Scientists have been warning for some time now that changes in the water temperature of the Atlantic Ocean could lead to the collapse of an enormous conveyor belt system known as the Atlantic Meridional Overturning Circulation, or AMOC, which includes the Gulf Stream. In his book titled Our Fragile Moment, author Michael Mann describes the AMOC as a “ribbon-like current system… driven by the sinking of cold, salty water in Baffin Bay, nestled between Greenland and northern Canada, and the Labrador and Norwegian-Greenland Seas.” This system “delivers warm subtropical waters to the high latitudes of the North Atlantic,” which ensure that the North Atlantic and its neighboring landmasses in North America and Europe are “warmer than they would otherwise be.” Mann explains that the AMOC “is tied to convective overturning, wherein cold, nutrient-and oxygen-rich waters from below mix into the upper ocean where they are otherwise constantly depleted by marine biota.”

According to Niklas Boers of the Potsdam Institute for Climate Impact Research outside Berlin, “The Atlantic Meridional Overturning Circulation really is one of our planet’s key circulation systems.” Boers is a foremost expert on the AMOC, “a crucial conveyor belt for ocean water and air, which influences weather,” reports USA Today. “Warm, salty water moves north from the tropics along the Gulf Stream off the U.S. East Coast to the North Atlantic, where it cools, sinks and heads south. The faster it moves, the more water is turned over from warm surface to cool depths.” 

But several recent studies have further confirmed that the AMOC could be in trouble. Based on a study from the University of Miami titled “Meridionally consistent decline in the observed western boundary contribution to the Atlantic Meridional Overturning Circulation” that appeared in Science Advances in April of this year, co-author Shane Elipot warns that “observations at the western boundary, in isolation from the eastern boundary, constitute the canary in a coal mine for the tendency of the AMOC.” Elipot told USA Today that “these findings support the evidence of a broader weakening of the AMOC.”  Another study by a group of European scientists that also appeared this past April in Science Advances titled “Observational constraints project a ~50% AMOC weakening by the end of this century,” finds that “most climate models underestimate its decline,” according to CNN. “The AMOC is on course to slow by more than 50% by the end of the century, a ‘substantial weakening’ that’s 60% stronger than that estimated by the average of all climate models.” According to Elipot, “A substantial weakening of the AMOC would impact natural and human systems globally, most notably leading to cooler temperatures over the North Atlantic Ocean, more winter storms in Europe, and a reduction in Sahelian and South Asian summer rainfall.”

Mann, whose investigations of global mean temperatures in the Northern Hemisphere gave rise to the famous “Hockey Stick” graph that compellingly depicted human-driven heating of the Earth’s atmosphere over the past two centuries, contends that the imminent collapse of the AMOC could be due to “a sort of ‘glitch’ in the climate system that can occur when a large amount of freshwater is suddenly released into the oceans.” His book analyzes a variety of “paleoclimate” data points from the Earth’s deep past, including “marine sediment silt data and planktic and benthic foram proxy data” that extend back about 1,600 years. The data can be used to reconstruct previous shifts in the ocean’s temperature that resulted in huge impacts on the Earth’s climate. One such shift occurred in the North Atlantic about 12,900 years ago and is known as the Younger Dryas. The name “younger” refers to “the more recent (and more pronounced) of two cold events that occurred toward the end of the last ice age” and “dryas” refers to a tundra wildflower (Dryas octopetala) “that is prevalent in high-latitude lake sediments dating to this time.” The Younger Dryas is believed to have been triggered by “rapid melting of the expansive Laurentide Ice Sheet, a continental-sized glacier that covered the upper half of North America, its southern edge reaching as far south as Chicago and New York City,” writes Mann. This rapid ice melt “led to a series of massive freshwater pulses exiting to the North Atlantic Ocean” which has led some to even speculate “that the flood myths found among various cultures … could have their origins in one of these meltwater events.” 

Mann goes on to explain that the “massive release of glacial melt water would have dramatically freshened the surface of the northern region of the North Atlantic Ocean.” This freshwater, which is “lighter than saltwater and wants to remain on top,” writes Mann, would have “halted the sinking of surface waters” that drives the Atlantic’s “conveyor belt of ocean circulation,” shutting down the warm current and cooling eastern North America and western Europe down to “almost … ice age-like conditions.” While a return to the Ice Age is unlikely anytime soon, Mann argues that the heating of the Earth’s atmosphere driven by greenhouse gas emissions from the burning of fossil fuels is similarly triggering the release of glacial melt waters from the Greenland Ice Sheet that “appears to be exceeding past model predictions.” Mann cautions that “the collapse of the ocean conveyor is … an example of a tipping point element in the climate system. Once it happens, there may be no bringing it back, at least on societal timescales.” As Mann sagely points out: “Tipping points force us to confront the tenuousness of climate stability, for we do not know precisely where they lie.” 

In explaining the connection between glacial meltwater and the collapse of the AMOC, Mann cites the work of Mikhail Budyko, the founder of modern climatology. Budyko’s work in the 1960s focused on polar amplification, the phenomenon whereby shifts in the Earth’s net radiation balance — through the intensification of the greenhouse effect in the planet’s atmosphere, for example — tend to result in larger temperature changes near the poles as compared to the planetary average. Budyko found that these changes would lead to polar sea-ice decline, unleashing the potential for runaway feedback loops within the climate system. According to Mann, Budyko recognized the “possibility of runaway ice melt … where warming begets ice melt, which begets more warming.” Among many of his predictions from as early as 1972, Budyko calculated that “fossil fuel burning would melt about half the Arctic ice cap by 2020,” writes Mann. “The prediction was spot on. He also predicted … an ice-free Arctic by 2050.” 

It’s too late to stop the melting that is already underway. But Mann argues that the complete disappearance of the Arctic ice cap “can still be avoided if we take dramatic measures to reduce carbon emissions … and preserve at least some of that Arctic ice.” Otherwise, we risk triggering an irreversible feedback loop: massive, sustained pulses of cold water into the North Atlantic Ocean, dilution of the warm waters in that same ocean, and destruction of the AMOC and the climate system it sustains. The result? “[M]assive sea level rise and coastal inundation … withering heat waves and droughts, unprecedented floods, and deadly superstorms.” As terrible as this prospect seems, warns Mann, it’s “where we’re currently headed in the absence of concerted action.”