A long-cooling patch in the North Atlantic—about 1 degree Celsius colder than it was since the 19th century—now appears tied to weaker ocean heat transport. New analysis suggests the “cold blob” is linked to declining heat supply from the Atlantic Meridional O
When the North Atlantic should have been warming in step with the planet, one patch did the opposite. Researchers say a “cold blob” in the region has cooled by about 1 degree Celsius since the 19th century—an anomaly that, even as oceans overall heat up, has stayed stubbornly cold.
The new clue is what’s happening to the heat. In a report published June 16 in Geophysical Research Letters, scientists argue that the cold area is not simply a byproduct of local weather or surface cooling. Instead, it points to a slowdown in how the Atlantic transports heat.
For years. prevailing wisdom held that the patch was getting colder because less heat was arriving by ocean currents—especially a massive circulation system called the Atlantic Meridional Overturning Circulation. or AMOC. The AMOC transports heat from the tropics northward toward Europe, shaping temperature and precipitation across Europe, North Africa and beyond.
But an alternative explanation also circulated: perhaps the water was losing large amounts of heat to the atmosphere, leaving behind colder surface conditions.
To sort out which story the data supported, physical oceanographer Stefan Rahmstorf and his colleagues reanalyzed decades of North Atlantic temperature and heat flux data. Temperature records in the region extend back to 1870, with satellite records taking over around 1993.
The logic was straightforward. If the AMOC had been holding steady—and if the surface was the source of heat loss—the data should show an uptick in heat flux to the atmosphere over time. That pattern never appears.
Instead. the analysis shows a marked decrease in the amount of heat escaping to the atmosphere over the last half-century. with the trend especially pronounced since 1993. The researchers say the largest drop in heat content is found in the top 1. 000 meters. aligning with the location of the AMOC itself.
Together, the results suggest the AMOC’s heat supply to this region has been declining over the last few decades, a conclusion drawn directly from the study.
For climate watchers, that connection lands like another warning sign—one that suggests the AMOC is slowing down and possibly nearing a tipping point, Rahmstorf says. He is affiliated with the Potsdam Institute for Climate Impact Research in Germany.
“The current’s shutdown could bring more cold and hot extreme temperatures to Europe and be devastating for agriculture,” Rahmstorf wrote June 9 on X.
In that same post, he described how his view of the risk had changed. For thirty years of his career studying it, he said he considered the “#AMOC tipping risk” a high impact but low probability risk for the future of humanity. “Recently,” he wrote, “I’ve changed my mind.”
The sequence of observations runs tightly together: a long-term cooling patch. a slowdown in heat escaping to the atmosphere. and the biggest heat-content drop in the same depths and region where the AMOC operates. With those pieces matching rather than contradicting each other. the question now becomes whether the ocean’s heat transport is simply weakening—or moving toward something harder to reverse.
AMOC Atlantic Meridional Overturning Circulation cold blob North Atlantic ocean heat transport climate change Geophysical Research Letters Rahmstorf tipping point extreme temperatures agriculture