Imagine a world where North America is locked in snow. Winter storms ravage Europe, while Australia suffers from permanent drought. This is not just the plot of Roland Emmerich’s 2004 disaster film “The Day After”. This is also what could happen if a crucial network of currents in the Atlantic Ocean were to close.
Unfortunately, research published August 5, 2021 in Nature Climate Change suggests that these currents are weakening due to man-made climate change. And if nothing is done to prevent it, they can completely collapse.
The pulse of the ocean
The Atlantic Meridional Reversal Circulation, or AMOC, plays an essential role in regulating ocean temperature. It’s made up of a network of currents branching out across the Atlantic Ocean like veins and arteries – some scientists compare it to the pulse of the ocean.
The AMOC works like a perpetually rotating treadmill. As the water warms in the tropics, it “rises” or rises to the surface, becoming more buoyant and less salty. This hot upwell rushes north to the polar regions, where it cools, becoming denser and more salty, before descending again. The cold, dense water is then brought back to the tropics to start the cycle again.
“The crucial ingredient is a positive feedback loop,” says Niklas Boers, researcher at the Potsdam Institute for Climate Impact Research in Germany. The density of the salt water maintains the churning of the AMOC. However, “it also means that when large amounts of fresh water are added to the North Atlantic,” Boers says, “the positive feedback can crumble.”
Such an event is known as a “tipping point”, a kind of ecological threshold which, once crossed, can take centuries or even millennia to stabilize. And the evidence suggests the tipping point is approaching. In a 2019 report, the Intergovernmental Panel on Climate Change (IPCC) estimated that AMOC “very likely” to continue to weaken due to climate change.
AMOC Run Amok
So what would a world with a collapsed AMOC look like?
Given that AMOC has been activated throughout human history, it is difficult to say for sure. But Earth’s ancient past offers some clues.
The last time AMOC closed its doors was during a period known as the Younger Dryas, some 14,500 years ago. The most recent ice age was drawing to a close; As temperatures rose, massive glacial melt from the North American landmass flooded the Atlantic, disrupting the flow of ocean currents. Then something strange happened: With no current to carry tropical waters to higher latitudes, the warming trend of the North Pole reversed. The northern hemisphere was plunged into near-glacial conditions for the next 3,000 years.
If this were to happen again, says Boers, “we will see much cooler temperatures in northern Europe.” Some areas could cool down to 14 degrees Fahrenheit (8 degrees Celsius). Unlike the period of the young Dryas, some of this frost can be offset by global warming trends; however, the collapse of AMOC and a warmer planet would permanently inundate some cities along the eastern seaboard of the United States.
In contrast, the southern hemisphere would experience even more warming, especially around Antarctica. Many parts of Europe would also experience severe flooding and winter storms. “For West Africa,” says Boers, “it has been suggested that a collapse of AMOC could lead to permanent drought conditions.”
So, with that in mind, how close are we to “The Day After Tomorrow”?
A real 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 this region makes this possibility alarming, to say the least.
However, the effects would not be as immediate or severe as the disaster movies show. Many full climate models do not predict a complete closure of the AMOC for 250 to 300 years under current emissions, although they do predict a host of negative consequences as the currents slowly slide toward collapse.
Climate catastrophe is a slow-burning problem, but it is a problem that has a clear solution. “Avoid emitting greenhouse gases,” says Boers. We are living at a turning point in Earth’s history, a time when our species is faced with a choice: we can be either the hero of the film or the disaster.