A new study has found fresh evidence that a vital system of ocean currents in the North Atlantic—crucial for stabilizing the global climate—is being disrupted by human-driven warming.
Published in Science Advances, the research used chemical signatures preserved in the growth rings of clams to trace changes in the ocean south of Greenland over the past 150 years. The findings suggest that an influx of freshwater from melting ice has been weakening the region's subpolar gyre—a vast swirling system that distributes ocean heat—since at least the 1950s.
The study's lead author, Beatriz Arellano-Nava of the University of Exeter, said the results add to growing evidence that climate change is pushing the planet toward potentially irreversible "tipping points." "We are moving outside the safe operating space for humanity," she warned.
A collapse or major slowdown of the subpolar gyre and related currents could upend global weather systems. Such a disruption would weaken the northward transport of warm water from the tropics, leading to stark regional differences: extreme heat and accelerated sea level rise in tropical regions, alongside colder, stormier, and more erratic conditions across northern Europe and the North Atlantic.
A system at risk of collapse
The subpolar gyre is a deep, three-dimensional circulation pattern that drives the Atlantic Meridional Overturning Current (AMOC)—a complex system of currents responsible for moving warm and cold water between the poles.
"The gyre is a central part of the deepwater formation that keeps the AMOC running," explained Anders Levermann, a climate physicist at the Potsdam Institute for Climate Impact Research. "The new paper is remarkable because it provides direct evidence that vital ocean circulations can shift into a new state under current conditions, not just in theoretical models or ancient climates."
Levermann was not part of the study but said the findings confirm long-held concerns that the subpolar gyre could abruptly switch from a strong to a much weaker state. "To find such recent evidence for tipping in a large oceanic system is worrisome," he said.
Tracing the climate's pulse through shell records
The scientists analyzed the width and isotopic composition of growth rings in bivalve shells—natural archives that record annual changes in ocean temperature, salinity, and nutrient availability.
"Bivalve records are like the tree rings of the sea," said Arellano-Nava. "They provide a continuous, annually resolved record of ocean conditions, allowing us to detect long-term changes that satellites can't capture."
Variations in oxygen isotopes revealed shifts in seawater temperature and the influence of Arctic waters, while narrower growth bands signalled colder conditions and reduced food availability during past climate transitions.
Early warnings from the deep
Co-author Tim Lenton, director of the Global Systems Institute at the University of Exeter, said the weakening of the subpolar gyre could serve as an early warning of a broader collapse of the AMOC—an event that would have "profound consequences" for weather, ecosystems, and food systems worldwide.
In a separate study published this week in Nature Geoscience, Lenton and colleagues identified signs of destabilization in other critical Earth systems, including the Greenland ice sheet, the Amazon rainforest, and the South American monsoon.
Arellano-Nava said the findings underscore the urgent need to better understand and monitor these thresholds. "Once a tipping point is crossed, it's too late to reverse," she said. "The problem is that you may not see any obvious changes until the transition has already happened—and then it happens abruptly."