https://phys.org/news/2019-03-ocean-conveyor-belt-foretold-abrupt.html

The team studied a key section of the ocean current pattern, known as the Atlantic Meridional Overturning Circulation (AMOC). They zeroed in on a section where water sinks from the surface to the bottom of the North Atlantic. They confirmed that the AMOC started weakening about 400 years before a major cold snap 13,000 years ago, and began strengthening again about 400 years before an abrupt warming 11,000 years ago.

"Our reconstructions indicate that there are clear climate precursors provided by the ocean state—like warning signs, so to speak," says lead author Francesco Muschitiello, who completed the work as a postdoc at Lamont-Doherty and now works at the University of Cambridge.