Uncertainty about weakening Atlantic currents isn’t a reason to wait but to act (commentary)

When a scientist says, “We don’t know yet,” it can sound like a shrug. In reality, it often means the opposite: We are worried enough to be careful. The public can reasonably ask why some climate risks, especially tipping points, don’t arrive with alarm and immediate action. George Monbiot recently voiced a frustration many people feel: Why has the possibility of an Atlantic Meridional Overturning Circulation (AMOC) shift not prompted a bigger political and media response?

Climate scientists are trained to avoid overclaiming and, instead, to communicate what the evidence shows, what it suggests, and what remains unresolved. That approach underpins my team’s recent research on ocean acidification, supported by the Frontiers Planet Prize. In that work, published in Global Change Biology, we found that large parts of the global ocean have already crossed into a “zone of risk” for ecosystem change.

That caution can serve to downplay the threat, but the latest research on the AMOC should be understood as a warning sign: The potential outcomes could be even more severe than projected, and the uncertainty around timing and thresholds is not a reason to delay, but an argument for action now.

Ocean life depends on AMOC

The AMOC is often described as a giant conveyor belt of Atlantic currents. Warm, salty surface waters flow north from the tropics to the subpolar North Atlantic. On its way, the water releases heat to the atmosphere, so that by the time it reaches the subpolar region, it has cooled and become denser. Here it sinks and returns southward into the deep ocean. This movement is part of the driving force for circulating water around the entire planet, which helps shape climate across the Atlantic region and beyond.

It is also an engine for ocean ecosystems. Ocean circulation redistributes heat, oxygen, carbon and nutrients, and those physical and chemical conditions set the stage for life, from microscopic plankton to commercial fish stocks and marine mammals. When circulation changes, the location and timing of productivity can change, too. Put simply: Currents don’t just move water; they move the conditions that marine food webs are built on.

That’s why discussions about AMOC weakening should not be confined to maps of temperature and rainfall. They should also be about biodiversity, fisheries, and the resilience of ocean ecosystems already under strain from warming, acidification and deoxygenation.

When uncertainty is mistaken for doubt

Climate science — and ocean science in particular — does not deal in absolute certainties about the future. The AMOC is a complex, dynamic system influenced by temperature, salinity, and freshwater inputs, all of which are changing under climate pressure.

Different models produce different timelines and outcomes. Some models project gradual weakening this century, while others point to the possibility of earlier, more abrupt changes. What unites them is not agreement on timing, but agreement on risk.

Yet in public debate, this nuance is often lost. A lack of consensus on exactly when or how a tipping point will be reached is too easily interpreted as disagreement about whether there is a problem at all. This is a misreading of scientific uncertainty. Uncertainty does not cancel risk, but amplifies it.

When scientists say a system could weaken dramatically, or may approach a tipping point, they are acknowledging the limits of prediction, not the limits of concern. In fact, the history of climate science shows risks have often been underestimated, rather than overstated.

In our research on ocean acidification, we revisited one of the Earth’s planetary boundaries beyond which environmental change risks becoming irreversible. Our findings suggest the ocean acidification boundary has already been crossed across significant portions of the global ocean. Our reassessment actually added more uncertainty to the original definition of the boundary, but in doing so, it allowed us to make a more thorough assessment of when and where the boundary is being crossed and what the risks look like. This makes the case for stronger action, not weaker, as it follows the precautionary principle and takes into account the inherent complexity of the ocean system.

However, the messaging can still be simple: By the year 2020, large areas of both surface and subsurface waters had entered a “zone of risk” with measurable consequences for marine ecosystems. Changes in ocean chemistry are already reducing suitable habitats for key species, including corals and shell-forming organisms that underpin entire marine food webs. Without immediate action, we will continue to move from lower risk to high risk of impact.

This matters for how we interpret AMOC research. The ocean is not a stable baseline waiting for a single tipping point; rather, it is already under pressure from warming, acidification, deoxygenation and biodiversity loss. A weakening of the AMOC would not occur in isolation. It would interact with these existing stresses, increasing the likelihood of abrupt and cascading impacts.

Waiting for certainty risks ecosystems

There is a natural instinct in politics, as much as in everyday life, to wait for clearer evidence before taking action. In the context of climate and ocean systems, that instinct can be dangerous. Tipping points, for instance, are difficult to predict precisely. Once crossed, they may commit us to changes that unfold over decades or centuries, beyond our ability to reverse.

If the AMOC were to reach such a threshold, the consequences could extend far beyond climate patterns. Shifts in plankton communities could ripple through the food chain, affecting fish stocks, seabirds and marine mammals. Fisheries that currently support coastal economies could become less reliable or collapse altogether.

These are not distant, abstract risks, but are built on processes we are already observing. Waiting for perfect certainty before responding is, therefore, not a cautious approach — it is a risky one. The question, then, is not whether we have absolute certainty about the future of the AMOC, but whether we have enough evidence of risk to justify action.

We already know that the AMOC is weakening, and that it has been more stable for thousands of years than it is today. We also know that it plays a critical role in regulating climate and sustaining marine ecosystems, and that the ocean is already undergoing rapid, human-driven change.

This evidence means we must act with urgency by investing in the observation systems that allow us to monitor these changes in real time. It also requires sustained support for ocean and climate science from prizes like the Frontiers Planet Prize, philanthropy and government funding so that we can reduce uncertainties and improve our understanding. Ultimately, it must also involve accelerating efforts to reduce greenhouse gas emissions, which remain the underlying driver of these changes.

A warning, not a silence

There is understandable frustration that warnings about systems like the AMOC have not prompted a stronger response. However, the issue is not that scientists are silent, but that the signals we are sending are not always being heard in the way they are intended.

If we wait for the language to become unambiguous, we may find that the system we are describing has already changed beyond recognition. The ocean is already telling us something important.

The question is whether we are prepared to listen, and act, while there is still time.

Helen Findlay is a biological oceanographer at Plymouth Marine Laboratory in the U.K. and the U.K. winner of the Frontiers Planet Prize.

Banner image: Atlantic puffins like this one in Norway rely on healthy Atlantic currents to support their food chain. Image courtesy of Ujval Pasupuleti / Ocean Image Bank.

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Citation:

Findlay, H. S., Feely, R. A., Jiang, L., Pelletier, G., & Bednaršek, N. (2025). Ocean acidification: Another planetary boundary crossed. Global Change Biology, 31(6). doi:10.1111/gcb.70238

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Erik Hoffner Editor

This story first appeared on Mongabay

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