There's a paper out in Nature Communications, in which PhD student Chris Boulton and colleagues say they've detected signals that can be used as an early warning of a collapse of the Atlantic Meridional Overturning Circulation (AMOC). They used a coupled climate model that showed that these signals could become significant about 250 years before the start of any collapse, and, going by the charts in the paper, may start to be seen sooner than that.
The AMOC transports heat from the tropics northward in upper levels in the ocean in the northern hemisphere, sending cooler water southward at lower levels in the ocean. You can read more about it here at the Encyclopedia of Earth. Here's a video from YouTube:
