Melting ice could weaken currents in the Atlantic Ocean and change the climate

The ship, Arctic Sunrise, surrounded by ice floating in the ocean near Svalbard, Norway.Christian Aslund / Greenpeace

• The system of ocean currents in the Atlantic Ocean, called southern inversion circulation of the Atlantic (AMOC), is responsible for warm temperatures in Western Europe.
• However, according to a new UN report, rising water temperatures and changes in the salinity of the world's oceans weaken AMOC.

• Other recent research has shown that AMOC could slow down considerably if enough fresh water from melting ice enters the ocean.

• Lower current flow could result in more extreme weather conditions in the northern hemisphere.
• A version of this scenario is described in the movie "The Day After Tomorrow", although the film is not scientifically accurate.

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According to a new report, the unprecedented melting of Arctic and Antarctic ice sheets, combined with warming seawater, could raise sea levels by more than 3 feet by the end of the century.

The assessment was compiled by more than 100 authors from 36 countries as part of the United Nations Intergovernmental Panel on Climate Change (IPCC). The results suggest that melting glaciers bringing huge amounts of fresh water into the world's oceans, the current Atlantic system will likely weaken over time.

This system, called the Southern Turnover Atlantic Circulation (AMOC), moves ocean water north and south of the Atlantic and mixes surface water with deep water. This is partly responsible for the hot and humid climate of Western Europe.

The UN report concluded that "all climate change scenarios should result in a weakening of AMOC in the 21st century". This means that changes will occur in the North Atlantic, even if we significantly limit greenhouse gas emissions (and, therefore, global warming).

Read more: According to a new UN report, the sea level is expected to increase by 3 feet in the next 80 years. Hundreds of millions of people could be displaced.

"We are definitely going into a world where AMOC is weakening," BusinessCenter Francesco Muschitiello, author of a different study on AMOC, told Business Insider.

As this happens, Europe could become colder and drier and hurricanes could experience more violent hurricanes.

Why melting glaciers and warming oceans weaken ocean currents

Scientists compare AMOC to a kind of water conveyor belt.

Once the warmer water reaches the area around the UK, it cools and sinks to the bottom of the Labrador seas and northern seas. Then this cold water turns around and makes its way to the bottom of the ocean, up to the Southern Ocean of Antarctica.

When AMOC flows rapidly, Western Europe enjoys a warm and humid climate. But if it becomes slow and weak, the warm tropical waters have not risen and the North Atlantic is cooling down.

The southern overturning circulation of the Atlantic brings hot tropical water into the North Atlantic (in red), where it cools and lowers before descending to the south (in blue).NASA / JPL

The speed of AMOC depends on a delicate balance between salt water and fresh water. The salt water is dense, so it flows easily. But as the icecaps of Greenland and Antarctica melt, as well as glaciers around the world, more and more fresh water is joining AMOC.

This merger is happening quickly, confirmed the UN report. According to a study conducted in April, Greenland ice melts six times faster than it was 40 years ago, with pack ice collecting about 286 billion tonnes of ice a year. Two decades ago, the annual average was only 50 billion.

Read more: Greenland is approaching the threshold of irreversible meltdown and consequences for coastal cities could be disastrous

In Antarctica, meanwhile, the entire ice cap melts almost six times faster than 40 years ago. In the 1980s, Antarctica lost 40 billion tons of ice a year. Over the past decade, that number has risen to an average of 252 billion tonnes per year.

The addition of all this fresh water makes the salty surface water lighter and less likely to flow, which obstructs the flow of traffic and weakens the AMOC.

Weaker AMOC could mean more extreme weather

The authors of the UN report, which focuses on the state of the oceans and the cryosphere (frozen parts of the planet) around the world, say they are somewhat confident that AMOC is already weakened compared to the period 1850-1900.

According to a 2018 study, AMOC is the weakest for at least 1,600 years.

According to the report, this weakening is likely to cause changes in our global climate, including more storms in Northern Europe and a decrease in the amount of organic matter produced and circulating in the Atlantic marine food web. North.

This would also lead to a decrease in summer rainfall in the South Asian and South African Sahel so that some parts of West and Central Africa may experience more drought.

Four tropical cyclones, Katia, Lee, Maria and Nate, were seen in the Atlantic Ocean Basin on September 8, 2011.NASA

"We will certainly see more extreme weather conditions," said Muschitiello. "Europe will become colder and dryer in the long run.There will be a surplus of heat in the subtropics, which is important for the formation of a hurricane."

When subtropical waters are warmer, this creates more frequent and intense hurricanes in the Atlantic, because hot air retains more water vapor – and this extra moisture feeds hurricanes.

A weakening of AMOC would also cause sea level rise along the northeastern coast of North America.

Could AMOC ever stop completely?

In the 2004 film "The day after tomorrow," AMOC stops almost overnight and a glacial age descends on Europe and North America. People are dying of cold in the streets, helicopters are falling from the sky and a huge tidal wave invades New York.

These effects are not scientifically accurate and have been hyperbolized in the film, of course, but the idea that the circulation of Atlantic water could be interrupted is not totally out of reach of the possible. This is very unlikely.

In the 2004 film "The day after tomorrow", the AMOC stops completely.20th Century Fox

According to Muschitiello, there are "reconstructions suggesting that AMOC has completely stopped in the past and that these major distortions of AMOC have led to the coldest events ever recorded".

But such an event would require a huge amount of meltwater – the distortions of the past evoked by Muschitiello occurred after large swarms of icebergs broke off glaciers and floated into the ocean North Atlantic. When these iceberg armadas melted, an excess of fresh water was added to the ocean, wreaking havoc on AMOC.

However, this influx of freshwater was several orders of magnitude higher than current melting rates.

Melting ice occurs on a cliff in Landsend on the Cape Denison Coast in Antarctica on January 2, 2010.Pauline Askin / Reuters

A study conducted in 2017 found that if the concentration of carbon dioxide in the atmosphere doubled instantly from the 1990 level, AMOC could collapse in 300 years. Greenhouse gas emissions, however, are not expected to significantly increase this increase.

The authors of the UN report also concluded that "a collapse is very unlikely". They noted, however, that this would be even more unlikely in the future with less greenhouse gas emissions.