The ocean’s dominant feature, extending up to 2 miles deep and as much as 1,200 miles wide, is the Antarctic Circumpolar Current, by far the largest current in the world.
It is the world’s climate engine, and it has kept the world from warming even more by drawing deep water from the Atlantic, Pacific and Indian oceans, and pulling it to the surface. There, it exchanges heat and carbon dioxide with the atmosphere before being dispatched again on its eternal round trip.
Without this action, which scientists call upwelling, the world would be even hotter.
“From no perspective is there any place more important than the Southern Ocean,” said Joellen L Russell, an oceanographer at the University of Arizona. “There’s nothing like it on planet Earth.”
For centuries this ocean was largely unknown.
But more recently, a new generation of floating, autonomous probes that can collect temperature, density and other data for years — diving deep underwater, and even exploring beneath the Antarctic sea ice, before rising to the surface to phone home — has enabled scientists to learn much more.
They have discovered that global warming is affecting the Antarctic current in complex ways, and these shifts could complicate the ability to fight climate change in the future.
As the world warms, Russell and others say, the unceasing winds that drive the upwelling are getting stronger. That could have the effect of releasing more carbon dioxide into the atmosphere, by bringing to the surface more of the deep water that has held this carbon locked away for centuries.
In addition, the Southern Ocean is getting warmer, and that has another important climate effect. Some of this upwelling water, which is already relatively warm, flows beneath ice shelves on the Antarctic coast that help keep the continent’s vast, thick ice sheets from reaching the sea more quickly.
While the potential magnitude of all these effects remains unclear, oceanographers and climate scientists say that it is increasingly urgent to understand this interplay of powerful forces and how human activity is transforming them.
“There’s lots of questions left,” said Lynne Talley, an oceanographer at Scripps Institution of Oceanography in La Jolla, California.
Much of humanity’s limited scientific understanding of the Southern Ocean was long linked to an industry that saw money to be made there: whaling.
Beginning in the late 19th century, whaling ships began heading southward, to the Antarctic, in growing numbers as whale populations in the more hospitable waters of the Atlantic and Pacific oceans declined from overhunting.
But in time, overhunting became a problem in the Southern Ocean as well. And the British government decided more needed to be learned about the environment and behavior of the whales there in hopes of sustaining their numbers.
Which is why, in the late 1920s, George Deacon, a young London university graduate, received an intriguing job offer: sampling the waters of the Southern Ocean as part of an expedition to help preserve the whaling industry.
He spent the better part of the next decade aboard ships, analysing water samples from various depths. It could be dangerous work.
But Deacon overcame these obstacles, ultimately sampling enough of the ocean to gain a broad understanding of its mechanics. He combined his ideas with those of others in a 1937 book, “The Hydrology of the Southern Ocean,” that became the standard textbook describing the waters around Antarctica.
Around the 1950s, though, research efforts expanded. And by the late 1970s, polar-orbiting satellites began gathering data as well.
SENSORS AND SATELLITES
But the real revolution in Southern Ocean science began in the mid-2000s, with the use of drifting floats that can adjust their buoyancy, like fish, to move up and down in the water as they take readings.
The floats, part of a worldwide project called Argo, have helped transform oceanographers’ understanding of the Southern Ocean.
Oceanographers now know much more about the complex cycle of worldwide oceanic currents, of which the Antarctic upwelling is only a part. The waters circling Antarctica are completing an epic journey from the Atlantic, Pacific and Indian oceans, flowing southward and slowly cycling upward as if climbing an ocean-sized circular staircase.
Scientists better understand how closely intertwined the Southern Ocean is, despite its remoteness, with the rest of the world. The circular flow of water around Antarctica is, in effect, a climate engine spinning on a continental scale.
With this new knowledge, researchers are now growing increasingly alarmed about how the ocean and current may change as the Earth continues to warm.
‘THE ROT OF AGES’
One of the most important processes that occurs in the Southern Ocean is the exchange of carbon dioxide between the ocean and the atmosphere. And how this process may change as the world warms has huge implications for fighting climate change.
Global warming is mainly caused by carbon dioxide put into the atmosphere by the burning of fossil fuels. Oceans absorb large amounts of these emissions, while also absorbing heat from the atmosphere, serving as a critical buffer against climate change and keeping the world from otherwise becoming a practically unlivable hothouse.
By some estimates the oceans have taken up about 25% of the excess carbon dioxide, and more than 90% of the excess heat, that has resulted from burning of fossil fuels and other human activities since the 19th century. But the deep ocean water that upwells around Antarctica contains even more carbon dioxide — not from current emissions, but dissolved over centuries from organic matter including decaying marine organisms, tiny and immense, that sink when they die.
“It’s been accumulating the rot of ages,” Russell said.
When this ancient water reaches the surface, some of that carbon dioxide is released, or “outgassed,” as the scientists say.
Researchers have long thought that the Southern Ocean absorbs more carbon dioxide than it releases, with a beneficial effect for climate. But if more water upwells, more of this carbon dioxide could be outgassed, shifting this critical balance.
Upwelling is driven by those incessant Southern Ocean winds, which push surface water northward, drawing up deep water behind it. The winds are affected by warming, and they have already strengthened in recent decades.
A recent study suggested that the Southern Ocean is still absorbing more carbon dioxide than it is releasing. But many researchers think the ocean may already be outgassing more carbon dioxide than previously thought. And if the winds keep strengthening as the world warms, they say, the upwelling and outgassing could keep increasing.
UNDERNEATH THE ICE
Carbon, however, isn’t the only concern. The water that’s welling up in the Southern Ocean is also relatively warm, and warming more, which spells trouble for the planet in the form of sea level rise.
Some of that warm water reaches Antarctica’s continental shelf, where it flows beneath ice shelves, the tongues of ice at the ends of glaciers. These glaciers act as buttresses, helping to hold back the massive ice sheets that cover the continent and that are slowly moving toward the ocean.
But scientists discovered several decades ago that this upwelling water is melting the ice shelves from underneath. As the ice thins, the glaciers lose some of their ability to keep the ice sheets in check.
So far, their melting and thinning has contributed only a relatively small amount to rising sea levels. But the concern is that if the ice shelves melt too much, they could collapse, accelerating the movement of the glaciers, and eventually much of the West Antarctic ice sheet, to the ocean.
Today, scientists are on the brink of getting even more data. The Argo program is about to deploy globally a new generation of more sophisticated floats capable of measuring much more than basic temperature and salinity.
Despite all that has been learned, Russell said, “Unlike any other field of exploration, we are at the absolute frontier here.”
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