Rate of climate change affects stability of the AMOC | EurekAlert!

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image: The current in the Atlantic Ocean now (left) and in the future when the Atlantic Meridional Overturning Circulation (AMOC) is further weakened by global warming.

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Credit: IPPC AR6 WGI chapter 9

For several years, climate scientists have shown that the Atlantic Meridional Overturning Circulation (AMOC) could come to a halt if the world warms too much. New research by researchers from Utrecht University shows that this picture is incomplete: the pace of warming also determines whether the AMOC stays stable. The study was published in the scientific journal Nature Climate Change.

The Atlantic Meridional Overturning Circulation, or AMOC, is the system of ocean currents that transports warm water from the tropics northward. It plays a major role in redistributing heat across the planet, and helps keep the climate in Western Europe relatively mild.

Scientists have long suspected that this ‘heat engine’ of the Atlantic Ocean could reach a tipping point. This means that the system would shift from its present-day strong state to a much weaker state within decades. Such a tipping event could be triggered by external influences, such as an increasing amount of meltwater from the polar regions, or global warming itself.

Temperature threshold

Until now, the AMOC was thought to tip and collapse around +4°C of warming. Researchers at the Institute for Marine and Atmospheric research Utrecht now show that there's more to the 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."

Slow versus fast warming

To investigate how the rate of climate change affects the strength of the AMOC, Van Westen and his colleagues ran a climate model twice. In their simulations, they used a gradually increasing amount of atmospheric CO2, but at different speeds. In one simulation, CO2 concentrations rose slowly (0.5 ppm per year); in the other, much faster (2.5 ppm per year), comparable to today's rate.

Under slow warming, the AMOC remained stable well beyond +4°C, and it did not collapse even at +5°C of warming. Under fast warming, the AMOC collapsed already 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.”

Resilience

The explanation lies in how the ocean is able to adjust to change. “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.”

Flying off the road

According to the researchers, the critical warming rate lies around 0.3°C per decade, a pace the world is already approaching. Van Westen compares it 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.”

In perspective

The same research group has published on AMOC stability several times in recent years, each time from a slightly different angle.

In 2024, the group showed that an increasing amount of meltwater in the North Atlantic makes the AMOC more unstable. This mechanism had long been suspected, but this study was the first to demonstrate it in a modern, complex climate model. The results confirmed that there is a critical meltwater threshold beyond which the AMOC becomes unstable. However, this threshold is unrealistically high, meaning that the present-day AMOC is unlikely to become unstable through this contribution alone. That study did not account for global warming or its pace.

A later study examined different global warming scenarios, and concluded that the AMOC would reach a tipping point around 2060 under both an intermediate- and high-emission scenario. In those simulations, the tipping point was reached at approximately 2.5°C of global warming.

The new study explains why the results of such studies can differ and why estimated temperature thresholds vary between climate models and emissions scenarios. While the AMOC has a critical threshold for meltwater, no universal temperature threshold exists. Instead, AMOC stability depends on the rate of warming: the faster the climate warms, the more vulnerable the AMOC becomes, whereas slower warming allows it to remain stable under substantially higher levels of global warming.

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