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"?