New research shows that present-day iceberg loss from Greenland stacks up to some of the most dramatic iceberg-slinging episodes in recent geological history. Such events involved the disintegration of an ice sheet over North America and coincided with the weakening or failure of vital ocean currents in the North Atlantic—as well as severe climate swings.
Despite this concerning parallel, there’s reason to think that modern iceberg loss from Greenland won’t disrupt ocean circulation within the next few decades, according to authors of the study, published in Science.
“Our study is a relatively rare piece of good news for the climate,” said paleoclimatologist Yuxin Zhou of the University of California, Santa Barbara. “We can still make a difference by acting” to combat ice loss.
Icebergs and the AMOC
The Atlantic Meridional Overturning Circulation, or AMOC, flows northward along the east coast of North America before veering east toward Scandinavia. Along the way, evaporation leaves behind water that’s increasingly cool and salty and therefore dense. In the Arctic, this dense, cold water sinks to join deepwater currents headed south to the Antarctic.
Shutting down the AMOC would wreak climate chaos. Without the current to transport heat, the tropics could boil, whereas temperate western Europe could start to look more like Siberia or northern Canada. Sea level would rise along the U.S. East Coast, and rain belts that sustain billions of people would reshuffle or disappear.
“If a sufficient amount of fresh water is added to the northern Atlantic, the AMOC could collapse.”
Such a scenario isn’t mere speculation: The AMOC has shut down multiple times, most recently during a period called the Younger Dryas that began some 13,000 years ago. This cold snap came right at the tail end of the last glacial period. For 1,300 years, it returned the defrosting world to a climate echoing that of the Last Glacial Maximum.
The causes of historical AMOC shutdowns are frustratingly mysterious, but the collapse of ice shelves is one of the prime suspects. Continental ice sheets such as the one covering Greenland are made of frozen fresh water, which is less dense than seawater. When this ice melts into the North Atlantic, it makes it harder for water in the current to sink, clogging the AMOC. Icebergs are especially problematic because they ferry fresh water far out to sea, where it can wreak the most havoc.
“If a sufficient amount of fresh water is added to the northern Atlantic, the AMOC could collapse,” said climate physicist Peter Ditlevsen of the University of Copenhagen. “The big research question for the present climate situation is, How much fresh water is needed for a shutdown?”
Ice-Rafted Debris
One way researchers have tried to answer this question is by using climate models. But most models simulate a climate that’s quite stable, requiring an “unrealistically high” amount of melt to shut down circulation, said Ditlevsen. That’s beginning to change, though, he added, as models begin to capture some of the real climate’s sensitivity to tipping points and drastic swings. Last year, Ditlevsen used modeling and statistical analysis of early-warning signals observed in AMOC to predict an imminent shutdown.
Zhou and his colleagues took a different approach, focusing on how much icebergs affect the current. “We went pretty far back in time to actually look at the periods when the AMOC was in severe decline,” Zhou said.
Counting long-melted icebergs is, of course, tricky business. So the researchers had to get creative. Icebergs release rocky debris when they melt, and researchers can determine how much of this debris was making its way into the ocean by measuring thorium and uranium isotopes in sediment cores.
The radioactive element uranium dissolves easily in water and is distributed very evenly throughout the ocean. It decays into thorium at a steady rate.
But unlike uranium, thorium prefers to be in solid form. It rapidly attaches itself to anything solid that happens to drift by—like a piece of debris raining out of an iceberg—and accumulates in sediments as quickly as it is produced. Thorium is produced at a constant rate via radioactive decay. So finding less of it than usual on the seafloor hints that lots of extra sediment was dropped into the ocean in a short amount of time, effectively diluting the constant flux of thorium.
By describing icebergs and the amount of debris they carry in a few simple equations, the team could back-calculate the number of icebergs launched into the Atlantic in the past.
“I like the fact that it’s a relatively simple model,” Ditlevsen said. It’s not a complicated, computational black box like many models are.
An Imperfect Analogue
Zhou and his colleagues focused on the most dramatic iceberg discharges of the last glacial period—Heinrich events—which occurred when North America’s now vanished Laurentide Ice Sheet launched veritable flotillas of icebergs all the way to Europe. Heinrich events happened during cold periods when the AMOC ground to a halt.
The new data suggest that modern iceberg loss from Greenland is comparable to such an event.
However, Zhou said this doesn’t mean the AMOC is headed for collapse—in fact, the new analysis suggests the opposite.
Heinrich events happened when the AMOC was already weak. Today’s AMOC is relatively strong but is expected to slow down as the planet warms, dwindling by about 20%–40% by the end of the century, which could make the current vulnerable to a shutdown. But by the time that happens, Greenland will have started to run out of icebergs, Zhou said. To put that in perspective, Heinrich events launched armadas of icebergs for 200 years on average.
“The past is never the perfect analogue of the future. But we can learn a lot from the past.”
The new findings hint that icebergs won’t disrupt the AMOC within the next few decades, Zhou said. But the more distant future is less certain—though its supply of icebergs will dwindle, the Greenland Ice Sheet will release more and more fresh water from land as the climate warms. This input of fresh water isn’t as disruptive as icebergs, but it can still weaken the AMOC.
Ditlevsen voiced caution about using Heinrich events as a yardstick for threats to the AMOC because Heinrich events happened during cold periods and not before them. Icebergs could have contributed to shutdowns, but they probably didn’t initiate them, he said. Some of the uranium-thorium data used in the new iceberg reconstructions also have large uncertainties.
But when it comes to understanding the ancient climate, caveats like that are nothing unusual, Ditlevsen said; being certain about the distant past is a hard problem to overcome. In this field, consensus builds over decades, not all at once.
“The past is never the perfect analogue of the future,” Zhou said. “But we can learn a lot from the past.”
—Elise Cutts (@elisecutts), Science Writer
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