A large, persistent cold patch in the subpolar North Atlantic, often referred to as the "cold blob" or "warming hole," has attracted significant scientific attention due to its potential implications for global climate. Despite worldwide trends of ocean warming, this region south of Greenland has cooled by nearly 1°C since the 19th century, a phenomenon first noted with concern about a decade ago.
Scientists have proposed two main explanations for the cold blob's formation: changes in surface heat fluxes, where more ocean heat escapes into the atmosphere, or disruptions in the Atlantic meridional overturning circulation (Amoc), a crucial system of ocean currents that moves warm, salty water northward and colder water southward. The Amoc is a key driver of global climate by redistributing heat, especially warming Europe through currents like the Gulf Stream.
Recent research led by Stefan Rahmstorf at Potsdam University supports the view that alterations in ocean heat transport, rather than surface heat exchanges, primarily drive the cold blob. The slowdown is thought to be linked to the influx of freshwater from the melting Greenland ice sheet. This fresh, cold water is less dense and remains at the ocean surface, hindering the sinking of denser, saltier water—a process essential for maintaining the Amoc’s circulation. The reduced sinking disrupts the "conveyor belt," leading to accumulated cold water in the North Atlantic.
This disruption affects atmospheric patterns, notably the jet stream, a fast-moving air current influencing weather across the Northern Hemisphere. The jet stream is deflected by the cold blob, causing weather extremes such as prolonged heatwaves and cold snaps over Europe. Some researchers suggest that the presence of the cold blob correlates with more frequent and intense summer heatwaves in the UK and parts of Europe. Over longer periods, the cooling of the North Atlantic could lead to a colder climate in northern Europe, possibly resembling the harsher conditions found in regions like eastern Canada, which lie at similar latitudes.
However, direct evidence for the Amoc’s weakening remains limited. Instrumental oceanographic data extend back only 25 to 30 years, while researchers state that at least 60 years of data are needed to establish clear trends. Much of recent analysis relies on reanalysis data—computer models integrating observations—which come with inherent uncertainties. This has led to differing interpretations among scientists, with some expressing concern about a possible imminent collapse of the Amoc, while others regard it as more stable.
Despite these uncertainties, climate models used in major assessments such as those by the Intergovernmental Panel on Climate Change (IPCC) consistently project that the Amoc will weaken during the 21st century due to ongoing global warming. The extent and timing of this weakening, and whether it might trigger abrupt changes, remain key questions under active study. The evolving situation underscores the complex interplay between ocean circulation, climate, and extreme weather events on a warming planet.