Simone Valesini July 25, 2023
One of our planet’s most important ocean currents could be at risk. And with it the climate of our hemisphere, at least as we have known it so far. This is indicated by estimates just published in Nature Communications by two University of Copenhagen researchers, physicist and climatologist Peter Ditlevsen and mathematician Susanne Ditlevsen, pointing to a possible collapse of the so-called Atlantic Meridional Overturning Circulation (AMOC) as early as 2025, at least in the worst-case scenario of global warming, with greenhouse gas emissions remaining flat for years to come.
The meridional overturning of the Atlantic Circulation is an ocean current that feeds the Gulf Stream and is part of a larger phenomenon known as the global thermohaline circulation, a sort of giant highway that snakes through the oceans and redistributes heat between the poles and the equator. The collapse of the Amoc is a well-known and studied phenomenon by climatologists because it has occurred in the past and appears to be related to one of the most sudden processes changing the climate of the Northern Hemisphere: the Dansgaard-Oeschger event, in which temperatures can rise as much as 10-15 degrees per decade, only to cool much more slowly over the course of a few centuries.
Currently, it is feared that the AMOC is in a deceleration phase due to the melting of the Greenland ice sheets, which will lower ocean salinity to the point that the warm and saline waters from the Gulf of Mexico will become denser and sink as it cools, fueling the meridional overturning of the Atlantic circulation. The influx of freshwater from the Greenland ice sheet could jeopardize this mechanism and lead to a slowdown or collapse of the AMOC, with far-reaching consequences for the entire planet’s climate. In Europe, it is believed that this would result in a sharp cooling of the northernmost countries (where temperatures are now relatively mild due to the effects of the Gulf Stream), a reduction in precipitation in the mid-latitudes and, more generally, a drastic change in climatic conditions across the continent.
The collapse of the AMOC is seen by experts as a climatic “tipping point” as it would have consequences of very large proportions that would take centuries to reverse. Because of this, it is widely studied and not a plausible event, at least for the current century, according to IPCC estimates. However, the estimates published in the new study paint a different picture. The two Danish researchers analyzed surface water temperatures in the North Atlantic between 1870 and 2020 (and considered them an indicator of AMOC strength) and recalculated the state of Atlantic currents. In doing so, they came to the conclusion that they are probably already slowing down. According to their calculations, the collapse could take place as early as 2025, but no later than 2095.
Of course, this is only true if the statistical model created by the two scientists turns out to be correct, which, as the authors themselves admit, cannot be taken for granted. Even if their calculations were correct – the two also point out – it is possible that the AMOC will never come to a complete collapse but face a partial halt that would have less dramatic consequences for global climate. In the model used by the two, CO2 emissions were the main cause of the decline in Atlantic currents, the concentration of which in the atmosphere increased almost linearly over the period studied. To counteract the danger (whether real or virtual), one should simply stick to the agreements already made at international level to reduce greenhouse gas emissions. And saving the Gulf Stream is just one more reason to get involved.