Residents of Chesapeake Bay, the 200-mile long estuary which stretches from Baltimore to the coast of Virginia, are no strangers to rising sea levels. Over the last century, locals have documented the disappearance of whole islands as the Atlantic Ocean slowly swallowed them up. Holland Island, whose population of 350 had fled by 1918, once boasted a town, a church schoolhouse and even a baseball team. Its last house, which had become a photogenic icon of the looming problem, collapsed into the sea in 2010.
While climate change is spurring the sea level rise, it’s not wholly responsible. Tide gauges dotted along the bay have shown that sea levels are rising at almost twice the global average, which suggests another factor is accelerating the change. Geologists at the University of Vermont have confirmed that the region of Chesapeake, once bunched up by the weight of the pre-historic ice sheet in the north of the continent, has been sinking back down in the hundreds of years since the ice has melted. Today, the slow land subsidence makes up for half of the bay’s observed sea level rise.
The culprit, the Laurentide ice sheet, is long gone. It covered millions of miles over most of Canada and stretched as far south as New York and Chicago. In a process known as ‘forebulging’, the Chesapeake Bay area to the south of the ice sheet rose as rock mantle was forced outward from under the heavy load. Just as former glaciated regions lift when the ice melts, areas around them might settle and sink.
Ben Dejong, lead author of the study, explains ‘it’s a bit like sitting on one side of a water bed filled with very thick honey; the other side lifts. But when you stand, the bulge comes down again. While it may seem crazy to us in our busy modern lives that this old ice sheet could have any impact, it’s important to recall the timescales over which geologic processes operate. When over 25 million cubic km of ice covers a sizeable portion of North America for an extended period of time, depressing the land surface and displacing mantle material deep in the Earth, it takes a very long time for that mantle material to return to its former state. As the ice only began its retreat around 21,000 years ago, the system has not had much time to re-equilibrate.’
By drilling into the coastal plains of Maryland state, the team has calculated how quickly this drop is happening and that it will continue to do so for millennia, regardless of human activity. They project that the area around the capital will drop by another six inches by 2100. Built on the banks of the Potomac river, Washington D.C. will be in danger of any sea level rise and storms which could exacerbate the land drop. In fact, areas of infrastructure, settlements and wildlife reserves all around the bay are at risk.
As higher waters elevates the risk of flooding, coastal and low-lying regions have begun to react. ‘Right now is the time to start making preparations,’ says DeJong. ‘Six extra inches of water really matters in this part of the world.’