The AMOC Collapse Debate: What CMIP6 Models *Actually* Tell Us About Resilience

The Atlantic Meridional Overturning Circulation (AMOC) is one of the most critical components of Earth's climate system, redistributing vast amounts of heat from the tropics to the Northern Hemisphere. Recently, a major scientific debate has erupted over its future:

• The "No Collapse" View: A recent Nature paper by Baker et al. (2025) argues that the AMOC "does not collapse" under extreme warming because a wind-driven remnant survives.
• The "Yes Collapse" View: Top climate scientists like Stefan Rahmstorf argue this is purely a semantic debate—the thermohaline (density-driven) component of the AMOC does collapse, bringing catastrophic cooling to the North Atlantic, regardless of what we call the remaining trickle.

To cut through the semantics, we need mechanistic evidence. Which models predict a "Baker-like" wind-dominated remnant, and which predict a "Rahmstorf-like" thermohaline collapse? And more importantly: can we predict a priori how a model will behave under extreme warming?

We analyzed 10 models from the CMIP6 ensemble to find out. Here is what we discovered.

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Methodology: Parsing the AMOC

The AMOC is not a single, uniform conveyor belt. It's driven by two distinct mechanisms:

1. The Thermohaline Component (T_n): Driven by cold, dense water sinking in the North Atlantic. This is the part vulnerable to freshwater influx (melting ice sheets) and has a known "tipping point" (Stommel, 1961).
2. The Wind-Driven Component (T_ek): Driven by the powerful westerly winds over the Southern Ocean. This component acts as a "floor"—it will persist for as long as the winds blow (Gnanadesikan, 1999).

We ran a two-phase analysis on CMIP6 models using Google Cloud computing resources:

Phase 1: The Historical Baseline

For the historical period (2000–2010), we extracted:

• Total Atlantic overturning strength (msftmz at 26.5°N)
• Southern Ocean zonal wind stress (tauuo), which we used to calculate the Ekman transport T_ek feeding the Atlantic.
• The Resilience Ratio (T_ek / T_n): A measure of how "wind-dominated" a model's AMOC is prior to any extreme warming.

Phase 2: The Crash Test (abrupt-4xCO2)

We then subjected the models to the ultimate stress test: the standard abrupt-4xCO2 experiment, where atmospheric CO₂ is instantly quadrupled and run for 150 years. We measured the AMOC response in the "late" period (years 130–150) to see how much of the circulation survived.

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📊 The Results

1. The AMOC weakens, but doesn't "break" (in 150 years)

Across the 5 models that completed the full pipeline, total AMOC declined by 18% to 36%. Under 4×CO₂, the AMOC bends but does not fully halt within 150 years. This aligns with the consensus that catastrophic collapse might take centuries to fully manifest.

However, looking at total AMOC masks the real story taking place beneath the surface.

2. The Thermohaline Component Takes a Beating

While the total AMOC dropped by ~30%, the thermohaline component (T_n) plummeted by 28% to 64%. The wind-driven Ekman transport stayed relatively stable, acting exactly as the "floor" theorized by Gnanadesikan.

3. The Key Finding: The Resilience Ratio Predicts the Character of Collapse

The most striking result of our analysis is that the Phase 1 resilience ratio predicts the Baker-vs-Rahmstorf character of the AMOC breakdown.

We classified the remnant AMOC under full warming into three categories based on its wind-driven fraction:

• &x1F7E0; Wind-Dominated Remnant (Baker-like)
  Example: CanESM5
  Result: The model had a high initial resilience ratio (0.87). Under 4×CO₂, its thermohaline circulation collapsed hard (−64%), but because it had strong wind support, the AMOC hit a hard floor. Today, 70% of its remnant AMOC is purely wind-driven. This perfectly illustrates Baker's "no collapse" argument physically.
• &x1F534; Sinking-Dominated Remnant (Rahmstorf-like)
  Example: MIROC6
  Result: The model had a low initial resilience ratio (0.23), meaning it was heavily dependent on northern sinking. Under 4×CO₂, the thermohaline circulation weakened, but because it lacked a strong wind-driven floor, only 29% of its remnant AMOC is wind-driven. This aligns closely with Rahmstorf's characterization: the thermohaline component remains the primary driver of the circulation's fate.
• &x1F535; Mixed Remnant
  Examples: CESM2, MPI-ESM1-2-LR
  Result: Models with moderate resilience ratios (~0.5) showed intermediate behaviour, settling into remnants that are roughly 40-50% wind-driven.

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The Takeaway

The Baker vs. Rahmstorf debate shouldn't be about whether the AMOC "collapses" or not. Our analysis shows that both sides are describing valid, physically distinct mechanisms that exist within state-of-the-art climate models.

The critical insight is that the models do not agree on which regime we are in.

If our real-world ocean resembles CanESM5 (highly wind-supported), the AMOC's absolute decline will be buffered, leaving a wind-driven remnant. If our ocean resembles MIROC6 (sinking-dominated), the circulation is fundamentally more fragile.

What's Next? (Phase 3)

To break this tie, we need to stop looking at models and start looking at reality. In our next phase, we will compare these model-derived resilience ratios against strict observational constraints using reanalysis wind data (ERA5) and direct AMOC observations from the RAPID array.

By grounding these models in real-world measurements, we hope to finally answer: Which of these simulated futures is most likely to be ours?