The heatwave currently impacting more than 100 million Europeans—many of whom are suffering in temperatures exceeding 35C—may have an unlikely partner-in-crime: a patch of cold ocean water located south of Iceland and Greenland. Often referred to as the “cold blob,” this swath of water in the North Atlantic has defied the broader global warming trend, cooling down even as temperatures rise across the rest of the planet due to human-induced climate change.
A recent study has reinforced concerns that this anomaly could signal a weakening of the Atlantic Meridional Overturning Circulation (AMOC), a key system of ocean currents that helps regulate the Earth’s climate. Scientists warn that a potential shutdown of this oceanic conveyor belt could result in harsher winters in northern Europe. Beyond cold winters, researchers are investigating the blob’s role in summer heatwaves, finding that extreme hot spells have historically coincided with periods when these waters west of Britain were unusually cold.
Gerard McCarthy, an oceanographer at Maynooth University in Ireland, notes that a cold Atlantic does not mean a colder Europe. He explains that the cold blob is not a get-out-of-jail-free card for global warming; rather, some hot extremes can be exacerbated by this phenomenon. While greenhouse gas emissions are the primary driver of climate change, Europe is warming faster than any other continent due to factors such as melting ice and changes in atmospheric circulation. Studies suggest the cold blob influences this circulation by altering the path and speed of the jet stream that flows west to east across the continent. When cooler and warmer waters meet, the sharp contrast creates a wavier and slower jet stream, allowing high-pressure systems like the current “heat dome” to park over Europe.
Marilena Oltmanns, a climate physicist at the University of Bremen, points to data showing a strong cold anomaly that acts as a guide for winds, causing the jet stream to bend northward around Europe instead of crossing it, which leads to the formation of a heat dome. Her 2024 study demonstrates that the melting of Greenland ice pours freshwater into the ocean, contributing to these colder surface waters. This sequence of events, originating from meltwater and the cold blob, helps explain why Europe is heating up more quickly than other regions.
Evidence for this trend is substantial; a 2016 study suggested that cold Atlantic anomalies were a common precursor to major heatwaves hitting Europe since the 1980s. A 2023 paper using computer simulations further confirmed that this anomaly results in longer and more intense heatwaves. According to a 2019 study, while global sea surface temperatures have increased by 1C on average since 1900, the cold blob region has cooled by up to 0.9C. Research published last month by Stefan Rahmstorf and his team clarified that this cooling is caused by ocean currents bringing less heat into the region, rather than more heat being lost through the sea surface.
While scientific debate continues regarding the speed of the AMOC’s decline, Rahmstorf, who heads Earth system analysis at the Potsdam Institute for Climate Impact Research, now estimates an over 50 percent chance of a future shutdown. He warns that the consequences of such a collapse would be massive, including tougher European winters, droughts in South Asia and parts of Africa, and higher sea levels around the North Atlantic. Scientists emphasize that recurring heatwaves are a clear marker of global warming, primarily caused by burning coal, oil, and gas, and they warn that these events are set to become more frequent, longer, and more intense.
