For thousands of years, the circulating currents of the Atlantic Ocean have continuously regulated the temperatures of Europe and North America, producing a warming effect that allows them to take advantage of relatively moderate weather conditions.
But the effects of anthropogenic climate change have decreased the throughput of this vast conveyor belt system, known as the Atlantic Meridional Turning Circulation (AMOC), and recent scientific research suggests it may even be on the point of collapsing.
The unprecedented slowdown of the vast system has been measured directly since 2004, but analysis of the indirect data suggests a longer decline, starting between the mid to late 19th century and accelerating after 1950.
One study, which examined ice cores and ocean sediments, determined that AMOC was “in its weakest state in over a millennium.”
“Everything points to a weakening of AMOC,” said Sybren Drijfhout, oceanographer at the University of Southampton.
The timeline of a potential AMOC collapse remains uncertain, but the consequences for Earth’s climate would be immense.
Temperatures in Europe and eastern North America would drop as much as 5 degrees Celsius (9 degrees Fahrenheit), resulting in more extreme winter conditions.
Coastal cities in North America would be inundated by rising sea levels. It would also disrupt the monsoons of West Africa and Asia, which provide vital rainfall for the crops on which tens of millions depend. of people.
How AMOC works
A huge system of ocean currents, AMOC is driven by the change in the density of water, which is determined by the salt content and temperature of the water.
In a process known as the “thermohaline circulation”, hot water moves northward across the Gulf of Mexico to Europe – the stretch known as the Gulf Stream – with temperature. surface area decreasing as evaporation occurs and salinity increases.
Becoming denser, the water then sinks into the North Atlantic and travels south along the ocean floor before rising to the surface far into the southern hemisphere.
The effects of global warming on AMOC are twofold. Warmer water is less dense, and freshwater runoff from melting ice in the polar region reduces salinity, further reducing density. These factors slow down the descent mechanism that propels the circulation.
The last time AMOC closed its doors was towards the end of the last ice age, around 14,500 years ago. Then, the melting glaciers flooded the North Atlantic with fresh water, collapsing the system and causing temperatures to drop in Europe.
The Intergovernmental Panel on Climate Change (IPCC) report released in August revealed with great confidence that AMOC is likely to weaken over the next several decades, but a full collapse before 2100 is unlikely.
“While it is highly unlikely that AMOC will collapse in the 21st century, its weakening can be substantial, which can therefore induce strong and large-scale climate impacts with potential large-scale impacts on people. natural and human systems, ”he said.
“Very massive impact”
Whether the decline in AMOC will continue in a linear fashion or reach a tipping point, after which the decline may accelerate precipitously, remains a matter of debate 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 a linear slowdown or a loss of stability. “
An article published by Boers in the journal Nature Climate Change in August analyzed eight separate indicators, using data on sea surface temperature and water salinity dating back to the 19th century.
He found that AMOC may have evolved from a period of relative stability to a “critical” transition that would signal a profound change in the global climate system. Such a tipping point could see AMOC coming to a complete halt over a relatively short period of several decades.
“We have a situation where there is a threshold… If we hit that threshold, then we’re going to have a very, very massive impact that is maybe virtually irreversible,” Boers said.
“Reduce emissions as quickly as possible”
Discrepancies between observed data and existing climate models remain, and there is still no consensus on the duration of a full shutdown. Some estimates suggest as long as several hundred years.
“All models agree that in warmer climates the AMOC will become weaker and weaker,” Drijfhout said. “It doesn’t necessarily mean a collapse. It could go very, very gradually.
In both cases, West Africa will have to adapt to declining rainfall and Europe to increasingly unpredictable winter conditions, in addition to other effects already produced by climate change.
Further advancements in climate modeling could provide a more accurate picture of things to come, but it’s already clear that reducing man-made global warming will be crucial to maintaining the stability of the Atlantic system.
The most important factor in the development of AMOC is the amount of greenhouse gases that will be released into the atmosphere over the next few years and decades, Boer said.
“There isn’t much room for compromise. So we really need to reduce emissions as much as possible – and as quickly as possible. “