The meridional circulation of the Atlantic (AMOC, in English acronym) is one of the major influences on Europe’s mild climate. It transports heated waters in the tropical regions on the western margin of this ocean, taking them northward where they are cooled again and return to the tropics.
By redistributing heat from the tropics to the Arctic, the AMOC grants Northern and Western Europe and also Iceland a climate more temperate than the latitude they would be allowed to have without it.
Several studies point out that the continued warming of the Atlantic and the melting of polar regions will eventually significantly weaken this current, perhaps within a few decades or less. If this happens, it is believed that average temperatures in Europe could fall substantially and that precipitation could become more irregular and uneven, with torrential rains in some regions and severe droughts in others.
The effects would also be felt in Africa, in South America and in Southeast Asia, and in North America sea level would rise markedly, with storms more frequent. As in other eras of the Earth, the thermal contrast between North and South would be far more pronounced: a North much cooler and a South much hotter.
But the future of the AMOC does not depend solely on the amount of warming, but also on the speed at which the temperature rises. While estimates point to the collapse of the current with global warming exceeding four degrees Celsius, scientists at Utrecht University (Netherlands) say that the stability of the AMOC depends mainly on how quickly the climate is changing.
Through climate models that simulate several hypothetical scenarios, the researchers, in a paper published in the journal Nature Climate Change, explain that a slower increase in atmospheric carbon dioxide globally would allow the AMOC to remain stable well beyond the four-degree rise in Earth’s average temperature. And they say it would not collapse even with a warming of five degrees.
On the other hand, in a scenario where carbon concentration rises rapidly in the atmosphere, the current would collapse even with an increase of two degrees in the planet’s average temperature.
The secret lies in how the ocean is able, or not, to adapt to the changes.
“In a slow-warming scenario, the entire ocean, from the surface down to the deepest layers, has time to gradually reorganize itself and adapt to the changing conditions,” explains Henk Dijkstra, coauthor of the study.
“With faster warming, the ocean simply cannot keep up,” he adds.
All things considered, the authors of this work estimate that the AMOC’s critical threshold will be an increase of 0.3 degrees per decade, a limit beyond which its collapse will be highly likely.
René van Westen, the article’s lead author, says that this is a limit we are quickly approaching. “When it comes to global warming, the world is still pushing on the accelerator with full force.”
