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.
To test the importance of warming speed, Van Westen and his colleagues ran two versions of a climate model. In both simulations, atmospheric CO 2 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."
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.
According to the researchers, the critical warming rate is around 0.3°C per decade. The world is alr…