In the movie The Day After Tomorrow , the world's weather systems are thrown into chaos when an important Atlantic Ocean current abruptly shuts down. Tornadoes rip through Los Angeles, sea levels rise and wipe out most of coastal New York, and large parts of the Northern Hemisphere are plunged into a deep freeze so severe that any exposure to the air kills people almost instantly. While the movie is probably the best-known example of cli-fi — climate fiction — its premise is loosely based on real science. And it is something scientists are taking very seriously. The Atlantic Meridional Overturning Circulation (AMOC), is one of several ocean conveyor belts that move water around the globe. It carries warm water from the southern Atlantic northward, where it cools and then sinks. This circulation helps give Europe a milder climate than parts of Canada at the same latitude. But there are fears that, as in the movie, it could collapse.…