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A study published in Science Advances suggests that Atlantic Ocean circulation could be on course to collapse.
Prof Tim Lenton, Director of the Global Systems Institute, University of Exeter, said:
“This work shows that the AMOC can tip in a complex climate model, which is consistent with abundant evidence that it has tipped repeatedly in the past. The study also shows that the AMOC could be close to a tipping point at present, which is consistent with some other recent studies (although the present study disagrees with them over what constitutes a reliable early warning signal).
“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.
“The study shows that in some places the changes in the climate caused by an AMOC collapse are much more abrupt than the collapse of the circulation itself — thanks to the faster response of the atmosphere — and are so severe that they would be near impossible to adapt to.
“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.
“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.”
Prof Jonathan Bamber, Director of the Bristol Glaciology Centre, University of Bristol, said:
“The authors of this study look at the stability of a key component of the ocean circulation called the Atlantic Meridional Overturning Circulation that has received a great deal of attention over the last decades as paleoclimate data suggests it is bistable and has an “off” mode and an “on” mode. The most recent and sophisticated generation of climate models, called GCMS, do not tend to reproduce this bistable behaviour but with this model they have managed to achieve it. They did this by imposing a huge freshwater forcing to the North Atlantic that is entirely unrealistic for even the most extreme warming scenario over the next century. Their freshwater forcing applied to the North Atlantic is equivalent to 6 cm/year of sea level rise by the end of the experiment, which is more than seen during the collapse of the ice sheet that covered North America during the last glaciation. To what extent such an experiment can be used to infer robust behaviour in the climate system is difficult to know but it is interesting that they do see a collapse in the AMOC, even in this artificial case.
“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.”
Prof Andrew Watson FRS, Royal Society Research Professor, University of Exeter, said:
“These authors study the collapse of the “Atlantic meridional overturning circulation” (AMOC) in a climate model that includes a full simulation of the atmosphere and the ocean. They say it suggests that “the present day AMOC is on route to tipping”. This sounds alarming, but it’s important to note that this is not the same as saying collapse is going to happen imminently. They have to run their model for a long time (1700 years) and push it quite hard to make the collapse happen, so, assuming they are right, the “route” could still be a long one and there may be time for us to change it.
“Models are not reality. The real system may be more, or less, prone to collapse than this model suggests. However this is a very useful study, because with a comparatively realistic model, the mechanism of AMOC collapse is revealed in some detail, and that allows the authors to suggest practical observations we can make that would help predict if a tipping point is close.”
Prof Robert Marsh, Professor of Oceanography & Climate at the National Oceanography Centre, University of Southampton, said:
“For the first time with a climate model of necessary complexity, representing interactions between ocean, atmosphere and ice, van Westen et al. have undertaken a simulation long enough to finally reach a tipping point, at which they find a complete reorganisation of the Atlantic currents (AMOC) that are essential for a stable climate across the northern hemisphere.
“This study is distinct from many to date in the very gradual way that this event is reached – the true definition of a tipping point. In contrast to previous simulated AMOC collapses, this happens here through subtle interactions between ocean, atmosphere and ice, triggered by freshwater input crossing an almost imperceptible threshold – rather than through sudden and unrealistic flooding of the North Atlantic.
“A sobering prospect is that even more realistic climate models – available now, but impractical for such a long simulation – may reveal that AMOC tipping is more imminent than currently thought, occurring here only after many centuries. As the authors emphasise, we need to look more closely at flows in and out of the South Atlantic, which they confirm as holding the key to potentially dramatic changes further north; it is gradual changes in flow between Africa and South America, observable now, that may herald AMOC collapse just a few years or decades into the future and already inevitable. The physics-based approach of Westen et al. essentially complements, and to an extent strengthens, recent statistical analyses of potential AMOC collapse.”
Prof Jeffrey Kargel, Senior Scientist, Planetary Science Institute in Arizona, said:
“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.
“A shut down of the Atlantic Conveyor would be felt globally, according to the model, from Europe to Antarctica. The shocking thing is that global warming, which is what underlies these hypothesized changes (and increasingly, observed precursory changes), is regionally suddenly and rapidly reversed, according to the model. Cooling of the sea surface around Europe of up to 18 degrees F is projected in one human generation! Sea ice in the Arctic, which has diminished drastically in recent decades, would expand in winter to the latitudes of Ireland, the U.K., and Denmark. Furthermore, the authors model that the wet/dry seasonal cycle of Amazonia would reverse, so that the current wet season will become dry, and dry becomes wet. Antarctic sea ice would diminish. With all the world’s ocean circulation and atmospheric circulation being interlinked, it is no exaggeration to say that the whole world will be drastically affected, with sharp regional differences. The authors draw support from wild gyrations of ice sheets and climate from the Ice Age. A frightening aspect of the authors’ warning is that a shut down or drastic diminishment of the Atlantic Conveyor could happen at any time this century, and it can take just a few years. It could happen even this decade or next.
“The projected changes in climate in Scandinavia and parts of Greenland and the United Kingdom and elsewhere may threaten those countries’ humanly habitability if glacier and ice sheet growth extends from the mountains and hill terrains to lowlands. Glacier advances and ice sheet development would take more than a few years; large glacier advances would take decades to begin, and many centuries would elapse before glaciers coalesce and form ice sheets that could spread into the lowlands. However, the climate shift itself could be overwhelming, immediately, for its impact on energy supplies and agricultural production. All of this would be happening while global warming continues. The effects might reduce global average temperature warming rates, but especially the southern hemisphere would see accelerated warming. If you consider what already has happened–for example the present years-long heat wave afflicting South America and Australia–this is all serious stuff.”
Prof Jon Robson, Research Fellow at the National Centre for Atmospheric Science, University of Reading, said:
“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.”
Prof Meric Srokosz, Marine Physics and Ocean Climate Scientist, National Oceanography Centre, said:
“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.”
‘Physics-based early warning signal shows that AMOC is on tipping course’ by René M. van Westen et al. was published in Science Advances at 7pm UK time on Friday 9 February.
Declared interests
Andrew Watson: I’m not associated with these authors and have no vested interests in this research. I am involved in research on the stability of the AMOC under climate change, in collaboration with colleagues at the Met Office.
Jonathan Bamber is funded by EPSRC, EU, German BMBF and DFG.
Jon Robson: I declare no conflict of interests.
For all other experts, no reply to our request for DOIs was received.