The Atlantic Meridional Overturning Circulation is the conveyor belt that transports warm water from the tropics northward and cold water south, balancing the heat distribution in the Northern Hemisphere and allowing for the relatively mild temperatures in Europe. New evidence has emerged this week from the Potsdam Institute providing further evidence that the AMOC is undergoing a loss of dynamic stability and may be on the verge of collapse.
The culprit is climate change, as lighter freshwater from melting land glaciers accumulates on the surface of the ocean, reducing the sinking action that drives this circulatory current.
The new report made headlines, but few seem to be providing the details about what this could mean for our future. The triggers of abrupt climate change in the paleo record remain a mystery, but the leading hypothesis is that these Dansgaard-Oeschger oscillations are related to changes in the AMOC. What seems to happen is that this fresh meltwater from a warm period sitting on the surfaces of the northern oceans causes this current to stall, which make temperatures in the Arctic plummet. Because freshwater freezes at higher temperatures than saltwater, this surface layer leads to an increase in sea ice, increasing the polar albedo, making the temperatures in the Arctic even colder, creating a feedback loop with cooling progressing southward toward the equator.
Another factor at play is the Beaufort Gyre, a clockwise-swirling current in the Arctic Ocean that traps cold freshwater. Historically, the gyre has reversed every 5-8 years or so, releasing that cold down into the North Atlantic, but the reversal is now a full 8 years overdue, and it’s holding more freshwater than all of the Great Lakes. When the reversal finally comes, it will release even more cold freshwater into the North Atlantic, sending Europe into a freeze and adding to the problem of the stalling AMOC. The effect is large enough that researchers have called the Beaufort Gyre a “ticking time bomb“—one that’s still ticking 4 years later.
The most extreme example of what an AMOC collapse can do may be the Younger Dryas, where temperatures rapidly fell 10 °C in Greenland and 5 °C Great Britain. With human-released CO2 tipping the scales in the opposite direction, no one really knows what’s going to happen. But it’s long been my opinion that it was this sudden climactic shift that drove the mammoths and mastodons to extinction, along with 70% of the rest of North American megafauna, and not human predation. The situation reminds me of this 19th century drawing of all the mammoth tusks from Siberia sitting on the trading docks in London. To me, it looks like Hokusai’s The Great Wave.
a frozen ocean