Unprecedented melting of the Arctic and Antarctic ice sheets, combined with warming ocean water, could cause sea levels to rise by more than 3 feet by the end of the century, according to new report.
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 findings suggest that as melting glaciers add vast amounts of much fresh water to the world's oceans, the current system in the Atlantic is likely to weaken over time.
This system, called the Atlantic Meridional Overturning Circulation (AMOC), moves ocean water north and south in the Atlantic and mixes surface water with that from deep below. It's partially responsible for western Europe's warm and wet climate.
The UN report concluded that "the AMOC is projected to weaken in the 21st century" under every climate-change scenario. That means changes are coming to the north Atlantic even if we significantly limit greenhouse-gas emissions (and, consequently, planetary warming).
"We are definitely going into a world where AMOC is getting weaker," Francesco Muschitiello, the author of a different study about the AMOC, previously told Business Insider.
As that happens, Europe could get colder, drier weather, and the tropics could experience stronger hurricanes.
Why melting glaciers and warming oceans weaken ocean currents
Scientists liken the AMOC to a kind of water conveyor belt.
Once warmer water reaches the area around the UK, it cools and sinks to the bottom of the Labrador and Nordic Seas. Then that cold water makes a U-turn and snakes its way along the ocean floor, all the way down to Antarctica's Southern Ocean.
When the AMOC is flowing quickly, western Europe enjoys a wet and warm climate. But if it gets sluggish and weak, warm tropical waters don't get moved up, and the north Atlantic cools.
The AMOC's speed depends on a delicate balance of salt and fresh water. Salty water is dense, so it sinks easily. But as Greenland and Antarctica's ice sheets melt, along with glaciers around the world, more fresh water is joining the AMOC.
That melting is happening quickly, the UN report confirmed. Greenland's ice is melting six times faster now than it was four decades ago, according to an April study— the ice sheet is sloughing off an estimated 286 billion tons of ice per year. Two decades ago, the annual average was just 50 billion.
In Antarctica, meanwhile, the entire ice sheet is melting nearly six times as fast as it did 40 years ago. In the 1980s, Antarctica lost 40 billion tons of ice annually. In the last decade, that number jumped to an average of 252 billion tons per year.
The addition of all that fresh water makes the salty surface water lighter and less likely to sink, clogging up the circulation's flow and weakening the AMOC.
A weaker AMOC could mean more extreme weather
The authors of the UN report, which focuses on the state of the world's oceans and cryosphere (the frozen parts of the planet) say they're somewhat confident that the AMOC has already weakened relative to the period from 1850-1900.
That weakening is likely to cause changes in our global climate, according to the report, including more storms in northern Europe as well as a decrease in the amount of organic matter produced and circulated in the north Atlantic's marine food web.
It would also lead to less summer rainfall in southern Asian and the central African Sahel, so parts of central and west Africa could experience more drought conditions.
"We'll see more extreme weather patterns for sure," Muschitiello said. "Europe will get colder and drier in the long run. There will be surplus of heat in subtropics, which is important for hurricane formation."
When subtropical waters are warmer, that contributes to more frequent and intense hurricanes in the Atlantic, since warm air holds more water vapor — and that additional moisture provides fuel for hurricanes.
A weakening in the AMOC would also cause sea levels to rise along along the northeast coast of North America.
Could the AMOC ever stop completely?
In the 2004 film "The Day After Tomorrow," the AMOC stops almost overnight and an ice age descends on Europe and North America. People freeze to death in the streets, helicopters fall out of the sky, and a massive tidal wave engulfs New York City.
Those effects are not scientifically accurate and were hyperbolized in the film, of course, but the idea that Atlantic water circulation could shut down isn't wholly outside the realm of possibility. It's just highly unlikely.
According to Muschitiello, there are "reconstructions that suggest that the AMOC stopped entirely in the past, and that these major distortions of the AMOC led to the coldest events ever recorded."
But such an occurrence would require a huge amount of melt water — the past distortions Muschitiello is referring to happened after large swarms of icebergs broke off glaciers and floated into the north Atlantic ocean. When those iceberg armadas melted, that added an excess of fresh water into the ocean, wreaking havoc on the AMOC.
However, that influx of fresh water was several orders of magnitude higher than today's melting rates.
A 2017 study found that if the concentration of carbon dioxide in the atmosphere instantaneously doubled from the 1990 level, the AMOC could collapse in 300 years. Greenhouse-gas emissions are not likely to increase that dramatically, though.
The authors of the UN report also concluded that "a collapse is very unlikely." They noted, though, that it would be even more unlikely in a future with less greenhouse-gas emissions.