19 June 2026
Scientists may never be able to say for certain whether, or when, a system of ocean currents in the North Atlantic will shut down, according to new research from the University of Reading.
The Atlantic Meridional Overturning Circulation, or AMOC, is a crucial regulator of the climate of the British Isles and Europe, as it carries warm water northwards through the Atlantic and keeps north-west Europe's climate mild. Ocean and climate scientists have suggested that rising carbon dioxide levels could push it towards collapse, bringing colder, drier conditions to the UK and northern Europe in a timeframe of decades. Not only could this turn around current warming trends in the UK, it is also expected to lead to harsher winter storms, accelerated sea level rise around the Atlantic coast and shifts in rainfall patterns worldwide.
The study, published today (Thursday 18 June) in Philosophical Transactions of the Royal Society A, used a climate model that sits in between simplified mathematical models and the highly sophisticated earth system models used for climate predictions.
“This allowed us to explore not only the climate states corresponding to a strong and a weak AMOC, but also the boundary that separates these two possible worlds,” said Dr Reyk Börner, who led the study as part of his PhD at the University of Reading's Department of Mathematics and Statistics.
The researchers found an unstable in-between state, or edge state, that reveals surprising dynamics where the AMOC strength and North Atlantic temperatures constantly alternate up and down over centuries. This shows that the barrier between today’s strong AMOC and a collapsed state is highly complicated.
When increasing the carbon dioxide levels in the model to values that the world could realistically reach within 20 years, the strong AMOC state and this edge state merge into a single, long-lived but unstable mix that the team calls a "ghost state". Once this happens, the ocean current can wander unpredictably in its direction and strength for several centuries, sometimes appearing to recover before finally collapsing.
This outcome helps explain why other major climate simulations, run with identical emissions, produce sharply different forecasts for the AMOC. Tiny, random weather differences inside each simulation can decide whether the current survives or collapses, even under the same levels of global warming.
"Our findings challenge the idea that the future of this crucial ocean current is simply on or off, determined by a sharp tipping point. Several different futures could be likely from where we stand today,” Dr Börner said.
“Predicting with certainty which of these futures will play out may be mathematically impossible. Yet, despite all uncertainties, one thing is clear from our analysis, that the risk of severe AMOC changes is lower with every ton of emissions we cut as soon as possible.”
Professor Valerio Lucarini from the University of Leicester, Börner’s PhD supervisor and co-author of the paper, said: “This work shows for the first time a key mathematical mechanism behind the collapse of the AMOC and links complex geophysical processes of crucial real-life relevance with fundamental notions of dynamical systems theory. This is a major advance in the broad area of mathematics of climate and provides crucial specific insight for our understanding of how climate models represent the Earth’s tipping points. For the first time we also demonstrate that loss of stability does not imply rapid transition to a new state. The AMOC can linger on because of powerful transient effects.”
The findings build on themes from Dr Börner's 2024 Fairbrother Lecture at Reading, which questioned simple, yes-or-no thinking about tipping points. Instead of pinning down one exact threshold, the researchers suggest preparing adaptation plans for several plausible AMOC futures, from collapse to temporary weakening and recovery, while continuously monitoring via an ocean observation system which scenario is unfolding.
The study was carried out with colleagues from Politecnico di Torino in Italy and the University of Leicester.
Full reference: Reyk Börner, Oliver Mehling, Jost von Hardenberg, Valerio Lucarini; Global stability of the Atlantic overturning circulation: edge state, long transients and boundary crisis under CO forcing. Philos Trans A Math Phys Eng Sci 18 June 2026; 384 (2322): 20250087. https://doi.org/10.1098/rsta.2025.0087
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