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Climate change may shut down the North Atlantic warm keeping current

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Both Europe and North America are warmed in the winter by currents circulating in the Atlantic — but climate change threatens this source of warmth, reported The Verge.

If the Earth warms too much, it’s possible that this current could collapse entirely, new research says. That would mean frigid winters for countries along the North Atlantic, expansion of the sea ice in the Greenland, Iceland, and Norwegian seas, and a shift in rainfall across the world.

The current is called the Atlantic Meridional Overturning Circulation (AMOC) and it’s like a conveyer belt that brings warm water from the tropics to the cooler reaches of the North Atlantic. There, the water loses its heat to the atmosphere. Because water gets denser as it gets colder, it sinks. This lower band of cool water circulates back to the tropics where it warms and repeats the process all over again. If you’ve ever watched soup boil up, spread, and then sink, it works kind of like that.

“It is a major player in the climate system, important for Europe and North America. So it’s a big deal,” says Tom Delworth, a scientist with National Oceanic and Atmospheric Administration who was not involved in this recent study. The AMOC is the same current that collapsed and triggered an icy apocalypse in the 2004 movie The Day After Tomorrow (featuring Verge favorite Jake Gyllenhaal). “It wasn’t quite accurate in the science, but that’s okay, it was entertaining,” Delworth says.

It might not have been accurate, but it wasn’t complete fantasy either. There are signs in the paleoclimate record that melting ice sheets may have once slowed or stopped the AMOC for decades at a time — triggering massive shifts in monsoon rainfall in Africa and India, changes in hurricane patterns, and even mini ice ages.

Most climate projections assume that the AMOC might weaken, but would still persist even as global temperatures creep steadily upward. But climate scientist Wei Liu at Yale University suspects that these models overestimate the AMOC’s stability, according to a new study recently published in the journal Science Advances.

Liu’s work suggests that most models don’t accurately reflect how much freshwater (which, in the ocean, just means slightly less salty water) travels in and out of the Atlantic. Freshwater is hard on the circulation because it’s not as dense as saltier water, so it doesn’t sink very well. Too much of it, and you lose the churn that keeps the AMOC going. “So it’s about how well models represent the movement of freshwater out of the Atlantic,” Delworth says. “Models don’t represent that very well, they may be missing a potentially important positive feedback.”

Basically, according to Liu’s work, the AMOC is a camel, and it’s already carrying a boatload of straws that current climate models don’t consider. Global warming could be the straw that breaks the metaphorical AMOC-camel’s back. That’s because if the air is too warm in the North Atlantic, the water in the AMOC won’t be able to cool down by transferring its heat to the atmosphere, so it won’t sink and circulate back to the tropics like it does now. In fact, Liu calculated that if atmospheric carbon dioxide increases to 710 parts per million it could be enough of a blow to collapse the AMOC within 300 years of that spike. For comparison, last week’s levels came in at 405 parts per million. That’s up from 355 ppm in 1990.

If the AMOC does collapse, winters in the North Atlantic will be frigid — they could drop as much as 7 degrees Celsius in the wintertime. “That’s a huge cooling,” Liu says.

His findings echo other scientists’ recent results that suggest that if carbon emissions continue their steady climb, the AMOC has a 44 percent chance of collapsing as soon as the year 2300, Hakai Magazine reports.

“I think it’s very, very important work,” Delworth says. “It identifies a clear weakness in our models, and it identifies a way in which that weakness may translate into a systematic underestimation of the risk of future climate change.”

Like all models, climate projections are still evolving as the field develops and new data comes in. But one thing’s for certain: if we don’t curb our carbon-emitting ways, we’ll be in for some nasty weather.

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