Is the AMOC really on tipping course?

A new research at the university of Utrecht suggests that the Atlantic Meridional Overturning Circulation (AMOC) is on a tipping point course.

“Physics-based early warning signal shows that AMOC is on tipping course’”by René M. van Westen et al. was published in Science Advances on Friday 9 February. 2024

Topographic map of the Nordic Seas and subpolar basins with surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures. (R. Curry, Woods Hole Oceanographic Institution/Science/USGCRP.)

The AMOC is an important component of Earth’s climate, and plays a key role as an ocean conveyor belt. The AMOC powers the poleward transport of heat and nutrients from the tropics to the Northern Hemisphere and across the equator – and draws down heat and carbon to the deep ocean.

Once in the Northern Hemisphere, the water brought up by AMOC from the tropics becomes cooler and then denser. The denser water sinks and then travels south, where it will eventually warm and resurface again.  This ocean conveyor belt enables heat to be distributed around our planet, and is important for regional and global climate, as well as changes in weather around the world.  The AMOC accounts for around 90% of the total northward ocean heat transport across the Atlantic Ocean. (Natural Environment Research Council.)

Modelling such a scenario is no easy task, however, and previous attempts relied on introducing unrealistically large amounts of freshwater, unrealistically quickly. The new simulations, which are the most sophisticated yet in considering how melting ice sheets could cause the AMOC to collapse, introduce freshwater gradually rather than all at once, says Westen. For example, between model year 0 and model year 2220, the model linearly increases the flux of freshwater at latitudes between 20°N and 50°N at a rate of 3 × 10−4 Sverdrups (Sv) per year. ( In oceanography, the sverdrup (symbol: Sv) is a  unit of volumetric flow rate, with 1 Sv equal to 1 million cubic metres per second.)

(A) The AMOC strength at 1000 m and 26°N, where the yellow shading indicates observed ranges (60, 61). The cyan-colored lines indicate the magnitude of FH. The red arrow indicates the AMOC tipping point (model year 1758; fig. S1, A and B), and the blue sections indicate the 50-year periods used in (B) to (D). Inset: The hosing experiment where fresh water is added to the ocean surface between 20°N and 50°N in the Atlantic Ocean (+FH) and is compensated over the remaining ocean surface (−FH). The black sections indicate the 26°N and 34°S latitudes over which the AMOC strength and freshwater transport (FovS) are determined, respectively. (B to D) AMOC streamfunction (Ψ) and Atlantic meridional heat transport (MHT; see also fig. S2) for model years 1 to 50, 1701 to 1750, and 2151 to 2200. The contours indicate the isolines of Ψ for different values.

On a path towards tipping

The researchers found that the strength of the AMOC decreased gradually over the first 400 years they modelled. Then, after model year 800, a clear negative trend developed because of the increasing amounts of freshwater being introduced. At the 1750-year mark, the team observed the AMOC collapse from a flow rate of about 10 Sv to 2 Sv by model year 1850. The flow rate eventually became slightly negative after model year 2000.

Such a fast AMOC response is, Westen says, “spectacular considering the small flux of freshwater we introduce in our model”. What is more, Westen adds that present-day real-world data on the AMOC indicates that its strength is already decreasing. “This means we are moving closer to the tipping point and are thus on route for tipping,” he tells Physics World.

According to the team’s calculations, if the AMOC did shut down, temperatures in London, UK could cool by 10 °C on average, while Bergen, on Norway’s west coast, could see a 15 °C drop. Sea levels would rise by 70 cm along the US East Coast, while the Amazon region would see its rainy and dry seasons flip, severely disrupting its ecosystem.

“One of the most important findings in our work is that AMOC tipping is possible and that it is not just a theoretical concept,” Westen emphasizes. “We hope that other research groups will conduct their own simulations with different models and confirm our results.”

Expert reactions to the study

We have to plan for the worst
Prof Tim Lenton, Director of the Global Systems Institute, University of Exeter:

“The research makes a convincing case that the AMOC is approaching a tipping point based on a robust, physically-based early warning indicator. What it cannot (and does not) say is how close the tipping point, because as it shows that there is insufficient data to make a statistically reliable estimate of that.”
“As the authors show, the AMOC is actually too stable in the model they use compared to observations – meaning that they have to force it slowly for a long time before anything happens. Whilst the study is a significant advance over previous work in terms of the spatial resolution of the model used it is still not high enough resolution to resolve all the fine-scale structure of important aspects of the ocean circulation.”
“We have to plan for the worst.  We should invest in collecting relevant data and improving estimation of how close a tipping point is, improving assessment of what its impacts would be, and getting pre-prepared for how we could best manage and adapt to those impacts if they start to unfold.”

Expect the unexpected
Prof Jeffrey Kargel, Senior Scientist, Planetary Science Institute in Arizona:

“The new research by René van Westen and colleagues presents what they consider to be a confirmation of the long hypothesized potential for a sudden shut down or rapid reduction in the strength of one of the world’s great climate regulators–that of the Atlantic Meridional Overturning Circulation; we can chew that mouthful down to the ‘Atlantic Conveyor’. A controversy is whether the present climatic paradigm is one where that shift can occur, which the authors say is imminent; furthermore, the warning signs are said to be present and similar to what preceded previous sudden changes in the Atlantic Conveyor.

“My suspicion is that the hypothesized imminent shut down of the Atlantic Conveyor will remain somewhat controversial until, one year, we know that it is happening. It’s similar mathematically to wild gyrations of a stock market that precede a major crash. Nobody knows what is a reversible fluctuation or is a precursor to a crash. The authors say that a shutdown is imminent, and precursory instabilities are happening. What I know, from glacier studies and global weather and climate measurements and hundreds of models, is that the Earth’s climate is in upheaval, and there are tipping points galore baked into the system. The hypothesized shut down of the Atlantic Conveyor is a big one. Greenhouse gas-driven climate change, spurred by carbon fuel combustion, is everywhere. But as the authors of the new article point out, and I’ll add below, some of the impacts of a shutdown of the Atlantic Conveyor are counter intuitive. So: expect the unexpected.

We need to see if this freshwater diagnostic is really a robust early warning indicator of abrupt AMOC changes

Prof Jon Robson, Research Fellow at the National Centre for Atmospheric Science, University of Reading:
“This study brings further worrisome evidence about the potential for abrupt and disruptive changes to the North Atlantic region. Such an AMOC collapse would have profound impacts on society and ecosystems through changes in regional temperatures, rainfall and winds.

“Although the paper underlines that climate models can simulate such abrupt AMOC weakening events, it is important to keep in mind that this study is based on only one imperfect climate model. Therefore, we need to see if this freshwater diagnostic is really a robust early warning indicator of abrupt AMOC changes. However, taking the freshwater early warning indicator at face value, it is still unclear how near we really are to such a tipping point due to our limited observations in the past.

“Nevertheless, it is clear that as carbon dioxide concentrations continue to rise in the atmosphere we are only increasing the likelihood of major disruption to the AMOC. It’s crucial that we continue to bring all tools available to bear in trying to understand if we are really approaching the brink of an AMOC collapse.”

The advantage of a physics-based indicator of potential AMOC tipping is that it better represents what is actually happening in the ocean circulation and should therefore provide a more robust indicator.”

Prof Meric Srokosz, Marine Physics and Ocean Climate Scientist, National Oceanography Centre:

“This is a careful study using a model to derive a physics-based and potentially observable early warning indicator of AMOC’s tipping into another state (rather than a statistics-based indicator, as in other recent studies). Examining this observable indicator (FovS – freshwater transport at 34˚S in the Atlantic) in re-analyses (model+data products) suggests that the AMOC may be on course to a tipping point. However, the authors are careful in the presentation of their results and, while suggesting a tipping point might be near, note that future observations in the South Atlantic are required to estimate when this might occur as existing re-analysis products provide insufficient information. The advantage of a physics-based, rather than statistics-based, indicator of potential AMOC tipping is that it better represents what is actually happening in the ocean circulation and should therefore provide a more robust indicator.”

Prof Jonathan Bamber, Director of the Bristol Glaciology Centre, University of Bristol thinks the title of the paper should read like this:

“Physics-based early warning signal shows that the AMOC may be on a pathway to a tipping point at some unknown time in the future”

“The authors also investigate a parameter related to freshwater transport by the AMOC that has been linked to its stability. They find that this parameter is declining and when it reaches a minimum this is an indication of an impending AMOC collapse. While that may correct, it is unclear when and if the parameter will reach a minimum and whether it will continue to decline. In fact, some observational data indicate it is not decreasing so the title of the paper is a little misleading. It would be more accurate to say that “Physics-based early warning signal shows that the AMOC may be on a pathway to a tipping point at some unknown time in the future”. it would be interesting to know why this GCM shows bistability when others have not and whether a more realistic experiment with a more realistic present-day climate and AMOC strength also shows bistability. No doubt this work will spur on further study on this important topic.”

I find this conclusion very convincing, as it highlights both the uncertainties of the process and the reliable findings.

Bernd Riebe, April 8, 2024

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