The AMOC is not a single current but a planetary-scale circulation powered by differences in temperature and salinity. Warm, salty surface water flows northward, releasing heat to the atmosphere. In the subpolar North Atlantic, cooling increases the water’s density, allowing it to sink and join a deep southward flow. This overturning helps regulate climate around the North Atlantic, influences rainfall and wind patterns, redistributes carbon and heat, and affects sea level and marine ecosystems. A major weakening would not simply mean a slower Gulf Stream; it would represent a reorganization of the climate system with consequences extending across both hemispheres.

Scientists have long known that the AMOC can, in some models, exist in more than one stable state. Under broadly similar external conditions, the circulation may occupy a vigorous “on” state or a much weaker “off” state. The possibility of these alternative equilibria makes the AMOC a potential climate tipping element: a system that can shift abruptly when a gradual change in forcing pushes it beyond a critical threshold. Freshwater entering the ocean is especially important because it dilutes seawater, reduces density, and can interfere with the formation of deep water in the North Atlantic. The resulting slowdown can itself alter salt transport, creating feedbacks that either stabilize or destabilize the circulation.

The new study, by Amber A. Boot of Utrecht University and the Technical University of Denmark and Henk A. Dijkstra of Utrecht University, tests whether earlier findings from a relatively simple climate model also hold in a more detailed ocean model. The researchers compared simulations from CLIMBER-X, an intermediate-complexity Earth system model with a coarse 5-degree ocean grid, with simulations from the Parallel Ocean Program version 2, or POP2. POP2 represents the ocean at a nominal horizontal resolution of 1 degree, includes 40 vertical layers, and incorporates parameterized processes such as mesoscale mixing and overflow in the North Atlantic and Southern Ocean. It is also used as an ocean component in the Community Earth System Model family.

The comparison was designed to isolate the influence of freshwater errors rather than reproduce a perfect modern climate. The researchers established baseline simulations and then imposed artificial freshwater biases in the Indian Ocean, the Atlantic Ocean, or both basins. The perturbations were deliberately large enough to expose the circulation’s response: a change of 1.50 millimeters per day, equivalent to roughly 0.24 to 0.27 sverdrups over the affected regions. One sverdrup represents one million cubic meters of water flowing every second. The added freshwater was balanced elsewhere so that the overall salt budget remained conserved.

The models were then subjected to a controlled “hosing” experiment, in which freshwater was added to the North Atlantic between 20 and 50 degrees north. This forcing was increased gradually until the AMOC collapsed. In the faster experiments, the model was allowed to adjust for 600 years after the bias was introduced, and the freshwater forcing rose at 0.30 sverdrups per thousand years. The researchers also ran slower CLIMBER-X experiments, allowing 10,000 years for adjustment and increasing the forcing six times more slowly. Such long integrations are necessary because ocean salinity and circulation respond over centuries to millennia, and a simulation that appears settled after a few decades may still be evolving internally.

Despite their different resolutions and physical complexity, POP2 and CLIMBER-X produced the same broad result. A negative freshwater bias in the Indian Ocean—effectively making that ocean saltier—tended to weaken and destabilize the AMOC. A positive Indian Ocean bias, which freshened the region, generally strengthened and stabilized it. The Atlantic response was reversed: adding freshwater to the Atlantic tended to weaken the AMOC, while removing it had a stabilizing effect. The researchers traced these responses to how salinity anomalies move through the connected ocean basins and alter the density contrast that helps drive overturning.

The physical chain is counterintuitive. When the Indian Ocean becomes saltier, the anomaly can influence the South Atlantic, changing the density structure there and reducing the north–south density difference across the Atlantic. A weaker density gradient means less pressure-driven overturning. The weakened circulation then transports less salt northward, leaving the North Atlantic fresher and further reducing the density of water available for sinking. In the Atlantic itself, additional freshwater directly lowers salinity and density, making deep convection more difficult. These processes are reflected in the freshwater transport of the overturning circulation across roughly 34 degrees south, a diagnostic measure tied to the salt-advection feedback.

That feedback is central to the study’s warning. Observational estimates generally indicate freshwater export characteristics associated with a destabilizing salt-advection feedback, whereas many climate models simulate the opposite sign. In those models, the feedback helps restore the AMOC after a disturbance, making the circulation appear more resistant to collapse. The discrepancy is linked partly to salinity errors and surface freshwater forcing, including precipitation patterns. One well-known example is the “double Intertropical Convergence Zone” bias, in which models produce two overly strong tropical precipitation bands. Such errors can redistribute freshwater between ocean basins and alter the large-scale salinity field that controls circulation stability.

The simulations did not show that every freshwater bias causes an immediate or inevitable collapse. Instead, the biases shifted the amount of additional North Atlantic freshwater required to reach the tipping point. In POP2, most experiments collapsed under less freshwater forcing than the baseline case. The main exception was the simulation with positive freshwater biases in the Indian Ocean, which required stronger forcing before collapse. The slower CLIMBER-X experiments showed a similar pattern. Some Atlantic-bias cases collapsed in two stages: first, deep convection in the subpolar gyre weakened, then the system entered a weak, oscillating state before a complete collapse under still stronger forcing.

One especially complex result emerged when opposing biases were applied across both the Indian and Atlantic oceans. In the POP2 simulation with negative biases in both basins, the AMOC became unusually variable and then highly sensitive to additional freshwater near a forcing of about 0.155 sverdrups. The researchers linked the behavior to a reorganization of the South Atlantic circulation, including changes in mixed-layer depth, sea-surface temperature and sea-surface salinity. Rossby waves generated near the Mid-Atlantic Ridge grew in amplitude as freshwater forcing increased, while changes in gyre transport amplified the weakening. This case illustrates why the circulation cannot be understood from a single regional salinity value: basin geometry, waves, gyres and density feedbacks can interact in nonlinear ways.

The team also separated the simulated AMOC decline into several mechanisms, including surface freshwater forcing, gyre transport, northern-boundary freshwater transport and the salt-advection feedback. Before the tipping point, direct surface freshwater forcing produced most of the weakening, with gyre changes contributing several sverdrups of additional decline. The salt-advection feedback played only a minor role at first but became the mechanism that drove the final collapse after the threshold was crossed. POP2 differed from a previous Community Earth System Model analysis mainly in the behavior of the gyre feedback, which was more destabilizing before the tipping point. Yet the gyre response was broadly similar among the different freshwater-bias experiments, suggesting that it did not erase the central effect of the biases.

The researchers caution that POP2 is still not a complete Earth system model. It is an ocean-only simulation driven by prescribed atmospheric conditions, and therefore lacks two-way interactions with a dynamic atmosphere and sea ice. In the real world, a weakening AMOC would reduce northward ocean heat transport, potentially expanding Arctic sea ice. More ice could insulate the ocean from the atmosphere, alter heat loss and suppress deep convection. Atmospheric changes in wind, moisture and heat transport could either reinforce or counteract the circulation’s decline. Southern Ocean winds are also important because they drive upwelling and help shape the pathways that connect the Atlantic to the global ocean.

The study likewise does not calculate a precise date or probability for an AMOC collapse. Its experiments use idealized freshwater forcing under preindustrial conditions, while the actual climate is being pushed away from equilibrium by rising greenhouse-gas concentrations. Real-world tipping could involve several processes at once, including gradual forcing, random fluctuations and changes in the background stability of the circulation. Freshwater variability itself may also contain model biases in its amplitude and distribution. The authors therefore present their results as evidence of a structural vulnerability in current modeling rather than as a forecast of imminent collapse.

To help test that vulnerability in more expensive Earth system models, Boot and Dijkstra propose a streamlined experiment. A model would first be run under baseline conditions, then branched before the tipping point. A freshwater bias would be introduced gradually over several centuries, allowing the ocean to adjust without triggering artificial rate-dependent effects. The simulation would then resume increasing North Atlantic freshwater forcing. In their POP2 proof-of-concept, a negative Indian Ocean bias was added over 400 years after a 500-year equilibration period. The modified simulations tipped within decades of renewed forcing and before the unbiased baseline, supporting the proposed protocol.

The broader implication is that model resolution alone may not solve the AMOC problem. The same qualitative response appeared in a coarse coupled model and a substantially more detailed 1-degree ocean model, indicating that the mechanism survives increased ocean complexity. Yet the authors stress that only experiments in fully coupled Earth system models can establish whether the bias pattern consistently makes the AMOC too stable. Until then, projections showing no complete collapse this century should not be interpreted as proof that the risk is negligible. If models are systematically giving the circulation an overly strong safety margin, correcting freshwater distributions could turn a seemingly remote tipping possibility into a more serious climate risk.

The ITF serves as one of the world’s most significant oceanic conduits, influencing the heat and salinity budgets of both adjoining ocean basins. By facilitating the transfer of vast water volumes, it helps modulate the regional climate of Southeast Asia and plays an indispensable role in shaping weather patterns throughout the Indo-Pacific region. However, the research team led by Hu, Lu, and Sprintall reveals that the ITF is not immune to the accelerating perturbations wrought by anthropogenic CO2 concentrations.

According to their comprehensive analysis, which integrates satellite observations, in-situ measurements, and state-of-the-art climate model simulations, the ITF has already approached a critical turning point where its flow rates are poised to decline sharply. This turning point arises from the interplay between rising CO2-driven ocean warming, altered wind stress patterns, and changing sea surface height gradients. As these factors interplay, they disrupt the delicate balance that sustains the ITF’s steady transport.

In particular, the study highlights how ocean warming induces stratification changes that limit the vertical mixing necessary for maintaining ITF strength. As surface waters warm and freshen due to increased precipitation in the tropical Indo-Pacific, the density contrasts that drive throughflow weaken. This stratification acts as a barrier, dampening the momentum of water masses that typically traverse the complex Indonesian archipelago.

The researchers underscore the significance of wind stress modifications caused by altered atmospheric circulation patterns resulting from CO2-induced climate change. Regional trade winds that once reinforced ITF current velocities are expected to shift in intensity and direction, further undermining the throughflow system’s robustness. Reduced wind forcing diminishes the pressure gradients that push Pacific waters through narrow Indonesian straits, effectively restricting flow volume.

Moreover, sea level rise, which is non-uniform across the Pacific and Indian Oceans, exacerbates the pressure imbalances crucial for ITF dynamics. Satellite altimetry data reveal that Pacific basin sea levels climbing faster than the neighboring Indian Ocean create a lessened pressure difference, thereby weakening the current’s driving force. This subtle but impactful change threatens the ITF’s ability to maintain its historical transport efficiency.

The implications of an attenuated ITF are far-reaching. This current acts as a vital heat and salt conveyor, regulating sea surface temperatures and influencing monsoon patterns, particularly over Southeast Asia and the broader Indo-Pacific region. A diminished ITF may lead to warmer sea surfaces in the Pacific Ocean, with cascading effects on regional climate phenomena such as El Niño-Southern Oscillation (ENSO) events, which drive extreme weather worldwide.

Furthermore, the study posits that the decline of the ITF could severely impact nutrient cycling and marine biodiversity in the Indonesian seas. Reduced water exchange hampers the delivery of oxygen-rich and nutrient-laden waters into Indian Ocean basins, potentially triggering hypoxic conditions and disrupting fisheries that millions depend upon. This biogeochemical fallout could ripple through marine food webs, threatening fisheries stability and food security.

Crucially, this research employs advanced coupled climate-ocean models enhanced with high-resolution bathymetric data, allowing unprecedented simulation fidelity of the Indonesian archipelago’s intricate straits and passages. Such fine-scale modeling reveals previously unrecognized sensitivity of the ITF to relatively modest changes in oceanic forcing, providing compelling evidence of an imminent rapid decline linked directly to rising CO2 levels.

The researchers emphasize that this phenomenon reflects broader ocean circulation vulnerabilities to climate change, underscoring the interconnectedness of ocean basins through complex current systems. The ITF’s projected weakening may serve as a bellwether for other key currents expected to undergo transformational shifts, thereby challenging existing paradigms of ocean circulation stability under anthropogenic climate pressures.

Importantly, this study also pinpoints the CO2 concentration threshold at which the ITF enters its downturn phase, suggesting that current global emissions trajectories might precipitate this decline within the next few decades. This finding carries a sense of urgency, as it delineates a concrete climatic tipping point with tangible oceanographic consequences.

The authors call for enhanced monitoring efforts across the Indonesian seas, advocating for expanded deployment of autonomous underwater vehicles and moored sensor arrays to track evolving ITF characteristics in real-time. Such observational networks would provide critical validation data, improving forecasting capabilities and informing mitigation strategies.

In summary, the imminent rapid decline of the Indonesian Throughflow marks a momentous shift in ocean circulation with global repercussions. The link between reaching a CO2 turning point and this change underscores the profound influence of greenhouse gas emissions on marine systems. This discovery not only deepens scientific understanding but also highlights the urgency of climate action to preserve ocean health and resilience.

As the scientific community digests these findings, the broader geopolitical and ecological stakes become clearer. Southeast Asia’s economies and livelihoods, tied intimately to the stability of maritime conditions influenced by the ITF, face heightened risks. Policymakers must grapple with integrating this new knowledge into coastal management, fisheries regulation, and international climate policy frameworks.

Ultimately, this pioneering research redefines how we perceive ocean-climate interactions in the 21st century. It reveals that ocean currents long considered stable may be far more sensitive to anthropogenic forcing than previously appreciated, demanding a reevaluation of climate models and adaptation plans. The Indonesian Throughflow’s fate may prove emblematic of the broader challenges confronting the planet’s life-supporting ocean systems in the era of human-driven climate change.

Subject of Research: Ocean circulation dynamics, climate change impacts, Indonesian Throughflow, CO2 concentration effects

Article Title: Imminent rapid decline of the Indonesian Throughflow after reaching a turning point of CO2 concentration

Article References:
Hu, S., Lu, X., Sprintall, J. et al. Imminent rapid decline of the Indonesian Throughflow after reaching a turning point of CO2 concentration. Nat Commun (2025). https://doi.org/10.1038/s41467-025-66746-0

Image Credits: AI Generated