“In the last five years or so,” he says. “I really unfortunately had to change my view about the probability of this happening.”
His argument begins with seawater. The AMOC depends on dense water sinking in the high North Atlantic. Cold and salt help make seawater dense.
As warm, salty surface water travels north through the Atlantic, evaporation makes it saltier. Further north, it cools. Together, those processes can make the water heavy enough to sink into the deep ocean, helping to drive the AMOC’s overturning flow.
Could the AMOC reach a tipping point?
Rahmstorf’s concern is not just that global warming could weaken that process. It is that, beyond a certain point, the weakening could begin to feed on itself.
Warmer surface water is lighter. Extra rainfall and meltwater from ice make the ocean fresher. Fresher water is lighter too.
If the water in the high North Atlantic becomes too light, less of it sinks. If less water sinks, the AMOC weakens. And if the AMOC weakens, it brings less salty water north.
