In 2018, two separate investigations came to the same conclusion: the planet’s circulatory system was weakening. The main set of ocean currents that carry huge amounts of water from the tropical seas to the north is slowing down due to the impact of climate change. The latest report by United Nations experts (the IPCC) published this year reached the same conclusion. But now, new work goes further, concluding that the so-called Atlantic meridional overturning circulation (AMOC) will collapse in the coming decades if greenhouse gas emissions are not reduced now. They even put a date on it, around 2057. However, other scientists maintain that there is not enough data to expect the collapse.
All those who swim these days at the beach can get an idea of how the AMOC works. When you go a few meters into the water, you will notice that the most superficial layer is hot, while the deepest ones are colder. At that point it is due to the direct impact of solar radiation. But on a global scale it is somewhat more complex. Seas with equatorial waters are warmer, and the warmer the less dense and heavy the water, which travels in the form of currents like the Gulf to higher latitudes. On its way, it tempers the North and South Atlantic and the climate of Western Europe and the American East. At the extreme of this circulatory system, the opposite happens: the coldest waters of the arctic zones sink to the bottom and travel to the equatorial zones. Despite its name, the AMOC does not stay in the Atlantic. Due to the higher relative temperature and salinity of the Pacific and Indian Oceans, the Atlantic circulation also reaches these oceans. Although its most obvious impact is on the climate, it also affects the distribution of waste or nutrients throughout the planet’s seas.
“The AMOC went from a weak state to the current one with the end of the last ice age, 12,000 years ago,” recalls Susanne Ditlevsen, a researcher at the University of Copenhagen (Denmark), co-author of the new study on the possible collapse. Twelve millennia ago, climatic conditions changed so much that they also facilitated the great revolutions carried out by humans that came later: expansion throughout the planet, agriculture, urbanization… “The increase in the amount of fresh water [por el deshielo] it is slowing down the AMOC, which is slowing down until it passes into a weak state”, adds the mathematician. Arctic freshwater, although cold, is less dense than saltwater, so it sinks worse, interfering with the circuit. “The problem is evaluating the amount of fresh water,” she ends.
Those who have studied the evolution of the AMOC are clear that the destabilizing factor is the melting of Greenland and, to a lesser extent, the accelerated loss of Arctic sea ice, both caused by global warming. The difficult thing is to determine its specific impact on ocean circulation. Direct data on the state of the currents has only been available since 2004, thanks to depth sensors, buoys or ships. But 20 years is too short to differentiate between natural variability or a process caused by human emissions. So one has to look for indirect indicators of the past state of this oceanic conveyor belt (thermohaline circulation). Ditlevsen and his brother Peter, a climatologist at the same Danish university, have used sea surface temperature records in the North Atlantic for nearly two centuries as a clue.
“From the end of the 19th century there was a drastic change. Since 1880 and every decade more, in a situation that cannot be compared with the pre-industrial situation”, says the mathematician from the Niels Bohr Institute of the Danish university. Based on these data and using complex statistical tools, the Ditlevsen brothers show in the results of their work, published in Nature Communications, that the AMOC could collapse long before the end of the century. His numbers say that, with a very high probability, the transition from one state to another would happen around the year 2057. “I know that it is the most controversial part of the job and I would like to be wrong. But, if the emissions continue as before, the results we obtain are what they are”, concludes Susanne Ditlevsen.
Doubts among other scientists
Alexander Robinson, an expert in ocean currents at the Institute of Geosciences (IGEO) of the Complutense University of Madrid, highlights the strengths of this study in which he has not participated: “They use recently developed statistical methods to provide early warning signals of when a system could collapse or go into a new state.” For Robinson, the key (and a possible weakness of this work) is the indirect indicator they have used to see the evolution of the circulation: “To the extent that temperature anomalies in the North Atlantic can be considered a good indicator of the AMOC, then this work convincingly shows that a significant change in its state is likely due to global warming this century.”
![An Atlantic humpback whale at the Silver Bank Whale Sanctuary, Dominican Republic.](https://imagenes.elpais.com/resizer/Trj_Yc8M6huc0R6XUKSjNRVxcog=/414x0/cloudfront-eu-central-1.images.arcpublishing.com/prisa/DK6SVTMDKZE4RMYCHHBNIEDXZE.jpg)
Another who has been studying this flow of currents for years is the climatologist Pablo Ortega. And he does it with the support of the computing power of the Barcelona Supercomputing Center (National Supercomputing Center). Orteqa is one of the researchers who detected the weakening of the Atlantic ocean current in 2018 and has spent years studying the impacts of the melting ice masses of Greenland. “Between 2004 and 2012 we detected that it was slowing down,” he says. “But in recent years the trend is not so clear,” he adds. Ortega considers that the AMOC and its connection with the global climate is too complex to entrust its fate to projections based on anomalies in the surface temperature of the northern seas. Ortega finds it hard to think that it could collapse in this century.
The SMC scientific information service has asked a round of questions with experts on the Atlantic ocean current. There is almost unanimity. The work of the Ditlevsen brothers is novel due to its support in statistical tools and not so much in climate models. It is also important for detecting possible early warning signs that would indicate the transition from a strong to a weak state of the AMOC. But they share Ortega’s idea that there is a lot of uncertainty, and basing the change in ocean circulation on a single indicator is risky. As Penny Holliday, Principal Investigator of OSNAP, an international program to study the AMOC, puts it: “Its collapse would profoundly impact every person on Earth, but this study exaggerates the likelihood of it occurring in the next few years.”
What they all agree on is that such a collapse would have global consequences. “The AMOC controls the transport of heat almost on a planetary scale,” says Ortega. So the end of this thermal sharing would cool most of the northern hemisphere, especially western Europe, and warm the already hot equatorial oceanic portions. Beyond the climate, the Atlantic ocean current is essential for the distribution of nutrients and sediments that sustain all the biodiversity that lives in the seas, especially in the Atlantic.
The following could be said by a climate doomsday, but Hollyday declared it to the British division of the SMC: “Heat would accumulate in the southern ocean and the south Atlantic, but in the southern continents, temperatures would also decrease. The main rainfall zones would shift, leading to much less rain in Europe, North and Central America, North and Central Africa, and Asia, and more in the Amazon, Australia, and southern Africa. Sea ice would extend south from the Arctic to the subpolar North Atlantic and Antarctic sea ice would extend north.”
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