The Atlantic meridional overturning circulation, often termed the “AMOC,” constitutes a collection of ocean currents located within the Atlantic oceanic basin that significantly influences the regulation of Earth’s climate by transferring heat from the Southern to the Northern Hemisphere. Additionally, the AMOC affects local weather patterns, from the mild summers experienced in Europe to the monsoon periods in Africa and India. Climate models have historically indicated that global warming will lead to a reduction in the AMOC’s strength, with some estimates suggesting a notable decline approaching a near-collapse compared to the current strength of the AMOC. Such a decline could result in substantial repercussions, including alterations in regional sea level rises and significant climate shifts, such as cooler conditions in northern Europe and drier spells in sections of the Amazon and West Africa.
Nevertheless, a recent investigation from Caltech reveals that while the AMOC is expected to weaken in the face of global warming, the extent of this weakening is likely to be considerably less than what present projections imply. The research team formulated a simplified physical model based on essential principles of ocean circulation—specifically, the relationship between density variations and the AMOC’s depth—which also accounts for actual measurements of the ocean current’s intensity collected over two decades through monitoring systems and other observational tools within the Atlantic basin. The findings suggest that the AMOC may diminish by approximately 18 to 43 percent by the conclusion of the 21st century. Although this indicates some degree of weakening, it does not align with the severe reductions suggested by more extreme climate models. This fresh insight significantly narrows the projected range for future AMOC weakening, addressing a long-standing ambiguity in climate science.
The study is detailed in a paper published in the journal Nature Geoscience. The research was carried out in the laboratories of Tapio Schneider, the Theodore Y. Wu Professor of Environmental Science and Engineering, and Andrew Thompson, the John S. and Sherry Chen Professor of Environmental Science and Engineering, director of The Ronald and Maxine Linde Center for Global Environmental Science, and executive officer for Environmental Science and Engineering.
Paleoclimate data, such as ocean sediments that document historical climate scenarios, reveal that the AMOC has undergone weakening in the past, for instance during the Last Glacial Maximum (around 20,000 years ago), resulting in significant climate fluctuations that impacted North America and Europe. Modern climate models demonstrate considerable disparities in their projections of AMOC weakening for the 21st century: some forecast significant AMOC reductions, while others anticipate only minor declines. The recent study, led by former graduate student Dave Bonan (PhD ’25), sought to deepen understanding of the physical processes influencing AMOC dynamics in climate models, aiming to reconcile these variations.
The research illuminates a long-standing and previously unexplained characteristic of climate models: the connection between the current and future intensity of the AMOC. Climate models that depict a stronger current today tend to forecast greater reductions under climate shifts. The researchers discovered that this correlation arises from the depth of the AMOC. A more robust AMOC generally reaches greater depths, allowing variations in surface water temperature and salinity—induced by global warming and freshwater flows—to penetrate deeper into the ocean, thereby driving greater weakening. In essence, a climate model with a stronger and deeper AMOC is less durable to surface alterations and experiences proportionally more AMOC reduction than one with a shallower current. Climate models showcasing a shallower present-day AMOC still reflect weakening under climate change, but to a lesser degree than their deeper counterparts.
The new study employs this comprehension to refine future AMOC projections by constructing a simplified physical model and integrating actual measurements of the ocean current’s strength. The outcomes indicate that the AMOC will likely face only minor weakening, even under the most extreme emissions scenarios. The study indicates that much of the prior uncertainty and some of the more severe AMOC weakening estimates arose from biases in how climate models represent the current state of the ocean, particularly its density layering.
“Our findings suggest that, rather than undergoing a considerable drop, the AMOC is more likely to experience a limited decline throughout the 21st century—still resulting in some weakening, but not as drastic as earlier estimates proposed,” Bonan states.
Bonan underscores the necessity of exploring higher-resolution climate models that also take into account more complex processes. These advanced models may provide deeper insights into AMOC dynamics and enhance projections regarding its future modifications. The study lays out a framework for examining and assessing more sophisticated models.
While conducting this research at Caltech, Bonan received funding from the National Science Foundation Graduate Research Fellowship Program (NSF-GRFP).
“The NSF-GFRP allowed me the freedom to innovate and explore,” he remarks. “There is immense significance in pursuing fundamental research — it can offer a clearer understanding of what the future might hold, as our study demonstrates.”
The paper is titled “Observational constraints imply limited future Atlantic meridional overturning circulation weakening.” In addition to Bonan, Schneider, and Thompson, co-authors include Laure Zanna from New York University, Kyle Armour from the University of Washington, and Shantong Sun from Laoshan Laboratory in Qingdao, China. The research was supported by the NSF, the David and Lucile Packard Foundation, and Schmidt Sciences LLC.