For years, climate scientists have warned that the Atlantic Meridional Overturning Circulation (AMOC) could eventually shut down if global temperatures rise too far. New findings from researchers at Utrecht University suggest that temperature alone does not determine the fate of this major ocean circulation system. The speed at which the planet warms also appears to play a critical role in whether the AMOC remains stable. The study was published in the scientific journal Nature Climate Change.
The Atlantic Meridional Overturning Circulation, or AMOC, is a vast network of ocean currents that carries warm water northward from the tropics. By moving heat around the planet, it strongly influences the global climate and helps maintain the relatively mild conditions found in Western Europe.
Scientists have long viewed this Atlantic ‘heat engine’ as a system that could cross a tipping point. If that happened, the AMOC could transition from its current strong circulation to a much weaker state within decades. Possible triggers include growing amounts of meltwater entering the ocean from polar regions as well as global warming itself.
Rethinking the AMOC Temperature Threshold
Researchers previously estimated that the AMOC could reach a tipping point and collapse at around +4°C of global warming. Scientists at the Institute for Marine and Atmospheric research Utrecht now say that this temperature threshold does not tell the whole story.
“Our results show there is not necessarily a fixed temperature beyond which the AMOC inevitably collapses,” says lead author René van Westen. “The stability of the circulation depends on how fast the climate is changing.”
The findings indicate that two worlds reaching the same eventual temperature could experience very different outcomes for the AMOC depending on how quickly the warming occurred.
Slow Warming Versus Fast Warming
To test the importance of warming speed, Van Westen and his colleagues ran two versions of a climate model. In both simulations, atmospheric CO2 increased gradually, but the rate of that increase differed substantially.
In one simulation, CO2 concentrations increased slowly (0.5 ppm per year). In the second, they climbed much more rapidly (2.5 ppm per year), which is comparable to today’s rate.
The contrast produced dramatically different results. When warming occurred slowly, the AMOC remained stable well beyond +4°C and did not collapse even after warming reached +5°C. Under the faster warming scenario, however, the AMOC collapsed at around +2°C.
“We deliberately looked at a scenario that is much slower than what we’re experiencing today,” explains co-author Reyk Börner. “That allowed us to isolate the effect of the warming rate alone, independent of how warm it eventually gets.”
Why the Ocean Needs Time to Adapt
The researchers say the difference comes down to the ocean’s ability to respond to changing conditions.
“Under slow warming, the entire ocean, from the surface down to its deepest layers, has time to gradually reorganize and adapt to the changing conditions,” says co-author Henk Dijkstra, professor of Dynamical Oceanography. “Under faster warming, the ocean simply can’t keep up.”
Slow climate change gives the ocean more time to adjust throughout its full depth. When temperatures rise more rapidly, those adjustments cannot happen quickly enough, leaving the circulation more vulnerable to instability.
A Critical Rate of Global Warming
According to the researchers, the critical warming rate is around 0.3°C per decade. The world is already approaching that pace.
Van Westen compares the situation to driving a car: “If you’re driving toward a wall, it makes sense to steer around it. To do that, you need to brake, otherwise you fly off the road. When it comes to global warming, the world is still pressing extra hard on the accelerator right now.”
The analogy highlights an important distinction. Avoiding dangerous climate changes may depend not only on limiting how warm the planet eventually becomes, but also on slowing how quickly it gets there.
Previous Research on AMOC Stability
The same research group has investigated AMOC stability from several different perspectives in recent years.
In 2024, the researchers found that increasing amounts of meltwater entering the North Atlantic make the AMOC less stable. Scientists had suspected this mechanism for years, but the research was the first to demonstrate it using a modern, complex climate model.
Those results showed that a critical meltwater threshold exists, beyond which the AMOC becomes unstable. However, the threshold was unrealistically high. This suggests that the present-day AMOC is unlikely to become unstable through this contribution alone. That study did not include the effects of global warming or the pace at which warming occurs.
A later study explored several global warming scenarios. It concluded that the AMOC could reach a tipping point around 2060 under both an intermediate- and high-emission scenario. In those simulations, the tipping point occurred at approximately 2.5°C of global warming.
The latest research helps explain why studies can produce different estimates for when the AMOC might reach a tipping point, as well as why temperature thresholds vary across climate models and emissions scenarios.
The AMOC does appear to have a critical threshold for meltwater, but the researchers find no universal temperature threshold for its collapse. Instead, its stability depends partly on how quickly the planet warms. Faster warming leaves the AMOC more vulnerable, while slower warming gives the ocean more time to adjust and allows the circulation to remain stable under substantially higher levels of global warming.
What the Findings Mean for Climate Policy
The findings suggest that slowing the pace of warming could reduce the near-term risk of an AMOC collapse by giving the Atlantic Ocean more time to adapt.
That could have important implications for climate policy. Much of current climate policy, including the Paris Agreement, focuses on limiting the eventual peak in global temperature.
Some strategies involve so-called overshoot pathways. Under these approaches, global temperatures would temporarily rise beyond a target limit, with the expectation that future technologies could later reduce temperatures again.
The new findings suggest that the path taken toward a given temperature may matter alongside the temperature itself. The faster global warming occurs, the less time the Atlantic Ocean has to adjust, potentially increasing the vulnerability of one of the planet’s most important circulation systems.
