New Research Highlights Importance of Warming Speed for Atlantic Meridional Overturning Circulation Stability

New findings from researchers at Utrecht University indicate that the stability of the Atlantic Meridional Overturning Circulation (AMOC) is influenced not only by rising global temperatures but also by the rate at which those temperatures increase. The study, published in the journal Nature Climate Change, challenges previous assumptions that a fixed temperature threshold guarantees AMOC collapse.

The AMOC is a critical ocean current system that transports warm water from the tropics to the north, significantly affecting global climate and maintaining temperate conditions in Western Europe. Scientists have long warned that this ‘heat engine’ could face a tipping point, with potential shifts to a much weaker circulation predicted to occur within decades. Factors such as increasing meltwater from polar regions and global warming contribute to this risk.

Shifting Perspectives on Tipping Points

Historically, estimates suggested the AMOC might reach a tipping point and collapse at approximately +4°C of global warming. However, lead author René van Westen notes, “Our results show there is not necessarily a fixed temperature beyond which the AMOC inevitably collapses. The stability of the circulation depends on how fast the climate is changing.”

To examine this further, Van Westen and colleagues modeled two scenarios with different rates of atmospheric CO2 increase: one gradual (0.5 ppm per year) and one rapid (2.5 ppm per year). The findings revealed stark differences in AMOC stability. In the slow warming scenario, the AMOC remained stable well beyond +4°C. In contrast, under the rapid warming scenario, the AMOC collapsed at around +2°C.

The Ocean’s Adaptation Challenge

Co-author Henk Dijkstra explains the ocean’s adjustment capabilities, stating, “Under slow warming, the entire ocean, from the surface down to its deepest layers, has time to gradually reorganize and adapt to changing conditions. Under faster warming, the ocean simply can’t keep up.”

According to researchers, a critical warming rate is roughly 0.3°C per decade, which the world is approaching. Van Westen likens the scenario to steering a car toward a wall, stressing that necessary adjustments to avoid disaster must include both reducing peak temperatures and the speed of temperature rise.

Implications for Climate Policy

The findings underscore the need for climate policies that not only focus on limiting peak global temperatures but also on slowing the rate of warming. With current strategies, including overshoot pathways, global temperatures could temporarily exceed target limits, awaiting future technologies to lower them again. However, this research highlights that the trajectory toward a given temperature is equally crucial for the stability of the AMOC.

The study suggests that by slowing warming, the immediate risk of an AMOC collapse may decrease, giving the Atlantic Ocean more time to adapt to changes.