For years, science has been concerned with the question of whether a huge heat pump in the ocean, the so-called Atlantic Overturning Circulation (AMOC), is slowing down due to man-made global warming – and at some point threatens to stop completely. However, what could happen if the system actually collapses is also very frightening: For northwestern Europe, for example, a collapse of the AMOC could mean a drastic cooling of the climate by two to five degrees, and some estimates predict significantly more. A catastrophe whose effects can hardly be imagined.
In the 2019 Oceans Special Report The Intergovernmental Panel on Climate Change (IPCC) estimates the chance of AMOC being canceled to be less than ten percent. However, various recent studies show evidence of much higher probabilities. It is an enormous scientific challenge to create forecasts from the data obtained in a wide variety of ways and at a wide variety of sections and to feed the climate models in such a way that they do justice to the complexity of this huge circulation system. The AMOC runs through almost the entire Atlantic from the Norwegian Sea to the Southern Ocean and is connected to ocean currents around the globe. Many components of the system are not yet fully understood.
:The current that can change everything
Europe’s climate is determined by a huge heat pump in the sea. If it breaks off, the continent could become significantly colder. And then?
One piece of the puzzle to better interpret the AMOC is measurement data from the Florida Current. The current brings warm water masses from the Gulf of Mexico north along the east coast of the USA and therefore also affects the Atlantic Overturning Circulation. Valuable measurement data on the strength of the Florida current has been available since 1982, which is obtained almost every minute via an undersea telecommunications cable that runs between Florida and the Bahamas. By measuring electrical charges on the cables, something can also be said about the masses of water flowing through them. Analysis of this data so far appears to show that the Florida Current has become significantly weaker, particularly over the past twenty years. This weakening, in turn, was also interpreted as an indication of a general weakening of the AMOC.
What was overlooked was that the Earth’s magnetic poles have shifted
Now scientists led by Denis Volkov from the University of Miami have published in the journal Nature Communications published a study that reinterprets the measurement data from the Florida Current. This means that the electricity may actually be much more stable than previously thought. In the analysis of the data so far, it has been overlooked that the earth’s magnetic poles have shifted over the corresponding time and this in turn influences the voltage in the telecommunications cables. This has not yet been taken into account when interpreting the data, which has led to a misjudgment of the Florida current. If one takes into account the effect of the geomagnetic field on the measurements on the cables over the past forty years, the attenuation would hardly be significant, according to the authors.
And what does that mean for the AMOC? Less than you might initially think. Because the supposed slowdown of the Florida current has always been just one of many pieces of the AMOC puzzle. “This study does not refute the possible slowing of the AMOC, but rather shows that the Florida Current, a key component of the AMOC in the subtropical North Atlantic, has remained stable over more than 40 years of observation,” lead author Denis Volkov said in a statement.
Jochem Marotzke, director at the Max Planck Institute (MPI) for Meteorology in Hamburg, considers the result for the relevant area of the AMOC to be convincing: “The study has merit, and the conclusions are robust: Gulf Stream transport has not decreased, and the weakening of the AMOC was marginal.” However, Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK) emphasizes: “Even in the corrected data for the Florida Current, the recent weakening of the AMOC is steeper than the long-term trend. This data correction does not result in any significant change to the risk assessment for the AMOC as a whole.”