The Atlantic Meridional Overturning Circulation (AMOC) stands as one of the climate system's most critical components, acting as a gigantic conveyor belt that transports warm surface waters northwards and cold, dense waters southwards in the Atlantic Ocean. Its strength and stability not only influence regional climates, especially across Europe and North America, but also play a pivotal role in the global heat budget and carbon cycle. Over recent decades, climate scientists have increasingly expressed concern over the potential weakening of the AMOC under twenty-first-century warming scenarios. Yet, the projections of this weakening from climate models have varied widely, leaving substantial uncertainty around the magnitude and impacts of future changes. A groundbreaking study now offers important clarity by linking this uncertainty directly to fundamental oceanographic dynamics and observational constraints.
The research illuminates a fundamental physical relationship that governs AMOC strength -- the thermal-wind balance. This principle connects the large-scale circulation to the meridional density gradient (difference in water density between northern and southern latitudes) and the vertical extent of the overturning circulation, commonly conceptualized as the 'overturning depth.' By expressing the AMOC's strength as a function of these two parameters, the authors reveal that much of the intermodel spread in predictions of AMOC weakening resides in changes in the overturning depth rather than the density difference itself. This insight reframes the problem from one of disparate model responses to one rooted in accurately representing stratification and vertical ocean structure.
Delving deeper, the overturning depth emerges as a crucial diagnostic and mechanistic link between the present-day ocean state and the projected future circulation. Climate models that simulate a relatively stronger and deeper overturning today tend to forecast greater weakening and a more pronounced shoaling, or reduction, in overturning depth as the century progresses. This is tied intrinsically to the stratification state of the North Atlantic. A less stratified present-day North Atlantic allows surface buoyancy forcing -- the changes in heat and freshwater fluxes at the ocean surface driven by global warming -- to penetrate deeply. This deep penetration fosters significant density changes at depth, consequently driving large AMOC weakening.
In contrast, models depicting a more stratified present ocean limit the influence of surface heating or freshwater input to shallower depths. This stratification acts as a barrier, impeding the transmission of buoyancy anomalies downward and thereby damping the AMOC response. By focusing attention on stratification -- the vertical layering and density gradients in the ocean -- this study highlights a key source of model bias and uncertainty. Models that fail to capture present-day stratification realistically produce more extreme and less likely AMOC weakening scenarios under warming.
This physical framework also offers a means to reduce uncertainty by incorporating observational constraints. Modern oceanographic measurements, including detailed density profiles and estimates of overturning strength and depth, provide benchmarks against which climate models can be evaluated. The research team applied such constraints to refine the bounds on plausible future AMOC changes. Their analysis suggests that, contrary to some alarming projections, the AMOC will experience a limited weakening of about 3 to 6 Sverdrups (Sv), equating to approximately 18 to 43% of current strength by 2100. This range holds regardless of different greenhouse gas emissions scenarios, highlighting a robust physical control on AMOC variability.
This conclusion has wide-reaching implications. The AMOC's influence on weather patterns, sea-level rise, and carbon uptake has fueled concern that a rapid or severe weakening could trigger abrupt, nonlinear climate shifts. However, the study's findings paint a more nuanced picture. While the AMOC is expected to weaken substantially due to anthropogenic warming, the magnitude is likely moderated by the ocean's internal stratification dynamics. This limits the risk of a complete shutdown or collapse within this century, which had been a looming worst-case scenario.
It is crucial to emphasize that this "limited weakening" still represents a significant alteration of the Atlantic Ocean circulation. A reduction of several Sverdrups would alter heat transport to the high latitudes, potentially modulating regional climate effects, including warmer winters in Europe and changes to tropical Atlantic rainfall patterns. Sea-level rise along the US eastern seaboard may also be influenced due to changes in ocean currents and density gradients. As such, the study does not imply the AMOC's future impact is negligible, but rather that the degree of change might be more predictable and less severe than previously thought.
The novel approach presented in this research hinges on the thermal-wind balance, a fundamental oceanographic concept that expresses how vertical shear in geostrophically balanced flow relates to horizontal density gradients. In the context of the AMOC, this principle ties the intensity of overturning flow -- measured in Sverdrups -- to the north-south density differences and the vertical scale over which they act. This mechanistic foundation thus serves as an elegant bridge, linking physical oceanography theory to complex climate model output and observational data.
Importantly, the study highlights how the present-day ocean stratification -- a product of temperature and salinity distributions shaped by atmospheric forcing, mixing, and large-scale circulation -- acts as a gatekeeper controlling AMOC sensitivity. In climate models where this stratification is more realistically represented and constrained by observations, the AMOC weakens within a narrower, more physically consistent range. Models that deviate in their baseline ocean state produce more extreme AMOC responses, underscoring the critical need for improved representation of ocean processes in Earth system models.
This insight calls for targeted efforts to better observe and understand vertical structure and stratification in the North Atlantic Ocean. Enhanced observational networks, including autonomous floats, moored instrument arrays, and satellite remote sensing, will be essential to refining our grasp of present-day ocean state and processes. Continuous monitoring will not only reduce model biases but also provide ongoing clues as to how the AMOC evolves in a warming world.
Furthermore, integration of these improved oceanographic constraints into coupled climate model development holds enormous potential. By anchoring the representation of AMOC-related processes in observed stratification and density structure, future projections can gain accuracy and robustness. This would aid climate risk assessments and adaptation planning for regions sensitive to AMOC-driven climate variability.
The study's implications extend beyond oceanography, touching on broader Earth system feedbacks. The AMOC's role in carbon sequestration, for example, depends partly on its strength and the depths to which surface waters are transported. Thus, accurately constraining AMOC weakening informs projections of the global carbon cycle under anthropogenic forcing. Similarly, understanding AMOC dynamics contributes to interpreting paleoclimate records where abrupt climate change events often coincide with changes in Atlantic circulation.
While uncertainties remain in quantifying finer-scale processes and potential tipping points, this research represents a major advance in reducing the "model spread" problem that has long bedeviled AMOC projections. By emphasizing the physical control of stratification and overturning depth, the authors provide a parsimonious and testable framework that can unify disparate modeling results and observations.
Looking ahead, the interaction between surface buoyancy forcing driven by greenhouse gases and the ocean's internal stratification will remain a central theme in climate science. Determining how this linkage unfolds will require sustained multidisciplinary efforts, including improvements in ocean physics, biogeochemistry, and climate dynamics. As climate change proceeds and longer observational records accumulate, ongoing refinement of AMOC projections will be vital to anticipate and manage possible impacts on global and regional scales.
This study's findings also carry a valuable message about climate model fidelity. Rather than simply focusing on reproducing trends or global temperature fields, attention to key mechanistic features and process-level realism can yield profound reductions in predictive uncertainty. Such an approach serves as a model for addressing other large-scale, complex Earth system components.
In summary, by unraveling the principal factors behind divergent AMOC projections, this research paves the way for more accurate and physically grounded climate predictions. The forecast that the AMOC is likely to undergo limited weakening by the century's end offers measured reassurance while underscoring the importance of continued observation and model improvement. Ultimately, the future of the AMOC and its influence on global climate will hinge on the subtle interplay of ocean stratification and surface forcing -- a dynamic now better understood thanks to this innovative work.
Subject of Research: Atlantic Meridional Overturning Circulation (AMOC) dynamics and future projections
Article Title: Observational constraints imply limited future Atlantic meridional overturning circulation weakening
Article References:
Bonan, D.B., Thompson, A.F., Schneider, T. et al. Observational constraints imply limited future Atlantic meridional overturning circulation weakening. Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01709-0