Explained: Why the Atlantic Ocean current system is slowing down and its implications

If you watched the 2004 film, Two days later, you may remember that a disruption to the circulation of the North Atlantic Ocean sent the planet into another Ice Age. A study published last week in Nature Climate Change notes that this circulation, officially known as the Atlantic Meridional Overturning Circulation (AMOC), is losing its stability. According to IPCC report (AR6) released on August 9, it is very likely that AMOC will decline in the 21st century.

Article author Niklas Boers explains in a statement: “The results support the assessment that the decline in AMOC is not just a fluctuation or linear response to increasing temperatures, but probably means approaching a critical threshold beyond which the circulation system could collapse. . “

What is AMOC?

AMOC is a large ocean current system. It is the Atlantic branch of the ocean conveyor belt or thermohaline circulation (THC), and distributes heat and nutrients to ocean basins around the world.

AMOC transports warm surface water from the tropics to the northern hemisphere, where it cools and flows. It then returns to the tropics and then to the South Atlantic as a bottom current. From there it is distributed to all ocean basins via the Antarctic Circumpolar Current.

What happens if AMOC collapses?

The Gulf Stream, which is part of AMOC, is a warm current responsible for the mild climate on the east coast of North America as well as in Europe. Without proper AMOC and Gulf Stream Europe will be very cold.

Modeling studies have shown that stopping AMOC will cool the northern hemisphere and decrease precipitation over Europe. It can also have an effect on El Niño.

An article published in 2016 in Science Advances noted: “The collapse of AMOC causes important and very different climatic reactions: significant cooling over the northern North Atlantic and neighboring areas, sea ice increasing over the Greenland-Iceland-Norwegian seas and south of Greenland, and extensive southward migration of the rain belt over the tropical Atlantic.

The study team suspected that previous models overestimated the stability of AMOC because they did not take into account the influence of fresh water. Fresh water from melting ice caps in Greenland and the Arctic region can weaken circulation because it is not as dense as salt water and does not sink to the bottom.

Has AMOC ever weakened?

“The strength of AMOC and THC has always fluctuated, mainly if you look at the Late Pleistocene period (last 1 million years). Extreme glacial stages saw weaker circulation and slowing AMOC, while ice terminations showed stronger AMOC and circulation, ”says Nirmal B, a PhD. researcher from Geoscience Research Lab, VIT Chennai, who studied the Atlantic paleoclimate.

“We know about these past fluctuations by studying paleoclimatic indicators such as sea surface temperatures (SST), salinity, and isotope signatures of single-celled organisms called foraminifera. But the changes we are experiencing in the last 100 to 200 years are anthropogenic, and these abrupt changes destabilize AMOC, which could bring the system down, ”he adds.

In February, researchers noted that AMOC was at its lowest in more than a millennium. The team has studied the evolution of AMOC over the past 1600 years. Stefan Rahmstorf, one of the authors of the study published in Nature Geoscience explained in a statement: “The results of the study suggest that it [AMOC] was relatively stable until the end of the 19th century. With the end of the Little Ice Age around 1850, ocean currents began to decline, followed by a second more drastic decline since the mid-20th century.

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Why is AMOC slowing down?

Climate models have long predicted that global warming can weaken the world’s major ocean systems.

Last month, researchers noted that part of the Arctic ice known as the “last ice patch” had also melted. Fresh water from melting ice reduces the salinity and density of the water. Now the water cannot flow as before and weakens the flow of AMOC.

A 2019 study suggested that the Indian Ocean may also be contributing to the slowdown in AMOC. The researchers said: As the Indian Ocean warms faster and faster, it generates additional precipitation. With so much precipitation in the Indian Ocean, there will be less precipitation in the Atlantic Ocean, resulting in higher salinity in the waters of the tropical part of the Atlantic. This saltier Atlantic water, when it arrives north via AMOC, will cool much faster than usual and sink faster.

“This would serve as a springboard for AMOC, increasing traffic,” author Alexey Fedorov said in a statement. “On the other hand, we don’t know how long this increased warming of the Indian Ocean will continue. If the warming of the other tropical oceans, especially the Pacific, catches up with that of the Indian Ocean, the benefit for AMOC will stop.

“If we continue to cause global warming, the Gulf Stream system will weaken further – from 34 to 45% by 2100 according to the latest generation of climate models,” notes Rahmstorf. “It could bring us dangerously close to the tipping point at which the flow becomes unstable.”

Niklas Boers added: “We urgently need to reconcile our models with the observational evidence presented to assess how far or how far AMOC really is at its critical threshold. “

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