An essential wingeed of the global climate, the AMOC, which also helps maintain rain in the tropics and store CO2, could weaken by 51% by 2100, according to a new French study.
This is an essential working wheel of the global climate, and it could falter more than expected. The main ocean circulation of the Atlantic, called the Atlantic Turn Meridian Circulation (or AMOC), could weaken by 51% by 2100 in a scenario of moderate greenhouse gas emissions (the trajectory currently followed by humanity), according to a study published in Science Advances, Wednesday, April 15. A much sharper decline than previous estimates, which counted on an average decline of 32%.
The AMOC plays a central role in the redistribution of heat on a global scale. This set of currents, of which the Gulf Stream is part, forms a huge loop from south to north of the Atlantic, with a flow rate of 18 million cubic meters per second. By transporting hot salt water from the tropics to the north of the basin, it helps maintain a mild climate in Europe. AMOC also affects the rainfall in the tropics, helps to store CO2 in depth and transports nutrients essential to marine biodiversity.
« Its weakening, especially if it is stronger, would have major consequences, » warns Didier Swingedouw, researcher (CNRS) at the Oceanic and Continental Environments and Paleo-Environments Laboratory in Bordeaux, one of the authors of the study published in Science Advance. In particular, it would lead to a drop in winter temperatures in Europe and a sharp drop in rainfall in the Sahel, affecting food crops and therefore food security.
Scale and pace of decline
All climate models agree on one point: this circulation will slow down during the 21st century under the effect of climate change. But they strongly diverge on the magnitude and pace of this decline. It is this uncertainty that the study has considerably reduced.
« The future projections of the AMOC have a huge margin of error, such that we do not know what the magnitude of the decrease in this circulation will be, » says Didier Swingedouw. The latest report of the Intergovernmental Panel on Climate Change (IPCC) gives a range of 3% to 72% decrease depending on different climate models and emission scenarios, with a margin of error of about 30% or 40% for each of them. Indeed, 80% of the uncertainty around the slowdown of the AMOC by 2100 comes from the models
climate themselves – well ahead of emission scenarios or the natural variability of the climate.
To refine the projections, French scientists forced the models with observations of the climate system. In particular, they used twenty-one different variables, such as the temperature or salinity of different areas of the Atlantic. They realized that many models underestimated the salinity of the South Atlantic, leading to overestimating the stability of the AMOC.
Result: the expected slowdown would be 60% greater than that suggested by the average of
Models. It would now be considered « substantial » in the IPCC classification. The margin
error has also been significantly reduced.
« Important and worrying study »
According to this new work, however, the AMOC would not collapse during this century – which corresponds to a reduction of more than 80%. The question of whether this circulation can reach a tipping point, a threshold beyond which it could not return to its initial state and would eventually stop, is hotly debated within the scientific community.
« This study is both important and worrying, » reacts Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research, who did not participate in it. This shows that pessimistic models are unfortunately the most realistic, as they are more consistent with observational data. In his eyes, the AMOC would then reach a « so low throughput » in 2100 that it would then be « very likely that the system would be in the pact of stopping completely ». The oceanographer and climatologist has argued for years that the AMOC is on the verge of collapse and that serious repercussions will be felt during this process.
Julie Deshayes, on the other hand, prefers not to get carried away ». This research director (CNRS) at the Pierre-Simon-Laplace Institute believes that it is not possible to quantify to what extent the results of the study in Science Advances imply more serious impacts for societies. She considers the study « very solid and very useful » thanks to a combination of oceanographic expertise and statistics. « It shows the urgency of improving models, but it does not give the keys to how to do it, » she emphasizes.
Many recent studies conclude that the latest IPCC reports underestimate the risks around the AMOC. Another study published on April 8 in Science Advances suggests that traffic may have begun to slow down. It is based on direct observations collected over the past twenty years by oceanic sensor networks. This signal, still difficult to distinguish from natural variability, is nevertheless, according to scientists, a « canary in the mine », an early warning of a potentially profound evolution of the oceanic system.
