Controversial prognosis: A research team has re-examined the development of the great overturning current in the North Atlantic – and has come to a daring prognosis. Accordingly, the North Atlantic Current is close to the tipping point and could tip over into a new state between 2050 and 2100, perhaps even collapse completely. However, this prognosis is controversial and has been criticized by peers. The world climate report and many other climate researchers see the danger of a partial or complete collapse from 2100 at the earliest.
The Atlantic Meridional Overturning Current ( AMOC ) is a motor of the global ocean circulation and ensures the distribution of heat between the poles and the equator. But this “district heating” of our continent is weakening. Due to the increased inflow of meltwater and the retreat of sea ice, the North Atlantic Current is weaker than it has been for a thousand years . In addition, there are increasing signs of a tipping over into a permanently weaker or even stagnant state.
When does tipping occur?
But when could the tipping point be reached? Determining this in advance is anything but easy. Because the overturning current is a complex system of surface and deep currents, the behavior and influencing factors of which are difficult to record and even more difficult to predict. In addition, precise satellite and sensor measurement data on current velocity, circulated water masses and other parameters have only been available since 2004.
So far, a more accurate forecast of the point in time when the tipping point will occur has hardly been considered possible. In the current world climate report by the IPCC, however, a progressive weakening of the North Atlantic Current in the 21st century is considered very likely. However, the report predicts a partial or complete collapse of the overturning flow only after 2100.
AMOC “fingerprint” as a forecast helper
A current forecast by the research duo Peter and Susanne Ditlevsen from the University of Copenhagen contradicts this. They extended the time series of AMOC measurements further back in time using an indirect “fingerprint”. “Analysis of observational data combined with a large number of climate model simulations have determined that sea temperatures in the subpolar gyrus of the North Atlantic are an optimal finder print for the strength of the AMOC,” the duo explains.
The decisive sea area is therefore between the coast of Newfoundland in the west, borders Greenland in the north and extends to Spitsbergen in the east. For their study, the researchers evaluated the sea temperatures of this subpolar current eddy from 1870 to the present day. They specifically looked for signs of the tipping over, including increasing fluctuations and increasing autocorrelation. The latter means that deviations from the mean increasingly result in similar rashes.
“Using new and improved statistical techniques, we have performed calculations that allow a more robust estimate of when the collapse of the thermohaline circulation is most likely to occur,” explains Susanne Ditlevsen.
Collapse already around 2050?
Based on these analyses, the team comes to the conclusion: The North Atlantic overturning current is closer to its tipping point than previously assumed. “We predict with a high probability that the overturn could take place as early as the middle of this century,” write Peter and Susanne Ditlevsen. Their estimated time is around 2050, although the range is from 2025 to 2095.
However, according to the Ditlevsens, it cannot be determined with certainty whether the circulation flow comes to a complete stop or only a partial collapse. They also admit that they deliberately used a simplified model of the system in their calculations. “Despite these limitations, this is a worrying result,” they say. This is especially true in view of the potentially serious consequences for the earth’s climate.
“Statement stands on feet of clay”
But what about this daring prediction? Colleagues of the research duo are rather critical of their conclusions. “In detail, the statistical analysis itself is correct. But I don’t think that the conclusion of the study is justified with the data basis and model quality,” comments Niklas Boers from the Potsdam Institute for Climate Impact Research (PIK), who researches AMOC himself. The uncertainties are so great that, based on the analysis of historical data, no statement can be made about the time of the tipping.
The criticism from Jochem Marotzke from the Max Planck Institute for Meteorology in Hamburg is even clearer. The renowned climate researcher states: “The statement made so confidently in the study now appearing that the AMOC will collapse in the 21st century stands on shaky ground.” In his view, the reduction of the complex system to a relatively simple model is both important as well as using only the regional surface temperature as a fingerprint are questionable and the conclusion therefore highly doubtful.
The tipping point is getting closer
The current debate thus once again underlines how difficult it is to understand and assess the complex interactions and reactions of the coupled ocean climate system. “There is still a great deal of uncertainty as to where the tipping point of the AMOC lies,” comments Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK), one of the pioneers of AMOC research.
At least there is agreement on one thing: the overturning flow is becoming increasingly weak and is thus approaching a tipping point. As long as climate change is not stopped, the time when it will happen will get closer and closer. “The new study adds evidence that it’s much closer than we thought a few years ago,” Rahmstorf said. (Nature Communications, 2023; doi: 10.1038/s41467-023-39810-w )