There is growing concern about the closure of the Atlantic ‘conveyor belt’ Climate Crisis News

For thousands of years, the circulating currents of the Atlantic have continuously regulated the temperatures of Europe and North America, producing a warming effect that allows them to enjoy relatively moderate weather conditions.

But the effects of anthropogenic climate change have slowed the flow of this vast conveyor belt system, known as the South Atlantic Upheaval Circulation (AMOC), and recent scientific research suggests it may even be headed for collapse.

The unprecedented slowdown in the vast system has been measured directly since 2004, but analysis of indirect data indicates a longer decline, beginning in the mid to late 19th century and accelerating after 1950.

One study, which looked at ice cores and ocean sediments, determined that the AMOC was “in its weakest state in more than a millennium.”

“Everything points to a weakening of the AMOC,” said Sybren Drijfhout, an oceanographer at the University of Southampton.

The timeline for a possible collapse of the AMOC remains unclear, but the consequences for the Earth’s climate would be huge.

Temperatures in Europe and eastern North America would drop by as much as 5 degrees Celsius (9 degrees Fahrenheit), leading to more extreme winter weather.

Coastal cities in North America would be flooded by rising sea levels. It would also disrupt the West African and Asian monsoons, which provide essential rainfall for crops on which tens of millions of people depend.

How AMOC works

An enormous system of sea currents, the AMOC is driven by changing water density, which is determined by the salt content and temperature of the water.

Under a process known as “thermohaline circulation”, hot water moves north through the Gulf of Mexico toward Europe – the stretch known as the Gulf Stream – with the surface temperature decreasing as evaporation occurs and salinity increases.

Becoming denser, the water then sinks into the North Atlantic and whips south along the ocean floor before “upstream” to the surface again far into the southern hemisphere.

The effects of global warming on the AMOC are twofold. Warmer water is less dense, and freshwater drainage from ice melting in the polar region reduces salinity, which reduces density even further. These factors slow down the sinking mechanism that propels the circulation.

The last time the AMOC closed was towards the end of the last ice age, about 14,500 years ago. Then glacial melting flooded the North Atlantic with fresh water, collapsing the system and causing temperatures in Europe to plunge.

The report of the Intergovernmental Panel on Climate Change (IPCC) published in August found with high confidence that the AMOC is likely to weaken over the coming decades, but a total collapse by 2100 is unlikely.

“Although the AMOC is very likely to collapse during the 21st century, its weakening can be large, which can therefore induce strong and large-scale climate impacts with potential widespread impacts on natural and human systems,” it said.

“Very massive impact”

Whether the decline of the AMOC will continue in a linear fashion, or reach some reversal point after which the decline could accelerate sharply, remains a point of discussion among scientists.

“That’s the million-dollar question,” said Niklas Boers, a researcher at the Potsdam Institute for Climate Impact Research. “Whether it’s just linear, slowing down, or whether it’s actually a loss of stability.”

An article published by Boers in the journal Nature Climate Change in August analyzed eight separate indicators, using sea surface temperature and water salinity data that extend back to the 19th century.

It found that the AMOC may have evolved from a period of relative stability to a “critical” transition that would signify a profound change in the global climate system. Such a tipping point could see the AMOC come to a complete halt over a relatively short period of decades.

“We have a situation where there is a threshold … If we reach that threshold, then we will have a very, very massive impact that is perhaps virtually irreversible,” Boers said.

“Reduce emissions as quickly as possible”

There remain differences between observed data and existing climate models, and there is still no consensus on how long a full-time shutdown could last. Some estimates suggest as long as a few hundred years.

“All models agree that in warmer climates the AMOC will become weaker and weaker,” Drijfhout said. “That doesn’t have to mean collapse. It could go very, very slowly. “

In both cases, West Africa will have to adapt to declining rainfall and Europe to increasingly unpredictable winter weather, in addition to other effects already produced by climate change.

Further advances in climate modeling could provide a more accurate picture of future issues, but the evidence is already clear that reducing man-made global warming will be crucial to maintaining stability in the Atlantic system.

The most important factor in how the AMOC evolves is the amount of greenhouse gases that will be released into the atmosphere in the coming years and decades, Boer said.

“There is not so much room for compromise. So you have to really reduce emissions as much as possible – and as quickly as possible. “

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