Sea levels are rising all over the world, but in some East Coast regions they're rising higher than in others.
Coastal communities along the Chesapeake Bay in Virginia, for instance, saw sea levels rise a foot-and-a-half over the 20th century, while seas in Portland, Maine, rose only about 6 inches. Boston lands in the middle, with seas rising about a foot over the same period. What gives? Isn't it all the same ocean?
A study published Thursday in the journal Nature and led by the Woods Hole Oceanographic Institution (WHOI) pins the difference on something called "post-glacial rebound," and notes that it will continue for the next couple hundred years, at least.
"A piece of work like this really improves our understanding of what's going on," said Rob Thieler, director of the U.S. Geological Survey Coastal and Marine Science Center in Woods Hole, who was not involved with the study. "As a nerdy scientist who has been working with this kind of problem for a long time, this is really neat."
Scientists have long puzzled over the different levels of sea level rise. "There were all these hypotheses floating around," said Woods Hole physical oceanographer Christopher Piecuch, lead author on the Nature study.
Piecuch said that globally, seas have risen about 6 inches over the past century or so, largely due to melting ice sheets and warming oceans. Scientists thought that local differences could be caused by a number of things: Maybe ocean circulation was shifting, or melting ice sheets had altered the Earth's gravitational field, or some ongoing geological changes were responsible. Piecuch and his colleagues decided to sift through the theories to figure out what was going on.
In lay terms: The Earth is still adjusting after being smushed by glaciers in the last Ice Age.
The researchers gathered data sets from a variety of sources, including tidal gauges, GPS satellite data and coastal marsh sediment records, and ran them though computer models. "We try to synthesize a lot of sources of information to try to get one coherent picture of what's going on," Piecuch said. The data pointed to one main culprit — a phenomenon called "glacial isostatic adjustment," or post-glacial rebound. In lay terms: The Earth is still adjusting after being smushed by glaciers in the last Ice Age.
At that time, a massive ice sheet covered Canada and parts of the northeastern United States. The ice was so heavy, Piecuch said, that it actually weighed down the Earth's crust. And as a result, areas around the edge of the ice sheet — places like the Mid-Atlantic coast — bulged up.
Thieler says the Earth acts like a yoga ball. "They've basically constructed, with data models, an experiment that's having a bunch of different people sit on a bunch of different yoga balls," he said. Each run through the computer model "tells you something about what the ice history might have been and what the Earth's response to that is."
As the ice gradually retreated, the land rebounded: The smushed parts bounced back, and the bulging parts sank down. The coastal areas that are sinking the most correspond to higher sea level rise.
"From the perspective of someone on the coast, you don't just care about how high the seas are, but specifically how high the seas are relative to the point you are on land," Piecuch said. "Whether the sea surface itself rises up, or the land beneath your feet sinks down, both amount to a rise in the sea level that you feel."
These geological effects don't explain all the difference, Piecuch added, "but they are certainly the most important factor."
Mystery solved. And even though the ice sheets disappeared 7,000 years ago, the rebound continues today. The Ice Age stays with us, even as the planet warms.
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