Atlantic Ocean Current Won’t Collapse This Century

A crucial ocean current system that regulates Earth’s climate will weaken moderately over the next 75 years but won’t experience the dramatic collapse predicted by some climate models, according to new research from Caltech.

The Atlantic meridional overturning circulation (AMOC) – which transports warm water northward and cold water southward across the Atlantic basin – will decline by just 18-43% by century’s end, far less than the near-total shutdown some models have projected. The finding addresses one of climate science’s most pressing uncertainties and suggests that while regional climate impacts are inevitable, the most catastrophic scenarios appear unlikely.

The AMOC acts like a massive conveyor belt, moving warm tropical waters toward Europe and Arctic regions while returning cold, dense water southward through the deep ocean. This circulation pattern helps maintain Europe’s relatively mild climate and influences rainfall patterns across Africa, South America, and North America.

Thermal-Wind Physics Reveals the Truth

The key breakthrough came from applying fundamental physics principles to understand why climate models produce such wildly different AMOC predictions. Lead researcher Dave Bonan, who recently completed his PhD at Caltech, developed a simplified model based on thermal-wind balance – the relationship between density differences across ocean basins and the depth at which circulation occurs.

Current climate models show bewildering variation in their AMOC forecasts. Some predict weakening of just 2 Sverdrups (a unit measuring ocean flow), while others project declines of 15 Sverdrups – enough to fundamentally alter global climate patterns. This sevenfold difference has made it nearly impossible to plan for future climate impacts.

“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 explained.

The research reveals that the relationship between present-day AMOC strength and future weakening stems from circulation depth. Models that simulate stronger, deeper ocean circulation today also predict more dramatic future declines. This occurs because deeper circulation allows surface warming and freshwater changes to penetrate further into the ocean depths, causing greater disruption to the density gradients that drive the current.

Stratification Holds the Key

A critical insight that emerged from the analysis involves ocean stratification – how water density changes with depth. Models with stronger present-day AMOC circulation typically simulate weaker North Atlantic stratification, meaning less resistance to vertical mixing between surface and deep waters.

When global warming increases surface temperatures and adds freshwater from melting ice, these changes penetrate much deeper in weakly stratified oceans. The result is larger density changes at depth and more dramatic circulation slowdowns. Conversely, models with stronger stratification limit how deeply surface changes can penetrate, naturally protecting the circulation system from major disruption.

The researchers discovered that North Atlantic stratification and AMOC strength show a strong negative correlation, with correlation coefficients reaching -0.89. This relationship explains why some models predict extreme weakening while others suggest modest changes – they’re essentially modeling different background ocean states.

Observational Reality Check

To move beyond model disagreements, the team incorporated real-world measurements from two key sources: the RAPID monitoring array that has tracked AMOC strength at 26.5°N since 2004, and the ECCO state estimate that combines ocean models with observational data from 1992-2015.

These observations suggest the present-day AMOC strength falls around 15-17 Sverdrups, toward the lower end of what climate models simulate. When the researchers applied their thermal-wind relationship using observed rather than modeled AMOC strength, the projections shifted dramatically toward more limited weakening.

The constrained projections indicate AMOC weakening of roughly 3-6 Sverdrups by 2071-2100, regardless of greenhouse gas emissions scenario. This represents moderate weakening that would still produce regional climate impacts but falls far short of the system collapse some studies have warned about.

Why Models Disagree

The analysis reveals that uncertainty in AMOC projections stems more from how models represent present-day ocean conditions than from differences in future emissions scenarios. Models with deeper simulated circulation consistently predict larger future declines, while those with shallower circulation project more modest changes.

This explains a puzzling feature of climate projections: AMOC weakening predictions remain remarkably similar across different emissions pathways within the same model, yet vary dramatically between different models using identical emissions scenarios. The answer lies in each model’s unique representation of background ocean stratification.

The finding suggests that improving present-day ocean simulation should be a priority for reducing uncertainty in climate projections. Models that better capture observed stratification patterns will likely provide more reliable estimates of future changes.

Beyond Simple Linear Relationships

The thermal-wind analysis revealed that the relationship between present-day AMOC strength and future weakening isn’t simply linear. Instead, it includes both linear and square-root terms that create a curved relationship where stronger circulation systems experience disproportionately larger declines.

This nonlinear behavior emerges from the physics of how overturning depth changes. The researchers found that models with deeper present-day circulation (larger H values) experience more shoaling under warming, with correlation coefficients around -0.61 between present depth and future depth changes.

The nonlinear relationship partially explains why some emergent constraint studies using purely statistical approaches may produce different results. The physics-based thermal-wind approach captures this curvature and provides more robust projections when extrapolated to observed conditions.

Climate Implications

While the study rules out AMOC collapse this century, moderate weakening would still produce significant regional impacts. A 20-40% decline could affect European temperatures, African monsoon patterns, and North American storm tracks, though less dramatically than total shutdown scenarios.

The research doesn’t address longer-term risks beyond 2100 or potential tipping points that might emerge from sustained warming. However, it suggests that frequently cited studies warning of imminent AMOC collapse may be overstating near-term risks.

What does this mean for climate adaptation planning? The findings suggest communities should prepare for gradual rather than abrupt changes in AMOC-related climate patterns over the coming decades.

Tapio Schneider, Caltech’s Theodore Y. Wu Professor of Environmental Science and Engineering and study co-author, emphasizes that the work highlights how fundamental physics principles can help resolve major uncertainties in climate science.

As Bonan noted, the research demonstrates the value of basic scientific investigation: “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.”