A patch of the North Atlantic Ocean just southeast of Greenland has been baffling scientists for over a century due to the steady cooling of its waters, despite the increase in temperature being experienced by the rest of the world’s oceans. While initially thought to be an influx of colder glacial meltwater from Greenland, or possibly even a result of pollution from shipping partially blocking sunlight, recent research suggests that the lower temperature of this region may be due to the North Atlantic Current slowing—a situation that could trigger a domino effect of climate changes across northern Europe and beyond.

Despite the marked increase in the temperature of the world’s oceans, the cold blob, sometimes referred to as the ‘blue blob’, that straddles the North Atlantic just southeast of Greenland, has cooled by about a half-degree Celsius over the past 120 years. Typically, the region would see thermal energy carried from the south by the Atlantic meridional overturning circulation (AMOC) being released here, but if the AMOC was weakening, that lack of warm water from the south might account for the regional drop in temperature.

As part of the network of currents that comprise the AMOC, the North Atlantic current—more commonly known as the Gulf Stream—zig-zags northward from the west coast of Africa across to the Caribbean where it picks up warm tropical waters, and then tracks northeast towards northern Europe.

Under normal circumstances, the warm water from this southern stream cools when it meets the currents flowing south from the Arctic, causing the water to sink into the depths where it joins another current that runs southward, deep under the Atlantic, to continue its three-year journey through the world’s oceans.

However, the accelerated melt of glaciers in Greenland have already caused a large amount of fresh water to flood into the ocean where this downward convection takes place, and this fresh water, being less dense than salt water, inhibits the sinking of the current’s waters, slowing the flow of the entire system.

The immediate effect of such an interruption to the AMOC would be that warm water from the south would no longer be delivered to the waters off the coast of northwestern Europe, right where the cold blob is currently situated.

Regardless of whether one considers such a scenario to be a symptom or a cause—or possibly both—the interruption of this key heat conveyer could cause the climate of northwestern Europe to cool enough to “potentially threaten the viability of agriculture in northwestern Europe,” according to an open letter presented to the Nordic Council of Ministers by a cadre of scientists concerned that enough wasn’t being done to address the situation.

“A 50 percent reduction would be far more substantial of an impact on our regional climate,” according to University of Southampton professor of oceanography and climate Robert Marsh, who is concerned that the cold-blob’s presence is an indicator that the AMOC is slowing. If the damage can somehow be mitigated, “a 10 percent weakening of the AMOC would be noticeable, but manageable,” Marsh added. “It probably would make our weather a little less mild in the wintertime, in particular.”

While there is speculation that the AMOC may grind to a halt at some point, Marsh stated that a full collapse “just doesn’t feel plausible to me,” but added that it doesn’t have to fail entirely to wreak havoc upon northern Europe. Further afield, this disruption would also cause the polar vortex to meander further south, bringing cooler Arctic air down along the northeast portion of North America; a cooler North Atlantic would also alter the monsoon rain patterns in the tropics, shifting the wet and dry weather patterns in East Asia, India, South America and West Africa.

“Right now, we have these regions that are used to getting this very intense rainfall in their wet seasons,” explained study lead Maya Ben-Yami, a researcher with the Technical University of Munich. These ecosystems “are adapted to these downpours, and people depend on rainfall for agriculture.”

In the meantime, researchers are looking for indicators that might provide a warning that the AMOC is about to reach a tipping point, such as an increase of fresh water flowing into the ocean off of the tip of South Africa, a situation that would precede a major interruption of the AMOC by at least 25 years. Although it used ocean salinity instead of fresh water as a marker, one study published in June 2024 predicts that this may happen around 2050.

But regardless of what indicators wind up being used, an early warning system for an AMOC interruption is vital, because “it could be that we need as much advanced knowledge as possible to prepare civilian, even military, assets” Marsh warned, to prepare communities, industries and infrastructure for the fallout of an AMOC slowdown.