In the past 20 years, sea levels ha

In the past 20 years, sea levels have risen about 3 millimeters per year, which has affected coastal flooding. How much more will the seas rise in the coming decades?The North and South poles are a good place to look for an answer. Melting ice in Greenland and Antarctica is a big cause of rising seas; Greenland has enough ice to raise sea levels by 21 feet. Loss of ice at the North Pole could shut down the Gulf Stream, plunging Northern Europe and Scandinavia into a deep freeze—a scenario depicted in the 2004 sci-fi movie The Day After Tomorrow.“It wouldn’t happen as dramatically as in the movie,” says physicist Thorsten Markus, who studies polar ice at NASA’s Godd ard Space Flight Center in Greenbelt,Maryland.“But that is basica lly the idea. If the circulation shuts down, everything changes dramatically.”Newsweek spoke to Markus, a project scientist for the NASA satellite ICESat-2. Launched in September, it measures polar ice by bouncing laser beams off the Earth’s surface. The elevation readings it obtains are accurate within the width of a pencil.