Britain’s mild winters depend on a current that’s now weakening

Britain’s relatively mild climate, compared with other places at the same latitude, is owed largely to a single ocean system: the Atlantic Meridional Overturning Circulation, or AMOC, according to a BBC report. That current now sits at the heart of one of the most closely watched debates in climate science, over whether it is beginning to break down.

The AMOC functions as a planetary conveyor belt, pulling warm surface water from the tropics toward the Arctic and sending colder, denser water thousands of miles back south through the deep ocean. It carries about one petawatt of heat northward, an amount equal to roughly 50 times the total power humanity consumes.

Researchers are tracking the system using uncrewed floats such as a bright yellow, human-sized probe drifting in the waters off Greenland, which dives deep, measures temperature, pressure and salt levels, then surfaces to beam its findings to a satellite. Scientists broadly agree the AMOC will weaken as the planet warms, though they disagree on how fast that will happen.

Some point to a patch of unusual cooling in the North Atlantic, known as the ‘cold blob,’ as early evidence of the slowdown. Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, who has studied the AMOC for three decades, calls the cold blob a clear fingerprint of a slowing current, driven by melting ice and heavier rainfall adding fresh water that is too light to sink the way colder, saltier water does.

A disrupted AMOC could plunge the UK and northwestern Europe into more volatile weather, including significantly colder and drier winters, and could also affect the West African monsoon and tropical rainfall belts that hundreds of millions of people in the Amazon and Africa rely on. Rahmstorf, who once viewed a full shutdown as a highly unlikely worst-case scenario, now believes the risk of crossing a tipping point is real.

Other researchers are more cautious. Andrew Watson of the University of Exeter has said the AMOC has changed before in Earth’s past and may reorganise its sinking zones rather than collapse outright, a view echoed by a UK Met Office study that found a full shutdown unlikely this century. The Intergovernmental Panel on Climate Change’s 2021 assessment expects the current to weaken in the coming decades without stopping before 2100.

Wikimedia Commons/by Bruce Miller, CSIRO