The Atlantic Meridional Overturning Circulation (AMOC) is a major ocean current that could be close to collapse resulting in severe climate impact (extreme weather, ocean levels rising).
The AMOC has two modes. A strong mode and a weak mode. Scientists believe they can observe early warning signs of the transition from strong to weak (the weak mode is the thing to worry about). All eight indices like sea-surface temperature and salinity data, showed that over the course of the last century, we are at a point close to a critical transition.
Read Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation published in Nature.