The Atlantic’s ‘cold blob’: a warning sign scientists can’t ignore

On global temperature maps that otherwise glow red with warming, one patch of the North Atlantic Ocean stands out as a stubborn blue anomaly. Scientists call it the ‘cold blob,’ and for a growing number of researchers, it has become the clearest visible sign that one of the planet’s most important ocean systems may be changing.

The system in question is the Atlantic Meridional Overturning Circulation, or AMOC, a network of ocean currents that functions like a giant conveyor belt. It pulls warm surface water from the tropics north toward the Arctic, then sends colder, denser water back south through the deep ocean, according to a BBC report.

Stefan Rahmstorf, an ocean physicist at the Potsdam Institute for Climate Impact Research who has studied the AMOC for three decades, has pointed to the cold blob as a clear fingerprint of a slowing current. The mechanism, researchers say, comes down to seawater density: as the planet warms, melting ice and heavier rainfall add fresh water to the North Atlantic, and that fresher water is too light to sink the way colder, saltier water does, weakening the circulation further.

The AMOC carries roughly one petawatt of heat northward, an amount of energy equal to about 50 times the total power humanity consumes, and it is why Britain enjoys a far milder climate than other regions at the same latitude. Scientists broadly agree the system will weaken as the planet warms; the disagreement is over how fast that will happen and what it will mean for the seasons.

Rahmstorf once viewed a full AMOC shutdown as a highly unlikely worst-case scenario, but recent data and climate modelling have led him to believe the risk of crossing a definitive tipping point is real, one that could make a shutdown self-amplifying and effectively unstoppable.

Not every scientist reads the cold blob the same way. Andrew Watson, a Royal Society Research professor at the University of Exeter, has noted that the AMOC has changed before in Earth’s history and argues it is a mistake to treat it as a switch that simply turns off; if the North Atlantic slows, he suggests, the system may reorganise rather than collapse. A UK Met Office study similarly found the AMOC is unlikely to stop working entirely within this century, while the Intergovernmental Panel on Climate Change’s 2021 assessment expects the current to weaken in the coming decades without shutting down before 2100.

Wikimedia Commons/by Bruce Miller, CSIRO