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The sea isn't just rising, the ground is sinking: 'Yoga mat' effect explains why the Chesapeake slowly sinking into water

The sea is rising, and this we can be sure of. Records from over a century of tide gauges worldwide show a steady upward trend, a few millimeters each year on average.

Strangely, these sticks in the water don't all tell the same story. In different places, the sea seems to be rising more slowly or faster.

The mid-Atlantic in particular has witnessed some of the fastest encroachment. Over the past century, the Intergovernmental Panel on Climate Change (IPCC) estimates that sea levels have been rising roughly 1.8 millimeters a year. In the region between New Jersey and Virginia, it's been double that.

Locals have long felt the tide inching closer and closer. The record is in their backyards, many of which now abut the ocean. Carolyn Cummins is a past chair of the Worcester County Planning Commission, which includes Ocean City and Assateague Island in Maryland. "I live in a house where the bay is from here to that window," she says, gesturing across the room maybe 20 feet away. "And that house was built in 1950."

In the past couple of decades, satellite measurements have confirmed what geologists have long suspected: It's not just that the sea is rising. The land here is sinking. linkbygging

The process is still something of a mystery, but in the Chesapeake region there are two main suspects.

First is the problem of groundwater withdrawal. Digging wells and drawing water from the ground has consequences. The earth compacts and the surface of the ground starts to sink, slowly, but noticeably. Near San Jose in California, for instance, the land can move up and down by more than an inch yearly, depending on how much underground water is removed, and how much is replenished.

Then there is a post-Ice Age phenomenon called, opaquely, "glacial isostatic adjustment." A better name might be the yoga-mat effect.

Imagine your foot is a glacier. Step on a yoga mat. Look at how the yoga mat bulges up near the edges of your feet. Now step off the yoga mat. Look at how the places that had bulged up are now sinking. That's glacial isostatic adjustment.

The Earth's surface is a lot like a yoga mat. It's a thin, crusty skin floating on squishy molten rock.

Tens of thousands of years ago, the Laurentide Ice Sheet covered most of Canada and reached as far south as Pennsylvania. In places it was up to two miles thick. The weight of all that ice caused the surface of the earth to squish up around the edges. Now that the glacier is gone, the land is slowly settling back. Canada is rising up - and the mid-Atlantic region is sinking.

In the Chesapeake region, it's believed that land subsidence has contributed to about half of the measured rise in sea levels in the last century. Some of that comes from removing water from underground, and some of that is glacial isostatic adjustment. Reducing fossil fuel emissions will tame some of the sea level rise, but it won't fix the natural sinking of the land. Which means that in places like the mid-Atlantic, communities will have to adapt to the increased flooding and inundated backyards.

In Worcester County, Cummins has been campaigning for years to put limits on coastal development. "Where my house is, when those lots were planned, there were two streets in front of me," Cummins says. "They're now underwater. It's teaching people how they're going to live with that."

Via - news.nationalpost.com