The Atlantic Meridional Overturning Circulation (AMOC) is a vital component of the Earth’s climate system, composed of a complex network of ocean currents confined within the Atlantic Ocean basin. This circulation is responsible for transporting vast amounts of heat from the Southern Hemisphere to the Northern Hemisphere, profoundly impacting global climate regulation. The AMOC influences regional weather patterns as well, playing a pivotal role in moderating European summers, shaping African and Indian monsoons, and affecting sea level changes across various coastal regions. Due to its extensive reach and climatic importance, any alteration to the AMOC’s strength or behavior has significant repercussions for both regional and global environments.
For decades, climate models have predicted that rising global temperatures driven by anthropogenic climate change will cause a substantial weakening in the AMOC. Some scenarios have even forecasted a near-collapse of this ocean circulation system by the end of the 21st century, which would signal catastrophic shifts in climatic conditions. Potential consequences of such a weakening include heightened sea level rise along the eastern coast of North America, prolonged droughts in parts of the Amazon basin and West Africa, and a reversal of the traditionally mild climatic conditions enjoyed in northern Europe. These projections have generated considerable concern among climate scientists, policy makers, and the public alike.
Despite these alarming predictions, new research emerging from the California Institute of Technology (Caltech) challenges the extent of anticipated AMOC weakening. The team, spearheaded by former Caltech graduate student Dave Bonan, has developed a simplified physical model that adheres closely to fundamental oceanographic principles. Rather than relying solely on the complex and variable outputs from traditional climate models, this approach integrates direct, empirical observations of the AMOC’s current strength amassed over two decades. These data stem from state-of-the-art monitoring arrays and a suite of oceanographic measurements, providing an unprecedented real-world constraint on the AMOC’s behavior.
The physical model developed by Bonan and colleagues focuses on the relationship between water density differences and the vertical structure of the AMOC, specifically how the circulation’s depth influences its sensitivity to surface changes in temperature and salinity. Their analysis reveals that the AMOC is projected to weaken by a more moderate margin—between 18 and 43 percent—by century’s end, even under high greenhouse gas emission scenarios. This refined projection contrasts sharply with the more severe declines suggested by a subset of existing climate models and significantly reduces the uncertainty surrounding future AMOC dynamics.
One of the longstanding puzzles in climate science has been the wide variation in AMOC weakening predicted by contemporary coupled climate models. By dissecting the physical mechanisms underlying these differences, the Caltech team discovered a critical link between the present-day strength and depth of the AMOC and the extent of expected weakening. States where models simulate a deeper and more vigorous AMOC currently tend to experience stronger declines when subjected to global warming. This is because a deeper AMOC facilitates more profound penetration of warming and freshwater anomalies into the ocean interior, destabilizing the circulation more effectively.
Conversely, models that simulate a shallower and weaker AMOC tend to project less drastic future weakening. This insight elucidates the physical basis for the variability in climate model outputs and offers a unifying framework to interpret divergent projections. Importantly, this understanding helps identify structural biases in climate models, especially relating to how they represent ocean density stratification and mixing processes, which are critical in governing the AMOC’s stability.
The significance of these findings extends beyond purely academic interest. Accurate predictions of the AMOC’s trajectory are essential for realistic climate impact assessments and adaptation strategies. The implications for sea level rise are particularly urgent since a weaker AMOC can exacerbate coastal flooding risks along populous urban areas. Additionally, shifts in precipitation patterns driven by AMOC changes could affect food security and water resources in vulnerable regions such as the Sahel and Amazon basin.
Bonan emphasizes the value of the new physical modeling approach that combines fundamental oceanographic theory with observational evidence. This dual strategy not only constraints future AMOC weakening projections but also provides a robust template for evaluating and improving more complex, high-resolution climate models. Such advanced models incorporate finer-scale ocean dynamics, atmospheric processes, and various feedbacks that may further refine our understanding of the AMOC under anthropogenic forcing.
While the study suggests a less dire scenario than prior extreme projections, it does not negate the importance of ongoing climate mitigation efforts. Even a moderate reduction in AMOC strength is expected to influence regional climates and necessitate thoughtful adaptation measures. Furthermore, since higher-resolution models may reveal additional nuanced behaviors of the AMOC, continued research and sustained observational campaigns remain critical to monitoring and anticipating future ocean circulation changes.
The paleoclimate record provides important context for current AMOC research. Sediment cores and proxy data indicate that the AMOC has undergone substantial fluctuations in the geological past. For instance, during the Last Glacial Maximum some 20,000 years ago, significant weakening of the AMOC coincided with abrupt and pronounced climate swings affecting both North America and Europe. Such historical perspectives underscore the AMOC’s sensitivity to global temperature and freshwater forcings, lending weight to the importance of improving future forecasts.
This latest work, described in the journal Nature Geoscience, is a product of interdisciplinary collaboration spanning oceanography, climate science, and environmental engineering. Alongside Bonan, principal investigators Tapio Schneider and Andrew Thompson led the project. Additional contributors include Laure Zanna of New York University, Kyle Armour of the University of Washington, and Shantong Sun of Laoshan Laboratory in China. Funding was provided by prominent sources including the National Science Foundation, the David and Lucile Packard Foundation, and Schmidt Sciences LLC.
The study’s nuanced conclusions provide a much-needed refinement to one of climate science’s pressing uncertainties. While confirming that the AMOC will indeed weaken in response to ongoing global warming, it restricts projections to a narrower and less extreme range than previously thought. This advancement in understanding equips scientists and policymakers with improved tools to anticipate future environmental changes, better manage associated risks, and craft effective climate resilience strategies worldwide.
In an era where climate anxieties run high, this research exemplifies the power of combining rigorous observation with theoretical innovation. It also highlights the importance of ongoing investment in basic climate science—an endeavor whose benefits, as demonstrated here, can dramatically enhance our grasp of Earth’s complex and fragile climate systems.
Subject of Research:
Atlantic Meridional Overturning Circulation (AMOC) under future global warming scenarios
Article Title:
Observational constraints imply limited future Atlantic meridional overturning circulation weakening
News Publication Date:
29-May-2025
Web References:
http://dx.doi.org/10.1038/s41561-025-01709-0
Keywords:
Climate change, Oceanography, Ocean physics, Climate modeling, Weather simulations, Anthropogenic climate change, Global temperature, Climate data
