Scientists have found evidence that the Atlantic Ocean’s great heat-transporting system can suddenly strengthen—even while it is already severely weakened. The discovery challenges a long-standing assumption that a faltering Atlantic Meridional Overturning Circulation, or AMOC, would simply remain weak for centuries or millennia. Instead, the circulation may behave more like a system capable of abrupt swings, producing repeated episodes of intensification that could rapidly reshape climate conditions across the Atlantic basin. The findings come from an analysis of ancient marine sediments collected off northeastern Brazil and offer a striking warning for a warming world: future climate disruption may not unfold as a single, predictable decline, but as a sequence of sudden changes in ocean circulation.
The AMOC is one of Earth’s most important climate-regulating systems. It acts as a vast, three-dimensional conveyor belt, carrying warm surface waters northward from the tropics and transporting colder, denser water southward through the deep Atlantic. By moving enormous quantities of heat, the circulation helps moderate temperatures in Europe and parts of North America while influencing rainfall across tropical Africa and South America. Its strength depends partly on the formation of dense water in the North Atlantic. When cold, salty surface water sinks near Greenland, it feeds the deep southward limb of the circulation. Global warming is now threatening this mechanism. Melting Greenland ice, increased Arctic precipitation and ocean warming add freshwater to the North Atlantic, reducing surface-water salinity and density and making it more difficult for water to sink.
Climate models and paleoclimate records have long suggested that continued freshwater input could push the AMOC toward a critical threshold, triggering an abrupt slowdown. Such a disruption could cool parts of the North Atlantic region, alter European climate, shift tropical rainfall belts and affect ecosystems, agriculture and coastal societies. Yet earlier research also indicated that once the AMOC entered a weak state, it might remain relatively stable for thousands of years. The new study presents a more dynamic picture. Led by Cristiano Mazur Chiessi of the University of São Paulo in Brazil and Stefan Mulitza of the University of Bremen in Germany, the research identifies two distinct episodes when the AMOC strengthened dramatically during Heinrich Stadial 1, a period when the circulation was generally much weaker than it is today.
Heinrich Stadial 1 occurred between approximately 17,800 and 14,800 years ago, during the final stages of the last Ice Age. The interval was marked by major disruptions to the climate system as huge ice sheets melted and released vast quantities of freshwater into the ocean. Against this unstable background, the researchers found that the AMOC intensified first between about 16,500 and 15,800 years ago. A second, far shorter surge occurred around 15,400 years ago and lasted approximately a century. During this brief event, the circulation became stronger than its modern level. The result is especially important because it demonstrates that a weakened AMOC is not necessarily locked into a single state. Even under conditions that suppress deep-water formation, the circulation can abruptly rebound, strengthen and then potentially weaken again.
To reconstruct these changes, the researchers examined a sediment core extracted from the equatorial Atlantic at a depth of 1,367 meters, roughly 189 kilometers off the coast of Maranhão, Brazil. The sample was collected in 2012 during a research expedition aboard the German vessel RV Maria S. Merian. Sediments deposited at the site preserve successive layers of microscopic marine remains and chemical signals that record past ocean conditions. The location was particularly valuable because intense rainfall over northeastern Brazil during periods of weak AMOC increased erosion and transported abundant material into the ocean. This created a rapidly accumulating archive with unusually fine temporal resolution, allowing the scientists to distinguish changes that occurred over centuries and, in the shorter event, over only about 100 years.
The central measurement was the ventilation age of deep water, determined through radiocarbon dating. The researchers analyzed shells produced by foraminifera, single-celled organisms that live either near the surface or on the seafloor. Planktonic foraminifera form their shells in surface waters that exchange carbon with the atmosphere, while benthic foraminifera form theirs in the deep ocean. Carbon-14, a radioactive form of carbon, is created in the atmosphere and enters the ocean primarily through surface exchange. When surface water sinks in the North Atlantic and travels through the deep ocean, it carries this radiocarbon with it. The longer the water remains isolated from the atmosphere, the older its radiocarbon signature becomes. Comparing the apparent ages of planktonic and benthic shells from the same sediment layer therefore reveals how long deep water has been separated from the surface—a crucial indicator of AMOC strength.
The modern ventilation age of deep water in the equatorial Atlantic is approximately 350 years. In the sediment record, the age difference between surface-dwelling and bottom-dwelling foraminifera was about 325 years immediately before Heinrich Stadial 1. During most of the stadial, however, the difference expanded to approximately 960 years, indicating that deep waters were circulating more slowly and had remained isolated from the atmosphere for much longer. During the first intensification episode, the difference fell to about 450 years, signaling a rapid acceleration of deep-water transport. During the second surge, it dropped even further, to approximately 200 years—evidence that the AMOC briefly exceeded its present intensity. These changes provide a direct physical record of abrupt circulation shifts rather than relying solely on indirect climate indicators.
The findings also align with independent evidence from South American rainfall records. The team compared the sediment results with paleoclimate data from stalagmites in caves in Mato Grosso do Sul and Bahia. During much of Heinrich Stadial 1, northeastern Brazil experienced heavier rainfall, while precipitation declined in the northern Amazon and other areas farther north. The two periods of AMOC strengthening coincided with transitions toward drier conditions over the continent, which would have reduced the amount of freshwater flowing into the Atlantic. That reduction may have increased ocean salinity and density, making sinking more efficient and helping the circulation accelerate. The timing of the surges also matches rises in atmospheric carbon dioxide recorded in Antarctic ice cores. The researchers propose that a stronger AMOC may have helped draw carbon-rich deep waters into the Antarctic Circumpolar Current, where the carbon dioxide could escape into the atmosphere.
The ancient events do not provide a precise timetable for what will happen in the modern climate system. During Heinrich Stadial 1, atmospheric carbon dioxide concentrations were substantially lower than in the pre-industrial era, ice sheets were enormous and freshwater delivery followed patterns unlike those produced by today’s human-driven warming. Those differences make the past an imperfect analogue. Nevertheless, the record reveals a previously underappreciated capability of the AMOC: even a weakened circulation can produce abrupt and powerful bursts of strengthening. Recent modeling based on improved observational data has suggested that the AMOC could weaken by roughly 43% to 59% by 2100, even if nations fulfill their current emissions-reduction commitments. The new evidence indicates that such a decline may not be smooth or permanent. It could include sudden reversals, potentially causing rapid changes in heat distribution, rainfall and atmospheric carbon dioxide.
The researchers say the discovery should encourage scientists to search for early warning signals of abrupt circulation shifts and should push policymakers to prepare for more than one type of climate emergency. A rapidly strengthening AMOC would not necessarily be harmless: any major reorganization of ocean heat transport could disrupt regional climate patterns just as profoundly as a slowdown. The study’s broader message is that the Atlantic circulation is not a passive victim of climate change but an active component of the climate system, capable of amplifying instability and transmitting its effects across continents. Better monitoring of ocean salinity, deep-water formation and rainfall patterns will be essential for detecting changes in real time. Reducing greenhouse-gas emissions remains the most effective way to limit the pressures driving the system toward dangerous thresholds, but the new sediment record suggests that resilience planning must also account for abrupt, repeated and unexpected changes in the ocean’s global conveyor belt.
Subject of Research: Atlantic Meridional Overturning Circulation, paleoceanography and abrupt climate change
Article Title: Centennial-scale intensifications of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 1
News Publication Date: 19-May-2026
Web References: https://doi.org/10.1038/s41467-026-73364-x; https://bv.fapesp.br/en/pesquisador/34247/cristiano-mazur-chiessi; https://www.agencia.fapesp.br/en/
References: Chiessi, C. M., Mulitza, S. et al., “Centennial-scale intensifications of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 1,” Nature Communications, published 19 May 2026, DOI: 10.1038/s41467-026-73364-x
Image Credits: Cristiano Chiessi
Keywords: Atlantic Meridional Overturning Circulation, AMOC, ocean currents, climate change, global warming, paleoceanography, Heinrich Stadial 1, radiocarbon dating, foraminifera, Atlantic Ocean, abrupt climate change, ocean circulation, deep-water formation
