The circulation is driven by the density of ocean waters in the North Atlantic – and that, in turn, means that it depends on both how cold and how salty they are. Cold salty water is very dense so it sinks in the North Atlantic and travels southward, becoming “deep water.” Meanwhile, at the surface, warmer water is pulled northward. The overall system thus leads to a considerable northward transport of warmth, but too much freshening in the North Atlantic could slow or shut down the circulation by reducing surface water density.
Using climate model simulations to mimic both conditions of global warming and an AMOC collapse, Drijfhout added a gigantic, continuous pulse of freshwater in the North Atlantic between the years 2000 and 2100. The result was striking – it was possible, at least temporarily, to shut down not only the circulation but the global warming trend.
For the first 11 years, globally averaged temperatures plunged by over 1 degree Fahrenheit, roughly comparable to the warming that occurred from 1880 up through the present, but a more rapid move in the opposite direction.
“This would affect hundreds of millions of people,” Drijfhout said, “At least temporarily, Europe would suffer conditions that would look like the ‘Little Ice Age’ of the Middle Ages.” In the Little Ice Age, temperatures dropped by roughly a degree Fahrenheit in the Northern Hemisphere.
The effect wasn’t because the factors driving global warming suddenly stopped – rather, it was due to the rapidity of the change brought on by the oceans. “The increase in greenhouse gases is too slow to compete with the response of an abrupt shutdown of the AMOC, which is reflected in the temperature response of the planet as a whole,” the study found. Europe, in particular, would see significantly colder temperatures in this scenario.
But only temporarily. The study found that after 15 years temperatures would start rising again, and then resume their upward trajectory – albeit with a considerable delay in comparison to a world in which the AMOC did not shut down.
The research mirrors scientists’ attempt to understand well documented temperature cycles within glacial periods, which were often punctuated by shifts from cold to warm and back again, a phenomenon that researchers think was also due to changes in the oceans. The difference now, however, is that there is an entirely new phenomenon added to the mix – all the greenhouse gases that humans have put in the atmosphere.
Thus, if the AMOC does shut down thanks to us, the overall consequences would be quite different than in the planet’s past. But the new research reaffirms that the AMOC is an incredibly powerful Earth system, and one that certainly should not be messed with. Or else, yes … global warming could indeed potentially cause short term global cooling.
Correction: A previous version of this article misinterpreted aspects of the study. The story has been corrected and substantially revised.