Taken from Reporterre
November 25, 2024
Par Vincent Lucchese
It is sometimes compared to a titanic treadmill. A complex set of ocean currents which cross the Atlantic – including the famous Gulf Stream – and which carry around 18 million m³ of water per second, or more than ten times the cumulative flow of all the world’s rivers. Called ” Atlantic meridional overturning circulation », or Amoc according to its English acronym, this system plays a crucial role in regulating the climate.
However, concern has been growing in the scientific literature for several years: we may have underestimated its weakening, or even its future collapse. The latest study, published on November 18 in the journal Nature Geoscience by researchers from the Australian University of New South Wales, concludes that Amoc could lose 30% of its power by 2040, twenty years earlier than previous estimates.
« This could lead to big changes to climate and ecosystems, including accelerated warming in the Southern Hemisphere, harsher winters in Europe and a weakening of tropical monsoons in the Northern Hemisphere. “, warn the authors.
Melting glaciers disrupt the ocean
In 2023, a study published in Nature Communications estimated that Amoc had a 95% probability of collapsing by 2095. And on October 21, around forty researchers from many countries signed an open letter warning the Nordic Council countries of the risk that we have “ greatly underestimated » the possibility of a weakening, or even a collapse of Amoc, which would have impacts “ devastating and irreversible » for many countries.
In its sixth assessment report, published in 2021 and summarizing the state of science on the subject, the IPCC [1] noted, however, with a degree of confidence “ AVERAGE “, that Amoc would not collapse by 2100. But a ” average confidence » leaves a worrying risk, underline the scientists in their open letter. And recent research published since tends to increase this risk, they write.
Currently, the scientific community is struggling to produce a consensual analysis of the situation. It is generally accepted that climate change is expected to weaken the Amoc. But when and with what intensity? The uncertainties and divergences of views on this question are commensurate with the extreme complexity of the phenomenon studied.
To understand this, let us return to the schematic functioning of the Amoc. One of its drivers is the dive towards the abysses of surface waters, in high latitudes. When warm currents from the tropics meet cold air masses in the north, some of the seawater freezes, leaving its salt behind. The remaining water thus sees its salt concentration increase. As colder, saltier water is denser, it sinks, causing the “ treadmill » from Amoc. This deep water then loops back south, where it rises and heats again at the surface.
We see, on this diagram of the Amoc, in red the warm surface currents, and in blue the cold currents circulating at depth. © NOAA
This system plays a crucial role in redistributing heat across the globe, via exchanges between the ocean and the atmosphere, and also contributes to the health of ecosystems, by transferring nutrients, carbon and oxygen across the ‘Atlantic. Climate change is disrupting all of this, notably by leading to the massive melting of Arctic glaciers in Greenland and Canada. By flowing into the ocean, this excess fresh water reduces the salinity, therefore the density and stops the engine of the Amoc which is the diving of cold waters into depth.
However, current climate models do not take into account this additional melting caused by human activities and struggle to reproduce the observed behavior of the Amoc. It is by integrating this font into their model that Australian researchers now claim to obtain better estimates.
Worrying uncertainties
Several researchers interviewed by Reporterre are, however, skeptical about the peremptory conclusions of this study, whose methodology could lack rigor, particularly in estimating the volume of fresh water resulting from the future melting of glaciers. The work of 2023 was also far from unanimous.
« It is very likely that climate change will slow down the Amoc over the century, but this weakening is estimated at -10 to -70% depending on the models, the uncertainty is enormous », recalls Didier Swingedouw, research director at CNRS, who studies these Atlantic currents closely.
The numerical simulations modeling the future of Amoc are all the more delicate as we still cannot properly represent the behavior “ normal » of the phenomenon, without taking into account climate change. “ Amoc results from a very subtle balance between numerous influences. Mixing zones between hot and cold water are inherently difficult to model. We must also succeed in representing the winds which will influence this convection, the precipitation and the evaporation levels which also play a role on the characteristics of these waters. », Lists Didier Swingedouw.
As factors evolve, there is a “cascade of uncertainties”. Pexels/CC/Laura Otte
To anticipate the future, we must add to the modeling challenge the evolution of these factors: how the increasingly warm tropics will increase evaporation and therefore the salinity of warm waters, how precipitation will evolve at high latitudes and in turn vary the salinity… “ A cascade of uncertainties », sighs the researcher.
It is not clear whether Amoc has already started to slow down. According to the modeling of the Australian study, the weakening would be 20% since 1950. But these results are the result of digital reconstructions: the observations on site have only been possible since 2004, and no clear trend has emerged. “ From direct observations of Amoc, what we measure is only strong seasonal, interannual and interdecadal variability “, but no clear signal linked to climate has been identified, underlines Sabrina Speich, oceanographer at the Dynamic Meteorology Laboratory.
Threats to Africa and Europe
Still, the trend seems to be moving towards increasingly pessimistic estimates. “ Before, we had “strong confidence” that Amoc would not suddenly collapse by 2100. The latest IPCC report moved to “medium confidence”. And then stopping at 2100 is arbitrary. Amoc could collapse in 2150. It is a slow system, its collapse time is probably on the order of a century », Underlines Didier Swingedouw, signatory of the open letter published in October.
The issue today is therefore less about understanding whether Amoc will weaken drastically, but when exactly it will do so. In any case, this could considerably cool northern Europe, surrounded by ever-warmer regions, leading to ” unprecedented extreme climates », calls out the scientists’ letter. This could potentially threatening the viability of agriculture in northwest Europe ».
« The countries of West Africa would especially be on the front line, worries Didier Swingedouw. The Sahel could become a desert, with up to a 30% drop in precipitation, and the now greener area south of the Sahel would become Sahelian. »
There is a doubly urgent need: to limit the extent of climate change as much as possible, but also to adapt to it. For the moment, the disasters linked to Amoc and its consequences on our societies are today neither anticipated nor even seriously evaluated, deplore the authors of the open letter.
Awareness will perhaps rise as climate projections become more refined regarding these phenomena. Researchers unanimously plead for the accumulation of data and additional work. The main measurement systems on site of Amoc massively involve American research institutes, whose future hangs on the inauguration of Donald Trump. The US president-elect plans to dismantle environmental agencies, including NOAA, the National Oceanic and Atmospheric Observation Agency.
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