Most people can probably name a few particularly prominent effects of global warming on the Earth system: The polar ice caps are melting and the permafrost is thawing. However, it is far less known that the so-called Atlantic Overturning Circulation (abbreviated Amoc for Atlantic Meridional Overturning Circulation) is weakening. The suspected consequences are at least as threatening.
The situation would be particularly bad if this current system in the Atlantic Ocean, which also includes the Gulf Stream, collapsed completely. Experts speak of a so-called tipping point; the Amoc would come to a standstill within a few decades and would not recover even under favorable conditions.
Basically, the Amoc – very roughly speaking – transfers heat from the South to the North Atlantic and thus contributes to a comparatively mild climate in Western and Northern Europe. Whether and under what circumstances this flow system could collapse is being intensively discussed among experts.
However, there is increasing evidence that this is both possible and becoming more likely. Dutch researchers recently showed in the journal “Science Advances” that they can simulate a collapse of the Amoc under certain conditions in a more complex climate model. The work was considered solid by several experts, but there was also criticism of certain assumptions made by the Dutch group.
This also presented a type of early warning system that, according to the researchers, shows that the North Atlantic current is developing towards a tipping point.
According to the analyses, the consequences would be dramatic: in some European cities, the annual mean temperature could fall by a few to 15 degrees within 100 years, depending on the region. It falls particularly sharply in winter and in the northwest. It could get colder in February by more than three degrees per decade in Bergen, Norway. One can only imagine the devastating effects of such rapid and extreme changes on nature and agriculture.
There could be accelerated warming in other regions. For the Amazon, the model shows a drastic change in precipitation patterns. “It is also predicted that the abrupt collapse of ocean circulation will cause sea levels in Europe to rise by 100 centimeters,” said lead author René van Westen from Utrecht University, according to the statement.
In order to understand the effects of the climate crisis on the Amoc, you have to take a closer look at the system. To put it simply, it consists of two opposing currents. Warm water is transported near the surface from the southern regions of the Atlantic to the north. There it cools down and sinks near the poles. As a cold current, it flows south again at depth.
The drivers of this system are differences in the density of the water. The water becomes particularly heavy near the poles because it is cold and salty. This causes it to sink into the depths and thus ensures dynamism.
However, according to experts, global warming is having a slowing effect on this system. On the one hand, the temperature of surface water in the far north is rising. On the other hand, the entry of fresh water, for example from melting ice sheets, makes the water there less salty. Both phenomena reduce the water density of the northern surface water, as a result of which the Amoc forcing becomes weaker. In the worst case, the event becomes increasingly self-intensifying until the flow system collapses.
The question is under what circumstances such a collapse could take place – and above all: when. Van Westen’s group, like many other experts, does not give a direct answer.
Danish researchers, who dared to make a prediction in the journal “Nature Communications” in July last year, are different. Their analyzes showed that the Amoc is very likely to collapse between 2025 and 2095. The response from experts was prompt and the study was heavily criticized. Many researchers considered the predictions to be untenable for methodological reasons.
Niklas Boers from the Technical University of Munich, who himself is conducting intensive research into weakening the Amoc, criticizes the fact that existing uncertainties were not sufficiently taken into account in the Danish model. The work makes assumptions that are far too simplistic to predict the future development of the Amoc from historical data alone, he told the DPA. On the other hand, the Dutch researchers led by van Westen write about the findings of their Danish colleagues: “Their estimate of the tipping point could be correct.”
Johanna Baehr, head of climate modeling at the Institute of Oceanography at the University of Hamburg, also emphasized the uncertainties of a forecast in an interview with the DPA: “We don’t know if and when such a collapse will come, whether in 50, 100 or 1,000 years.” However, with regard to the Dutch study, Baehr also says: “The possibility of a collapse can no longer be completely dismissed.” It is now the task of science to further narrow down a possible time frame.
For Baehr, the sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC) remains the benchmark. It says: The Amoc will very likely decrease over the course of the 21st century – regardless of the climate protection scenario. In addition, one is relatively confident (medium confidence) that this will not lead to a collapse before the year 2100.
The Amoc is not the only system that is being discussed as a so-called tilting element. Late last year, the Global Tipping Points Report highlighted five major natural systems facing potentially irreversible upheaval. However, in individual cases it is difficult or impossible to specifically state how close a tilting element actually is to collapse. Too many components play a role.
With regard to the Amoc, according to Boers, it is unclear how much additional fresh water would actually come into the northern Atlantic under a given level of global warming, whether from melting polar ice caps or more input from rivers and additional rain. Where exactly the fresh water comes in also plays a big role. On the other hand, there is concern that the models portray the Amoc as too stable. The current is currently weaker than ever before in the past 1000 years.
For Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK), the risk of collapse must be reduced at all costs. “The question is not whether we are sure this will happen. The problem is that we have to rule it out 99.9 percent,” he writes in a blog post. As soon as there is a clear warning signal, it is too late to do anything about it.
