28 avril 2016

La côte est de l'Amérique s'enfonce...

Fascinante découverte:

Geological changes along the East Coast are causing land to sink along the seaboard. That's exacerbating the flood-inducing effects of sea level rise, which has been occurring faster in the western Atlantic Ocean than elsewhere in recent years.

New research using GPS and prehistoric data has shown that nearly the entire coast is affected, from Massachusetts to Florida and parts of Maine.

(...) Engelhart drew on data from prehistoric studies and worked with two University of South Florida, Tampa scientists to combine it with more modern GPS data to pinpoint the rates at which parts of the Eastern seaboard have been sinking.

Their study revealed that Hyde County — a sprawling but sparsely populated farming and wilderness municipality north of the Pamlico River — is among the region's fastest-sinking areas, subsiding at a little more than an inch per decade.

Taken together, that suggests the sea has been rising along the county's shorelines recently at a pace greater than 4.5 inches per decade — a globally extraordinary rate. Similar effects are playing out in places that include Sandy Hook in New Jersey and Norfolk in Virginia, the analysis shows.

(...) The main cause of East Coast subsidence is natural — the providential loss of an ice sheet. Some 15,000 years ago, toward the end of an ice age, the Laurentide Ice Sheet stretched over most of Canada and down to modern-day New England and the Midwest. Its heavy ice compressed the earth beneath it, causing surrounding land to curl upward.

Since the ice sheet melted, the land beneath it has been springing back up. Like a see-saw, that's causing areas south of the former ice sheet to sink back down, including Maryland, Virginia and North Carolina.

The data suggests that some land in coastal Maine, New Hampshire and Massachusetts, on the other hand, is rising slightly, although not quickly enough to keep up with the global rate of sea level rise.

The study shows that subsidence is occurring twice as fast now than in centuries past in a hot spot from Fredericksburg, Va. south to Charleston, which the scientists mostly blame on groundwater pumping.

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