"In particular, many scientists have been paying close attention to the Atlantic Meridional Overturning Circulation (AMOC), an ocean pattern that transports warm water from the tropics up to the North Atlantic and Nordic seas. (This is also sometimes referred to as the Gulf Stream system.)
This system is the reason why Europe has a relatively warm climate despite being so far north. But it's also a potential source of concern: paleoclimate evidence suggests the overturning circulation, or AMOC, has abruptly slowed or stopped in the distant past. Were that to happen again, it could be quite bad for both Europe and the United States.
Now, a new study in Nature Climate Change argues the Atlantic overturning already appears to be weakening. The researchers, led by Stefan Rahmstorf of the Potsdam Institute for Climate Research, created an index of regional climate conditions going back centuries. They find the weakness of the AMOC appears to be unprecedented in the past 1,100 years, possibly due to an influx of freshwater from Greenland's melting ice caps. (This contrasts with previous work suggesting the AMOC was still just fluctuating in natural cycles.)"