This topographic map shows the schematic circulation of surface currents (solid curves) and deep currents (dashed curves) of the Atlantic Ocean, which form a portion of the Atlantic Meridional Overturning Circulation (AMOC). The colors of the curves indicate approximate temperatures. Wikimedia Commons (CC BY 3.0)
Imagine a world where North America is locked in snow. Winter storms ravage Europe, while Australia bakes in permanent drought. This isn't just the plot of Roland Emmerich's 2004 disaster flick "The Day After Tomorrow." It's also what could happen if a crucial network of currents in the Atlantic Ocean were to shut down.
Unfortunately, research published Aug. 5, 2021 in Nature Climate Change suggests that these currents are weakening due to manmade climate change. And if nothing is done to prevent it, they may collapse completely.
The Pulse of the Ocean
The Atlantic Meridional Overturning Circulation, or AMOC, plays an essential role in regulating ocean temperatures. It is composed of a network of currents branching throughout the Atlantic Ocean like veins and arteries — some scientists compare it to the ocean's pulse.
The AMOC works like a perpetually turning conveyor belt. As water warms in the tropics, it "upwells," or rises to the surface, becoming more buoyant and less salty. This warm upwell rushes northward towards the polar regions, where it cools, becoming denser and saltier, before sinking once more. The cold, dense water is then whisked back to the tropics to start the cycle anew.
"The crucial ingredient is a positive feedback loop," says Niklas Boers, a researcher at the Potsdam Institute for Climate Impact Research in Germany. Salt water's density keeps the AMOC churning. However, "this also means that when large amounts of freshwater are added to the North Atlantic," Boers says, "the positive feedback might collapse."
Such an event is known as a "tipping point," a kind of ecological threshold that, once crossed, can take centuries or even millennia to stabilize. And evidence suggests that the tipping point is edging closer. In a 2019 report, the Intergovernmental Panel on Climate Change (IPCC) deemed the AMOC "very likely" to continue weakening under climate change.
The AMOC Run Amok
So, what would a world with a collapsed AMOC look like?
Since the AMOC has been "on" for all of recorded human history, it's difficult to say for sure. But Earth's ancient past offers some clues.
The last time the AMOC shut down was during a period known as the Younger Dryas, some 14,500 years ago. The most recent ice age was coming to an end; as temperatures rose, massive glacial melt off the North American landmass flooded into the Atlantic, disrupting the flow of oceanic currents. Then something strange happened: without a current to carry tropical waters to higher latitudes, the north polar warming trend reversed. The Northern Hemisphere was plunged into near-glacial conditions for the next 3,000 years.
If this were to happen again, Boers says, "we'll see much cooler temperatures in northern Europe." Certain regions could cool by as much as 14 degrees Fahrenheit (8 degrees Celsius). Unlike the Younger Dryas period, some of this freezing may be offset by global warming trends; however, AMOC collapse plus a warmer planet would permanently flood some cities along the United States' eastern seaboard.
In contrast, the Southern Hemisphere would experience even more warming, particularly around Antarctica. Many parts of Europe would also experience severe flooding and winter storms. "For Western Africa," Boers says, "it has been suggested that an AMOC collapse could lead to permanent drought conditions."
So, with that in mind, how close are we to "The Day After Tomorrow"?
A Real-life Disaster Movie?
Theoretically, the AMOC could be shut down by another massive influx of freshwater from a melted glacier the size of, say, Greenland. And the accelerated rate of glacial melt in that region does make this possibility alarming, to say the least.
That said, the effects wouldn't be as immediate or severe as disaster movies portray. Many comprehensive climate models don't predict a total AMOC shut down for another 250-300 years under current emissions, though they predict a bevy of negative consequences as the currents slowly slide toward collapse.
Climate catastrophe is a slow-burn issue — but it's one that has a clear solution. "Avoid emitting greenhouse gases," says Boers. We live at a tipping point in Earth's history, one where our species faces a choice: We can either be the hero of the movie, or the disaster.