the land surrounding Washington has risen and fallen. Compared with today’s elevation, the land surface has been higher at some times in the past, whereas at other times it has fallen much lower. Today, it’s on the way down, according to the study, which will be published in the August issue of GSA Today.
In the short term, the findings mean more water woes for a low-lying coastal area already threatened by rising sea levels. They come on top of forecasts that sea levels around the world could rise a meter or more in this century as a result of climate change. The sinking is “another addition to what’s going to come from melting Greenland and melting glaciers,” says Paul Bierman, a geologist at the University of Vermont in Burlington, who worked on the study. In this century “that might make a difference between flooding a subway tunnel and not.” A sea-level rise of just 10 centimeters could cause $2 billion in damages in the capital, according to a 2012 study. In the much more distant future, a 40-meter drop in elevation would swamp many of today’s landmarks. The base of the Washington Monument on the National Mall is 9 meters above sea level and the base of the Capitol sits 27 meters above the sea.
Even if climate change is slowed or halted, there is little to be done about the sinking ground. That was set in motion at the height of the most recent glacial period, called the Last Glacial Maximum (LGM). At the glacial period’s peak, a giant ice mass called the Laurentide Ice Sheet covered much of northern North America, extending as far south as Long Island, New York. Its weight pressed down the underlying earth. In response, land along the southern fringe of the ice mass bulged up, much like a balloon that expands in one spot when squeezed in another. The Washington, D.C., area—which is just below the southernmost extent of that ice mass—sits in this “forebulge.” When the ice sheet stopped expanding and began retreating about 27,000 years ago, that forebulge gradually began to sink back down.
The new study, based on sediment cores drilled in the Blackwater National Wildlife Refuge 80 kilometers southeast of the capital, is part of an emerging rethinking of just how big an impact this ice sheet had. Fueled by a new way to date geologic events, it suggests the mid-Atlantic coast has witnessed far more dramatic fluctuations in elevation than previously thought—and not just since the LGM.
This new view “is really going to open up how we understand the entire Atlantic coastal plain, at least the last couple million years of it,” says David Krantz, a coastal geologist at the University of Toledo in Ohio who wasn’t involved in the work. “It’s huge.”
Scientists have long known the region was gradually sinking, and the roughly 16-centimeter drop during the 21st century was already forecasted based on tide gauges that measure sea level compared with the land.
But for decades, it was widely accepted that in a previous episode, when the Laurentide Ice Sheet peaked approximately 150,000 years ago, it had a pretty shallow footprint, amounting to a slow loss of several meters from current levels over tens of thousands of years, Krantz says.
To better understand the region’s future, scientists have tried to reconstruct the rise and fall connected to that previous visit by the ice sheet 150,000 years ago. To gauge how high the land was at various times, geologists look for evidence of ancient shorelines now up on dry land, much like remnant fossils that recall a long-ago era. Using techniques to date these shorelines, they sync them up with what’s known about past global sea levels at that time to better understand how it compares with land heights today. For instance, if an ancient shoreline is 5 meters above sea level today, but it formed when global sea levels were approximately 10 meters lower, then that piece of land was once 15 meters lower than today.
Scientists had thought that ancient shorelines in the region were 125,000 years old, a time when sea levels were within 9 meters of today.
But in the last 2 decades, scientists have found that some former shorelines in the area are only 80,000 years old, coinciding with a time when sea levels were roughly 20 meters lower, says Ben DeJong, an author of the new paper who conducted much of the research while studying at the University of Vermont.
The new study dates the land’s low point even closer to the present. DeJong looked on dry land at the refuge, and found ancient shorelines that formed between 30,000 and 60,000 years ago, a time when global sea levels were 40 to 80 meters lower than today. Although not in Washington, D.C., the refuge lies at almost the same latitude, and there is no reason it would rise and fall differently, DeJong says.
The revised dates come courtesy of a technique that uses electrons trapped in quartz crystals as a kind of geologic stopwatch. Stray electrons collect in the crystals at a predictable rate, if the crystal is shielded from light. So the stopwatch starts when the sediments in shorelines are buried, letting scientists collect sediment cores and date when the shorelines formed. And then when they are exposed to light/once-buried shorelines are exposed—what happens to the stopwatch?
The new study aligns with other recent work in North Carolina and Virginia. Still, not everyone agrees with the new version of events. Milan Pavich, a retired geologist with the U.S. Geological Survey who was not part of the study, says some geologists express disbelief at his similar findings near the Potomac River, which haven’t been published yet.
“I’ve had people come up and tell me ‘No way.’ They don’t believe it. The paradigm of this stable coastal plain has been so strong,” Pavich says.
Kelvin Ramsey, a geologist at the Delaware Geological Survey in Newark, who reviewed the new study before it was published, cautions against taking the new account as the final word. “There is no consensus regarding interpretations” of how low the shorelines sank in the past, he writes in an email. “Hopefully, in the years to come, as more geologic mapping is conducted in the region, we can get a better handle on this.”