Major Ocean Current Tells Cautionary Climate Tale
A visual representation of the Antarctic Circumpolar Current. Illustration by Nicole Kurtz, courtesy Frank Lamy.
New sediment samples reveal the history of saltwater circulation in the Southern Ocean — and what the future of global warming has in store.
For the Antarctic Circumpolar Current, the biggest ocean current on the planet, looking to past behavior is the best way to predict future behavior.
The ACC, located around Antarctica, historically strengthened when temperatures rose — and that could spell serious trouble in today's warming world. With findings published this past March in the journal Nature, an international team of researchers utilized oceanic sediment sampling to unearth the geologic past of the ACC and determine what might become of it as climate change continues.
“The ACC is actually the only current that goes to thousands of meters of depth,” said Frank Lamy, the study’s lead author and senior research scientist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, in Germany. “The particles that are there on the floor get sorted in a special way, and that’s what we used.”
Bringing together the Pacific, Atlantic and Indian Oceans, the ACC operates on a worldwide scale, as it connects shallow and deeper oceanic waters, influences carbon dioxide changes in the atmosphere and affects the carbon cycle in the Southern Ocean. But global warming could make the current stronger.
Increased atmospheric and sea surface temperatures add to the strength of the ACC, which would in turn strengthen the Southern Westerly Winds that partially control the current through the forces of wind stress. As it relates to the ocean’s role as a carbon sink, the ACC continues to accelerate under climate change.
“When [the ACC] was more sluggish, you probably had more carbon dioxide that had been stored, independent of any anthropogenic influence,” Lamy said. “When it was stronger, the outgassing of natural carbon dioxide was higher.”
According to Southern Ocean expert and Submarine Scientific partner Veronica Tamsitt, who wasn’t involved in the research, the position of the ACC also matters, on top of its strength. For continents in the Southern Hemisphere, the current impacts weather patterns, from rainfall to drought. “There are also connections between what's happening in the Southern Ocean circulation and El Niño in the Pacific Ocean,” said Tamsitt. “This is what we call teleconnections — where these anomalies in one place can affect somewhere else.”
Dating as far back as 5.3 million years ago, the climate largely influenced the strength and weakness of the current. During glacial periods, otherwise known as ice ages, the ACC had a reduced flow, but the opposite effect occurred throughout warmer periods without large ice sheets — in general, the current became stronger in these so-called interglacial times.
“We know something about what the climate was doing, and now we know something about what the current was doing,” said Tamsitt. “That helps fit into that broader research on what controls the circulation and come up with hypotheses of what might happen.”
To collect data for the study, scientists spent two months in the rough, remote seas of the central South Pacific Ocean, drilling 3,600 meters deep at two sites to gather sediment samples. “The weather was bad, but there were some windows in between. Of course, it was great at the end, and we had very nice sediments,” said Lamy.
Back on land, specialized x-ray scanning technology at the Alfred Wegener Institute and Texas A&M University in the U.S. correlated the samples, but data did not lie in the sediment alone. After researchers washed and dried the bulk sediments, they used a wet brush to pick out samples of the benthic foraminifera — bottom-dwelling microorganisms — found within the samples. The chemicals in the remains of foraminifera shells serve as a climate proxy — essentially a form of preserved organic data, similar to tree rings on land.
Despite the ACC’s key role in telling the story of the climate's past, another vital oceanic current system has been under the spotlight as of late. A paper published earlier this year in Science Advances found that the Atlantic meridional overturning circulation could reach its tipping point sooner than expected, resulting in disastrous climate impacts across the world.
Still, Tamsitt emphasized that although experts can more easily access the North Atlantic Ocean, compared to the Southern Ocean, the ACC deserves a closer look. “We understand the North Atlantic a lot better than we understand the Southern Ocean, though in that sense, I think there's a little bit of urgency to pay more attention to what's happening, given the potential of Antarctic ice melt to contribute such a huge amount to sea level rise,” she said.
Indeed, the study authors did find a correlation between the strength of the ACC and the West Antarctic Ice Sheet. In phases when the current weakened, the ice sheet advanced, but in times when it strengthened, the ice sheet retreated — further assisted by the southward transfer of heat and upwelling of water from the deep.
The research team's reconstructions suggest that either the retreat or full collapse of the ice sheet might be connected to the ACC's flow, and if the current does collapse, which could be unavoidable at this point, it may account for a sea level rise of 10 feet.
But no matter what’s in store for the ACC, the study indicates climate change will play a significant role in its future, and the knowledge we have about what might happen next remains just as strong as the current itself. “We’re trying to understand the past and give some ideas about what our Earth naturally can do,” said Lamy. “It’s important to know, if possible.”
An aerial drone view of the RV Joides Resolution, the drill ship Lamy and his scientific colleagues used to sail and drill for sediment in the South Pacific Ocean. Photo by Brian Cimagala, courtesy Frank Lamy.