A massive system of rotating ocean currents in the North Atlantic is behaving extremely strangely, possibly because it is approaching a tipping point, a new analysis of clam shells shows.

The North Atlantic subpolar gyre plays a key role in transporting heat to the Northern Hemisphere, and it is a part of a much larger network of ocean currents called the Atlantic Meridional Overturning Circulation (AMOC). But new evidence suggests the subpolar gyre has been losing stability since the 1950s, meaning the gyre’s circulation could weaken substantially in the coming decades, researchers report in a study published today (Oct. 3) in the journal Science Advances.

“It’s highly worrying,” study lead author Beatriz Arellano Nava, a postdoctoral research fellow in physical geography at the University of Exeter in the U.K., told Live Science. “The subpolar gyre was recently acknowledged as a tipping element. We still need to understand more of the impacts of a subpolar gyre abrupt weakening. But what we know so far with the few studies that have been published is that it would bring more extreme weather events, particularly in Europe … and also changes in global precipitation patterns.”