A long-cooling patch in the north Atlantic—known as the “cold blob” or “warming hole”—is drawing fresh scrutiny. New research using climate reanalyses suggests ocean heat loss in the region has declined since 1955 and cooling extends 1000 metres deep, supporti
For years, one patch of the north Atlantic has refused to warm the way the rest of the planet has. It sits south-east of Greenland. In the data visualisation that traces average temperatures in 2015 against the 1951–80 average. the difference is stark: this “warming hole”—also called the “cold blob”—has cooled by as much as 1°C.
Scientists have argued over what it means. The latest evidence is pushing some of that debate toward a single culprit: the Atlantic Meridional Overturning Circulation, or AMOC—the vast system of currents that shifts warmth from the tropics toward Europe.
The AMOC is more than a chart feature. Warm, salty water flows from the Gulf of Mexico northward, cools in the north Atlantic, and sinks. It then returns south along the ocean floor. The concern is that the surge of freshwater from Greenland’s melting ice is making the water less dense. If it sinks more slowly, the circulation weakens—and the climate effects can ripple far beyond the ocean surface.
Some research has gone further, warning that the AMOC could cross a tipping point within decades, locking in a future collapse. The stakes of that scenario are sweeping: a collapse could freeze Europe and disrupt monsoon rains that millions rely on for agriculture across Africa and Asia.
Yet the hard part is time. There are only 22 years of direct observation of AMOC strength—far too short, scientists say, to tease out a clear trend from raw measurements alone.
That’s why the “cold blob” matters. Climate modelling has suggested that a slowing AMOC carries less warm water to the north Atlantic, helping explain the cooling patch. But other modelling has pushed much of the blame onto the atmosphere instead.
In 2022, Chengfei He at Northeastern University in Boston and colleagues offered one atmospheric pathway. They found that rapid warming of the Arctic has reduced the temperature difference between the pole and the tropics. That shift nudges the jet stream northwards into the cold blob region. When the strong westerly winds arrive. the work of the winds—more evaporation and a churn of the water—can draw heat out of the ocean.
Another strand of work suggested that increased evaporation can lead to more clouds. Those clouds, in turn, shade the cold blob from the sun’s warmth.
Now the argument is turning again, and it’s doing so by changing how the question is measured.
Stefan Rahmstorf at the Potsdam Institute for Climate Impact Research in Germany and colleagues investigated the cold blob using climate reanalyses—data products built on direct weather observations from satellites. buoys. and ships. rather than climate modelling alone. Their findings point to a decline in ocean heat loss from the surface of the cold blob since 1955. They also report that the ocean has been cooling not just near the surface, but down to 1000 metres. For them, that depth is the tell.
“That means that the AMOC is transporting less heat, not that winds are taking more heat away,” they argue.
Rahmstorf puts the implication plainly. Winds and clouds “only explain a modest fraction of the warming hole”. “Even if, in some modelling approaches, it seems possible that the cold blob is caused by the atmosphere, in fact, the data show it is caused by the ocean.”
He also says the finding shows Atlantic Ocean circulation has been changing for decades—an important difference from a sudden, short-lived phenomenon. The concern then widens beyond the AMOC itself to the subpolar gyre, a massive swirl of currents around the cold blob.
The subpolar gyre matters because it helps bring in salty surface water needed to feed the sinking of dense water that drives the AMOC. If the gyre shuts down, temperatures in the UK and nearby countries could fall more quickly than they would under a full AMOC collapse.
Rahmstorf adds a second timeline, with a warning that is hard to ignore: “The subpolar gyre passing this tipping point could already lead to serious climate impacts in western Europe as early as in the 2040s,” he says.
Even so, the limits of observation still shape the debate.
One key problem is that ocean surface heat flux hasn’t been directly measured. The Rahmstorf study inferred it through modelling. And a 2021 study, also drawing on some of the same reanalyses, found that stronger winds accounted for most of the cold blob.
“It’s challenging to try to use reanalysis to infer the energy budget in the cold blob,” He says.
For now, the new study is being treated as important—but unfinished.
David Thornalley at University College London calls it useful, but says it “won’t be the final word” on what is causing the cold blob. Because data is limited, alternative explanations can’t be entirely ruled out.
Neil Fraser at the Scottish Association for Marine Science points to one possibility that flips the story away from AMOC weakening as the only driver. A branch of the AMOC known as the Norwegian current may be strengthening, transporting more heat out of the cold blob area.
Fraser puts the bottom line in cautious terms. “The cold blob is consistent with a weakening AMOC,” he says. “But it is not conclusive evidence.”
So the cold blob remains what it has always been: a stubborn pocket of cooling in a warming world. The latest work shifts the emphasis toward the ocean and away from the atmosphere. But until direct measurements fill the gaps. the question of why this region is behaving differently—and what that means for Europe and the rains beyond—stays open.
Atlantic cold blob warming hole AMOC weakening ocean heat loss Greenland meltwater AMOC tipping point subpolar gyre climate reanalyses jet stream winds and clouds