New research confirms that the land under the Chesapeake Bay is sinking rapidly and projects that Washington, D.C., could drop by six or more inches in the next century — adding to the problems of sea-level rise.
This falling land will exacerbate the flooding that the nation’s capital faces from rising ocean waters because of a warming climate and melting ice sheets — accelerating the threat to the region’s monuments, roads, wildlife refuges and military installations.
For sixty years, tide gauges have shown that sea level in the Chesapeake is rising at twice the global average rate and faster than elsewhere on the East Coast. And geologists have hypothesized for several decades that land in this area, pushed up by the weight of a pre-historic ice sheet to the north, has been settling back down since the ice melted.
The new study — based on extensive drilling in the coastal plain of Maryland — confirms this hypothesis, and provides a firm estimate of how quickly this drop is happening. Additionally, the researchers’ detailed field data make clear that the land sinking around Washington is not primarily driven by human influence, such as groundwater withdrawals, but instead is a long-term geological process that will continue unabated for tens of thousands of years, independent from human land use or climate change.
Washington’s woes come from what geologists call “forebulge collapse.” During the last ice age, a mile-high North American ice sheet, that stretched as far south as Long Island, N.Y., piled so much weight on the Earth that underlying mantle rock flowed slowly outward, away from the ice. In response, the land surface to the south, under the Chesapeake Bay region, bulged up. Then, about 20,000 years ago, the ice sheet began melting away, allowing the forebulge to sink again.
“It’s a bit like sitting on one side of a water bed filled with very thick honey,” explains Ben DeJong, the lead author on the new study, who conducted the research as a doctoral student at UVM’s Rubenstein School of Environment and Natural Resources with support from the U.S. Geological Survey, “then the other side goes up. But when you stand, the bulge comes down again.”
The new research provides the first high-resolution data from the same latitude as Washington, D.C., DeJong said, of how this forebulge has subsided — and will continue to. “Until recently, the age of the thing was really poorly constrained,” he said.