Washington (AFP) – A mile-thick ice sheet in Greenland vanished around 416,000 years ago during a period of moderate natural warming, driving global sea rise to levels that would spell catastrophe for coastal regions today, a study said Thursday. In a dark room, scientists took interior strips of the ice core and exposed them to blue-green or infrared light, releasing trapped electrons that form a kind of ancient clock that shows the last time they were exposed to sunlight, which erases the luminescence signal. “And the only way to do that at Camp Century is to remove a mile of ice,” said Tammy Rittenour, a co-author of the study at Utah State University. " Plus, to have plants, you have to have light." Coastal cities imperiled The Camp Century core was taken only 800 miles from the North Pole, with the study showing the entire region would have been covered in vegetation.