Collapse of critical Atlantic current is no longer low-likelihood, study finds

From the article:

Climate models recently indicated that a collapse before 2100 was unlikely but the new analysis examined models that were run for longer, to 2300 and 2500. These show the tipping point that makes an Amoc shutdown inevitable is likely to be passed within a few decades, but that the collapse itself may not happen until 50 to 100 years later.

The research found that if carbon emissions continued to rise, 70% of the model runs led to collapse, while an intermediate level of emissions resulted in collapse in 37% of the models. Even in the case of low future emissions, an Amoc shutdown happened in 25% of the models.

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The study, published in the journal Environmental Research Letters, analysed the standard models used by the Intergovernmental Panel on Climate Change (IPCC). The scientists were particularly concerned to find that in many models the tipping point is reached in the next decade or two, after which the shutdown of the Amoc becomes inevitable owing to a self-amplifying feedback.

Here is the study:

Shutdown of northern Atlantic overturning after 2100 following deep mixing collapse in CMIP6 projections

Apparently there are some signs that it may be happening already. In the abstract, here is the part about what’s been measured:

Maximum mixed-layer depths in the observations are still dominated by internal variability but notably feature downward trends over the last 5–10 years in all deep mixing regions for all data products analysed. This could be merely variability but is also consistent with the model-predicted decline of deep mixing.

They don’t think it will be abrupt:

In IPCC language the term abrupt refers to events taking place over a few decades or less (<30 years) and persisting at least over a few decades. In our view the term abrupt is not applicable to the future shutdown of the overturning associated with North Atlantic deep water (NADW) as seen in climate models, which will be discussed further below.

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Observation-based data products of mixing depths in the northern Atlantic (figure 6) still show large variability, so that the decline of mixing depths seen during the past 5–10 years may or may not be the beginning of the end of deep convection. But this decline is consistent with what the models project. It is a matter of concern how convection in climate models switches from a phase of internal variability to a phase of terminal decline around the current time (figures 5(c) and (d)), even though mixed-layer depths in the models are overestimated.

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Of particular concern is our finding that deep convection in many models stops in the next decade or two, and that this is a tipping point which pushes the northern AMOC into a terminal decline from which it will take centuries to recover, if at all. As a result, CMIP6 models point to a significantly higher risk than previously assumed […]

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[W]hen projections ending in year 2100 do not show such northern AMOC shutdown this does not imply that the northern AMOC is not in the process of vanishing in those simulations, but simply that the length of the simulation was too short to show whether models are en route to a northern AMOC shutdown or not.

So, this is something that’s starting soon (in the next few decades) but the main effects are expected to be after 2100.