The Atlantic Ocean’s “conveyor belt,” a powerful current that drags warm water north before submerging it in the North Atlantic, has been humankind’s constant companion. For 8000 years, it has held steady, nourishing Western Europe with tropical warmth. But a new study of the current’s strength over the past half-million years suggests global warming may not shut down the current any time soon, as some scientists fear. Instead, it could trigger a replay of ancient events, when multiple bouts of warming caused rapid, centurylong swings in the current’s strength, sowing climate chaos that may have alternately chilled and warmed Europe.
“A strong circulation can also be a highly variable one. [That] might be the most important lesson,” says Ulysses Ninnemann, a paleoclimate scientist at the University of Bergen and a co-author on the new paper.
The Atlantic conveyor runs on salt. First, the Gulf Stream and similar currents bring salty, warm water thousands of kilometers north to the seas around Greenland and Iceland, where it cools and sinks to the sea floor. There, it slowly migrates south through the abyssal depths. The currents not only play a huge role in Europe’s climate, but they also help the oceans sequester much of the heat that humans have trapped in the atmosphere by dumping greenhouse gases into it.
However, buoyant freshwater can stall this engine by diluting the heavy saltwater, limiting how much dives down in the North Atlantic. That’s almost certainly what happened toward the end of previous ice ages, when the kilometers-thick ice sheets covering North America melted into the North Atlantic. But in the warm periods between ice ages, known as interglacials, scientists assume the Atlantic circulation is stable.
To find out whether this is truly the case, Eirik Vinje Galaasen, a paleoclimatologist at the University of Bergen, and his colleagues examined a 250-meter-long core of seafloor clay previously drilled off the southern tip of Greenland, at a spot known to capture sediments pulled down by plunging surface waters. Throughout the layers of dirt, representing 500,000 years of history, were the tiny shells of single-celled organisms known as foraminifera. Galaasen and colleagues flushed the foraminifera fossils from the different layers of mud and analyzed their chemistry to see what they said about the Atlantic circulation. “It was a stupid amount of work, years in the lab, to dig through mud at this level of detail,” he says.
Each fossil contained an isotopic fingerprint of the surrounding water’s travel history, integrated into their shells. There are only two spots on the planet where water descends from the surface to the ocean bottom: the Southern Ocean and North Atlantic. Waters that descended off nutrient-rich Antarctica had more carbon-12 than carbon-13, whereas those from the nutrient-poor North Atlantic have the opposite pattern. By examining the ratio of the carbon isotopes over time, Galaasen could determine when the current was strong, drawing down the North Atlantic, and when it was weak, allowing southern waters to dominate.
The fossils revealed that the strength of the Atlantic circulation dropped sharply before rebounding during periods of peak warming in three recent interglacials, they report today in Science. Those fluctuations, which occurred about 423,000, 335,000, and 245,000 years ago, sometimes lasted only 100 years. Although the team did not model how these swings would have changed the climate, the effects would likely have been “catastrophic,” says Guido Vettoretti, a climate scientist at the University of Copenhagen who was not involved in the study. Other models suggest circulation slowdowns severely cool northern Europe and dry out southern Europe.
In the samples, the slowdowns were often accompanied by iceberg-born debris—a sign that meltwater from the Greenland Ice Sheet could have caused this sputtering. The debris suggests Greenland’s fate today not only affects sea level rise; it could also modulate the climate. “The Greenland Ice Sheet may be extremely important to the stability of our climate system,” Vettoretti says.
How resilient is the Atlantic current today? Modern-day studies are limited in what they can say. Two decades of monitoring, for example, have revealed short-term swings in strength, but it is difficult to tease out a long-term pattern—or to know whether human warming is affecting the current. The new study could make such work even more difficult, complicating forecasts of how the circulation might change in the future, Ninnemann says. He adds that models should incorporate the possibility that global warming could cause the strength of the circulation to fall and quickly rebound.
What’s needed now, Ninnemann says, is continued observation of today’s current, along with a close study of what the ancient world looked like when it grew erratic. But such efforts have run up against funding and logistical difficulties, especially now with the coronavirus pandemic. Just this month, for example, the United Kingdom cut short a cruise that would have recovered moorings from an array observing the current. And although the program had funding to put out new arrays, it does not currently have money to recover them. “We’re running as close to the wire as we ever have,” Eleanor Frajka-Williams, the array’s chief scientist at the United Kingdom’s National Oceanography Centre, said in an interview prior to the cruise’s departure.
In some ways, it might seem like good news that the circulation can decline and rebound, rather than simply declining, or worse, shutting down completely. But Ninnemann notes that human systems of agriculture, trade, and settlement were not designed to cope with such fluctuations. “We have built everything we have in this relatively stable climate period [of the past 10,000 years],” Hinnemann explains. “But the geologic record shows us this may be an exception, rather than the rule.”