For more than a century, a patch of cold water south of Greenland has resisted the Atlantic Ocean’s overall warming, fueling debate amongst scientists. A new study identifies the cause as the long-term weakening of a major ocean circulation system.

Researchers from the University of California, Riverside show that only one explanation fits both observed ocean temperatures and salinity patterns: the Atlantic Meridional Overturning Circulation, or AMOC, is slowing down. This massive current system helps regulate climate by moving warm, salty water northward and cooler water southward at depth.