The Atlantic Meridional Overturning Circulation (AMOC), a major system of ocean currents that transports heat northward through the Atlantic, may be more sensitive to the rate of global warming than to the eventual temperature reached, a new study has found.
Researchers from Utrecht University reported that the AMOC could remain stable under substantially higher levels of warming if the temperature rise occurs slowly. However, rapid warming at rates comparable to those occurring today could push the circulation towards a tipping point at much lower levels of global warming.
The findings, published in Nature Climate Change, challenge the idea that the AMOC has a single, fixed temperature threshold beyond which it will inevitably collapse.
The AMOC acts as a large scale climate regulator, carrying warm water from tropical regions towards the North Atlantic while transporting colder water southward. Its circulation influences temperatures, rainfall patterns and climate conditions across large parts of the world, particularly in Europe.
Previous research has suggested that the AMOC could potentially collapse at around 4°C of global warming. The Utrecht University study indicates that such a temperature threshold cannot be considered universal.
The researchers used climate model simulations with two different rates of atmospheric carbon dioxide increase. In one scenario, CO2 concentrations increased by 0.5 parts per million (ppm) per year, while the second used a much faster increase of 2.5 ppm per year, comparable to the current rate.
The difference was substantial.
Under the slower warming scenario, the AMOC remained stable even after global temperatures rose beyond 4°C and continued to function at around 5°C of warming.
Under the faster warming scenario, however, the circulation collapsed at approximately 2°C of global warming.
Lead researcher René van Westen said the results indicate that there is not necessarily a fixed temperature at which the AMOC must collapse. Instead, its stability is influenced by how quickly the climate system changes.
The researchers attribute the contrasting outcomes to the ocean’s ability to adapt.
When warming occurs gradually, the ocean has more time to reorganise from its surface to its deepest layers. This allows the circulation system to adjust to changing temperature and other conditions.
Rapid warming, on the other hand, can outpace these adjustments. As a result, the AMOC becomes increasingly vulnerable to instability.
The study identifies a critical warming rate of about 0.3°C per decade. Researchers noted that the current pace of global warming is approaching this level, making the rate of temperature increase an important factor in assessing future AMOC risks.
A major weakening of the AMOC could have significant consequences for the climate system. Because the circulation redistributes heat across the Atlantic, changes in its strength could influence European temperatures, rainfall patterns and other regional climate conditions.
Scientists have also investigated the role of freshwater entering the North Atlantic from melting ice. Large amounts of freshwater can reduce the density of surface water, potentially interfering with the sinking process that helps drive the AMOC.
Previous research by the Utrecht group found that increasing freshwater input can destabilise the circulation. However, the researchers noted that the threshold identified in that study was considerably higher than present-day freshwater input.
The latest research adds another factor: the speed at which global warming occurs.
The findings suggest that climate policy should consider not only how much the planet eventually warms, but also how rapidly it reaches that temperature.
This could be particularly relevant to climate scenarios in which global temperatures temporarily exceed a target before later declining. Such “overshoot” pathways assume that future reductions in greenhouse gases could eventually bring temperatures back down.
The new study indicates that the path towards a particular temperature could influence the stability of the AMOC. Faster warming could leave the ocean with less time to adapt and potentially increase the risk of crossing a tipping point.
The researchers therefore argue that slowing the rate of global warming could provide the Atlantic circulation with more time to adjust, potentially reducing the near term risk of a major AMOC disruption.
The study, titled “Failure to track a stable AMOC state under rapid climate change”, was authored by René M. van Westen, Reyk Börner and Henk A. Dijkstra and published in Nature Climate Change.