Cross-hemispheric overturning outlasts North Atlantic deep-water formation by centuries in two models

The evidence for the headline result is narrow. The coupled-model run described in the Eos summary, which phys.org republished, is one model in one scenario ^[14]. That model is CESM2, run with carbon dioxide at four times its starting level, and the researchers present it as an illustrative example from a hierarchy of models ^[3][13].

The second model is what makes the result more than a curiosity of one simulation. The idealized reduced-gravity model keeps only the key dynamics governing overturning, and it gave the same results ^[4]. When a stripped-down model reproduces the complicated one, the mechanism can be read from the simple system. Here, according to the authors, the continued overturning is powered by a gradual deepening of density layers, a rearrangement of the ocean's stratification, and not by a steady balance of sinking and rising water ^[5].

That mechanism limits how far the "less absolute collapse" reading can go. A deepening of density layers is a finite adjustment. The authors say it may sustain the circulation for several centuries, until the ocean reaches a new density distribution ^[6]. The paper's title uses the word "Transient" ^[12]. We think the supportable claim is the narrow one: in these models, the end of deep-water formation and the end of cross-hemispheric transport are separate events.

As the overturning persists, the models produce a clockwise overturning circulation in the Indo-Pacific ^[8]. The authors say the changed circulation could send more heat to deeper layers, keeping the surface cooler at the expense of intermediate water ^[9].

The summaries do not include the size of the slowdown, its timing, or the increase in ventilation age. Coupled models already diverge on AMOC's fate ^[1], so the CESM2 behavior is the result to test in other coupled models before it is treated as general.