Atlantic ocean current system will weaken, but is unlikely to collapse

The Atlantic meridional overturning circulation, or AMOC, is a giant system of ocean currents that shuttles warm surface water northward and returns cold, dense water to the south at depth.

For years, climate scientists have worried that rising greenhouse-gas emissions could cripple this “conveyor belt” in the Atlantic Ocean.

A shutdown of the AMOC is prominently featured in worst-case climate scenarios, raising the specter of abrupt cooling in Europe, shifts in African and South Asian monsoons, and faster regional sea-level rise.

However, a new study led by the California Institute of Technology (Caltech) suggests that while the AMOC is indeed poised to weaken, it is unlikely to experience the dramatic near-collapse forecast by some computer models.

Atlantic models re-examined

The study combines a streamlined physical model of ocean circulation with two decades of direct observations collected by trans-Atlantic monitoring arrays.

Lead author Dave Bonan, who completed the work as a Caltech doctoral student, explains that standard climate models differ wildly in how they simulate today’s AMOC and therefore in how they project its future.

Instead of attempting to reconcile all of those complex simulations, Bonan’s team identified a single – but crucial – physical relationship: the link between the current’s strength, its depth, and its sensitivity to surface warming and freshening.

The analysis concludes that by 2100, the AMOC will slow by roughly 18 to 43 percent, depending on greenhouse-gas emissions. That is a significant reduction, but far short of the 60–80 percent declines, or near-total collapses, that some earlier studies implied.

“Our results imply that, rather than a substantial decline, the AMOC is more likely to experience a limited decline over the 21st century – still some weakening, but less drastic than previous projections suggest,” Bonan said.

Establishing the depth of the AMOC

The researchers discovered that present-day model biases stem largely from how deep each model’s AMOC extends.

A stronger, deeper overturning cell allows climatic changes at the surface – warmer temperatures and an influx of freshwater from melting ice – to penetrate farther downward. That penetration erodes the density contrasts that power the circulation, leading to a bigger slowdown.

Models that start with a shallower AMOC simulate less surface-driven change and therefore a milder weakening. Because real-world observations show that the actual AMOC lies somewhere in between those extremes, the team’s new estimate falls in the mid-range as well.

To anchor their model, the scientists drew on 20 years of data from moored instruments that continuously measure temperature, salinity, and current speed along 26.5° N and other latitudes in the Atlantic.

The records provided a reality check for the model’s present-day state and narrowed the plausible range of future outcomes.

Capturing the ocean’s structure

A weakening of roughly one-fifth to two-fifths still has consequences. Slower heat transport would cool parts of northern Europe, raise regional sea levels along the U.S. East Coast, and alter precipitation patterns in the tropics.

Yet the new forecast suggests those effects will be less abrupt than feared, giving societies more time to adapt. It also refocuses scientific attention on model fidelity: improving how models capture today’s ocean structure may be as important as tweaking how they respond to greenhouse gases.

Study co-author Andrew Thompson, a professor of environmental science and engineering at Caltech, argued that the work provides a “road map” for testing next-generation, higher-resolution climate models.

Those AMOC models resolve smaller eddies and better represent how winds, mixing, and ice melt influence the circulation. The team’s framework can now be used to gauge whether added complexity brings projections closer to observation-anchored reality.

Uncertainties in climate simulations

Bonan credits the freedom afforded by the National Science Foundation’s Graduate Research Fellowship Program for allowing him to pursue a question that sits at the intersection of theory and observation.

“The NSF-GFRP gave me the freedom to tinker and explore,” he said. By stripping the problem to its physics and cross-checking with real measurements, the team could sidestep many of the uncertainties that plague full-scale climate simulations.

Tapio Schneider, a professor of environmental science and engineering and senior author of the study, notes that fundamental research of this kind complements, rather than contradicts, traditional modeling.

Understanding why models diverge could let us improve them. It also helps policymakers see that while the AMOC is vulnerable, a catastrophic tipping point is not the likeliest outcome this century.

Future of the Atlantic ocean current system

The authors emphasize that their findings do not reduce the urgency of cutting emissions. A 20–40 percent slowdown in the AMOC would still amplify coastal flooding risks and disrupt weather patterns that billions of people rely on.

Moreover, other climate tipping elements – from West Antarctic ice shelves to Amazon rainforests – remain sensitive to continued warming.

“There is immense value in doing basic research – it can give us a better indication of what the future might look like, as our study shows,” Bonan said.

In short, the Atlantic ocean current system appears sturdier than some dire forecast models suggested, but it is far from immune to the mounting pressure of greenhouse gases.

Better models, grounded in real-world data, now paint a clearer picture of what lies ahead – and underscore the stakes of curbing global warming while there is still time.

The study is published in the journal Nature Geoscience.

Image Credit: NOAA

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