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The great Atlantic Current, known by the acronym AMOC (Atlantic Meridional Overturning Circulation), will lose 51% of its strength by 2100, with a margin of error of plus or minus 8%, even if all countries in the world meet their current CO₂ emission reduction promises. The conclusion comes from a recent study led by Valentin Portmann from the University of Bordeaux, France, published in the journal Science Advances, which drastically reduced the uncertainty of previous forecasts on the subject.
Until the last IPCC report in 2021, simulations could barely determine whether the weakening of the Atlantic Current would actually occur: the estimate was a 32% reduction, but with an uncertainty margin of 37%, so broad that it practically did not allow anything to be stated with certainty. Portmann’s study corrected biases that existed in climate models, especially in the simulation of South Atlantic salinity and the calculation of Greenland’s ice melt, and arrived at a much more assertive prediction. The 51% reduction of the AMOC means that the ocean circulation that transports heat from the tropics to Europe and brings cold water to the coasts of Brazil will operate at half its current power before 2100. For Brazil, the impact may directly affect the Amazon and the Caatinga, altering rainfall regimes that sustain these biomes.
The engine that drives the oceans is slowing down
The Atlantic Current functions as an engine that moves the entire global ocean circulation system. In the North Atlantic, cold and salty water sinks to the ocean floor because it is denser, and this sinking pulls warm surface water from the tropics, creating a continuous flow that distributes heat around the planet. This mechanism moderates European winters by transporting huge masses of warm water from the Southern Hemisphere to the North, and it alleviates summers in the equatorial region by bringing cold water to the coasts of Brazil and Africa.
The fuel of this engine is the difference in density between the waters, determined by temperature and salt concentration. When the water is cold and salty, it is heavy and sinks. When it is warm or diluted by fresh water, it becomes light and remains on the surface. The melting of Greenland dumps increasing volumes of fresh water into the North Atlantic, diluting the salinity, reducing the density, and slowing down the sinking that keeps the engine running. Less sinking means less circulation, and less circulation means the Atlantic Current loses strength.
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What the French study corrected in previous forecasts
The work led by Portmann brought precision to a field where uncertainty reigned. The main limitation of previous studies was the underestimation of Greenland’s melting and the inaccuracy in measuring the transport of salty water from the South Atlantic to the North, a crucial variable for calculating the salinity that feeds the AMOC engine. Without reliable data on how much salt reaches the North Atlantic, the models could not safely predict whether the current would weaken slightly or significantly.
“Climate models are not perfect. They are useful for predicting future climate, but to refine them, it is interesting to incorporate real-world observations, as we did,” explained Portmann. Correcting the salinity bias in the South Atlantic, combined with more recent data on the actual volume of melting, allowed the French model to deliver a forecast with a margin of error of only 8%, compared to the 37% uncertainty of previous works. The difference is between not knowing if the problem will happen and knowing that it is practically inevitable.
Amazon and Caatinga: Brazilian biomes in the spotlight
The weakening of the Atlantic Current is not an exclusively European problem. Ocean circulation directly influences rainfall patterns in the Southern Hemisphere, and an AMOC operating at half power can alter the position and intensity of the Intertropical Convergence Zone, the rain belt that feeds the Amazon during the wet season. If this zone shifts south or weakens, the Amazon may receive less precipitation, accelerating a savannization process that other studies already point to as a concrete risk.
The Caatinga, a semi-arid biome that already operates at the water limit, would be even more vulnerable to any reduction in rainfall. Entire communities in northeastern Brazil depend on precipitation cycles that are ultimately connected to the Atlantic ocean circulation. Cristiano Chiessi, a scientist from the University of São Paulo who researches the topic, confirms that the trend identified by Portmann is not isolated: “There are a number of other recent studies pointing in the same direction,” he stated, indicating that the scientific community converges on the conclusion that the weakening of the AMOC is real and significant.
The expectation for the next IPCC report
The conclusions of the French study have increased the expectation that the weakening of the Atlantic Current will gain prominence in the next IPCC assessment report, scheduled for 2027. In the 2021 report, the AMOC was treated with caution due to the imprecision of predictions, and the panel could not confidently assert the intensity or timing of the weakening. With data from Portmann and other researchers pointing in the same direction, the next report may classify the weakening of the AMOC as a high-probability scenario.
A change in classification by the IPCC would have practical consequences for climate policies worldwide. If the panel confirms that the current will lose half its strength by 2100 even with emission reductions, governments will need to include this variable in their climate adaptation plans. For Brazil, this would mean rethinking water management in the Amazon and Northeast, anticipating changes in agricultural patterns, and preparing infrastructure for scenarios of prolonged drought in regions that currently depend on rains connected to ocean circulation.
The Greenland Ice Melt that Fuels the Problem
The accelerated melting of the Greenland ice sheet is one of the main factors weakening the Atlantic Current. The icebergs that break off and the meltwater flowing into the ocean dilute the salinity of the North Atlantic, making the water lighter and reducing the sinking that drives the circulation. The more ice melts, the more freshwater enters the system, the more the engine slows down, and the more the current loses strength, in a self-reinforcing cycle.
Previous models underestimated the volume of ice melt and, consequently, its impact on the AMOC. With updated data on the actual rate of ice loss in Greenland, which has been accelerating in recent decades, predictions have become more accurate and more concerning. The incorporation of these real measurements into climate models is what allowed the French study to deliver a prediction of a 51% reduction with a narrow margin of error, replacing the uncertainty of previous works with an answer that the scientific community increasingly accepts as likely.
Half the Strength, Double the Concern
The great Atlantic Current will lose 51% of its strength by 2100, according to the most precise study ever published on the subject. The reduction of the AMOC could alter rainfall patterns in the Amazon and Caatinga, cool Europe, change agricultural crops, and force adaptations that cost billions. The next IPCC report, in 2027, should reflect this new scientific certainty, and the window to act before the effects become irreversible diminishes with each year of emissions that are not cut.
Did you know that an ocean current in the middle of the Atlantic influences the Amazon’s rainfall? Tell us in the comments what you think about the prediction of a 51% loss of AMOC strength, whether you believe Brazil is prepared for the impacts on biomes, and how you assess the difference between knowing the problem will happen and acting to mitigate it. We want to hear your opinion.
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