The Southern Ocean may be less of a carbon sink than we thought

The vast stretch of icy water that separates Antarctica from other continents is a dark mystery to most people. Polar explorer Ernest Shackleton, one of the few who have been to the Southern Ocean, regarded its storm-wracked seas with fear and awe. After ice floes trapped and crushed the three-masted Endurance in 1915, Shackleton made an epic rescue attempt, sailing 1,300 kilometers to bring help to his stranded crew. He crossed the Southern Ocean’s waters in a small open boat, threatened by what he called “uprearing masses of water, flung to and fro by Nature in the pride of her strength.”

Yet this remote, tempestuous ocean also benefits humankind. Scientists estimate that each year, the Southern Ocean slurps up more than 40 percent of the carbon dioxide that people release by burning fossil fuels for electricity, heat and transportation. That makes the ocean a powerful support system for slowing the buildup of heat-trapping greenhouse gases in the atmosphere. The more carbon this immense body of water takes up, the less accumulates in the atmosphere to warm the planet.

But certain spots in the Southern Ocean may be working against the waters’ carbon storage role. Scientists have begun to make ambitious new measurements of how much CO ₂ it absorbs, using deep-diving floats that travel to far corners of the ocean. Last September, with the new data in hand, researchers reported that rather than sucking up CO ₂ , parts of the ocean near Antarctica are actually burping the gas back into the atmosphere during the dark and cold of winter. That suggests the Southern Ocean is more of a fair-weather friend than scientists had hoped.

These details have oceanographers trying to flesh out a more complete picture of how much carbon the Southern Ocean can actually soak up, and how quickly. If less of that carbon is going into the Southern Ocean than scientists had thought, then it must be going somewhere else — either staying in the atmosphere, or being absorbed by a different ocean or by trees and other plants on land.

Researchers probably need to revise their ideas about where the planet’s carbon is flowing. “To me, that’s one of the most exciting things,” says Alison Gray, an oceanographer at the University of Washington in Seattle. “What are the implications for the global picture? Have we been missing something all along?”

Distinctive waters

Continents help shape how water circulates within the Atlantic, Pacific and Indian ocean basins. In contrast, water flows unimpeded all the way around the Southern Ocean (SN: 9/16/17, p. 36), which is often defined as extending from 60° S latitude down to Antarctica. “It’s unique in its geometry, which makes it unique in its circulation,” says Nicole Lovenduski, an oceanographer at the University of Colorado Boulder.

The pattern with the biggest effect on CO₂ levels in the Southern Ocean is the strong overturning circulation, which helps connect the deep waters with the surface.  

One set of currents pulls surface water down, carrying carbon and sequestering it from the atmosphere. Researchers track water by lowering sampling bottles into the ocean at different depths, shutting the bottles tight, then raising them to the surface to be tested in a laboratory. By measuring the isotopes, or chemical variations, of carbon in the samples, scientists can date how old the water is. Anything younger than the start of the Industrial Revolution, about 150 to 200 years ago, probably contains carbon that was belched out by coal-burning power plants or other human sources. Most of this human-made carbon in the Southern Ocean is tucked away in the uppermost 500 meters.

A second set of currents brings water from down deeper up toward the surface. This ancient water is too old to contain human-made carbon, but it does contain natural carbon from the remains of organisms like plankton that have lived and died in those depths. When the water reaches the surface, it releases some of that old, natural carbon into the atmosphere. “Oftentimes that water hasn’t seen the atmosphere for hundreds of years,” says Lovenduski, who coauthored a review paper about the Southern Ocean’s carbon variability in the January Annual Review of Marine Science.

These patterns of downwelling and upwelling are different all around the Southern Ocean, so some patches of water absorb carbon while others emit it. Oceanographers have been trying to figure out which of these patterns is more dominant — is the Southern Ocean overall releasing more or less carbon than it soaks up each year? Sometimes the conclusions depend on which part of the ocean the researchers are looking at.

Computer simulations suggest that early in the industrial era, the Southern Ocean may have been an overall source of carbon, putting out more than it absorbed. But sometime around 1930, Lovenduski’s calculations suggest, the levels of CO₂ in the atmosphere got so high that the ocean was essentially forced into absorbing the gas from the air — it switched from emitting carbon to storing it.

Researchers have had a hard time confirming this because there are so few observations in the Southern Ocean. Oceanographers would occasionally measure CO₂ by putting buoys into the water or by sampling from ships outfitted with specialized equipment. But very few ships dare ply the Southern Ocean other than along the Drake Passage, the relatively narrow route between the tip of South America and Antarctica; the seas are so rough even there that journeys are rare, and mostly limited to the summer.

Scientists took what observations they did have, and then combined those with simulations to estimate what might be happening in the parts of the ocean not directly studied. By the 2000s, scientists generally agreed that the Southern Ocean was overall a carbon sink. “That seemed like great progress at that point,” Lovenduski says.

But in 2014, researchers began dropping the first of 200 special floats all around these southernmost waters as part of a project known as Southern Ocean Carbon and Climate Observations and Modeling, or SOCCOM. These 1.3-meter-long yellow cylinders gather data on water temperature, salinity, oxygen content and pH, or acidity, which is used to estimate CO₂ levels. (When seawater absorbs carbon dioxide, it converts the compound into bicarbonate, a mild acid.) More than 150 floats have been deployed as of early May, with more than 130 of them still sending data.

SOCCOM floats drift into remote corners of the Southern Ocean throughout the year. The floats collect information as they bob up and down through the uppermost two kilometers of water. To transmit their data via satellite, they occasionally rise to the surface. Some floats even travel and gather data beneath the sea ice surrounding Antarctica. They can sense when ice is above them, so they don’t try to surface at the wrong time. “It’s just revolutionary,” Gray says. 

Analysis of the first three years of SOCCOM data has transformed scientists’ views of how carbon is flowing into and out of the Southern Ocean. Last September, in Geophysical Research Letters, Gray and colleagues reported data collected by 35 of the first SOCCOM floats from 2014 to 2017. In the coldest, darkest months of July through September, the ocean was belching CO₂ at various spots around Antarctica.

“The ocean in winter is a much stronger source of CO₂ than we expected,” says Peter Landschützer, a marine biogeochemist at the Max Planck Institute for Meteorology in Hamburg who’s been studying this area. Nobody had seen this before simply because nobody had ever looked during the harsh winter.

The localized CO₂ belches might be related to seafloor topography, Lovenduski says. When ocean currents hit an underwater mountain or ridge, they could be forced upward toward the surface, where they release their carbon. Lovenduski is modeling how this process might happen, and why it might be more common in winter than in summer.

So much CO₂ is being emitting from these hot spots that the Southern Ocean may not be doing much to help humankind after all. Before the initial SOCCOM results, researchers had calculated that the entire ocean was absorbing about a gigaton of carbon each year (roughly half of what humans produce). The SOCCOM data told a very different story: At least during those three years, the ocean was spitting out as much as it sucked up. “It hit us all a bit by surprise,” Landschützer says. Enough of a shock that not everyone believed it.

Creative data collection

To find out what’s really going on in Antarctica’s waters, researchers need more info. Gray has been working with Landschützer and others to see what new SOCCOM data from a larger number of floats might reveal. The scientists are also studying how to reconcile the float data with information gathered from ships, like the icebreaker Laurence M. Gould, that regularly ply the Drake Passage. These ships have typically measured less CO₂ being emitted from the Southern Ocean than the SOCCOM floats do. That may be because the ships mostly stay within that narrow path without venturing into the distant reaches of the Southern Ocean.

Combining the SOCCOM data with the ship-based estimates, the scientists think they can confirm that carbon belching is happening in certain parts in winter — though maybe not as powerfully as suggested by Gray’s team in September. Team member Seth Bushinsky, an oceanographer at Princeton University, will present these latest findings in Montreal in July at a meeting of the International Union of Geodesy and Geophysics.

Other groups are throwing all kinds of measurement tools at the Southern Ocean to assess CO₂. Dorothee Bakker, a marine biogeochemist at the University of East Anglia in Norwich, England, even wants to strap pH sensors to the heads of seals and let them collect observations as they dive deep for food during the Antarctic winter. “Anything to get more data,” she says.

In December, oceanographers with a U.K. government project called CUSTARD, for Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth, tethered a kilometers-long string of instruments to the seafloor off the southwestern tip of South America. This mooring is measuring things like oxygen, nutrients and CO₂ levels. The goal is to better understand how water downwells in this area, taking carbon with it.

Another new approach relies on a small, uncrewed sailboat built by Saildrone of Alameda, Calif. The Saildrone craft navigates autonomously across the seas, snapping photographs, clocking wind speeds and wave heights, and measuring CO₂ levels hourly. In January, two bright-orange Saildrones left New Zealand on a private expedition to circumnavigate 15,000 nautical miles, or about 28,000 kilometers, around Antarctica. Heavy seas damaged one and forced it to return to port, but the second had already made it halfway around Antarctica by May, soldiering on through waves as high as a three-story house.

The Saildrone will occasionally rendezvous with SOCCOM floats, which will bob up to the surface to measure CO₂ at around the time the Saildrone passes by. Comparing the CO₂ numbers from the two devices offers an accuracy cross-check, says Adrienne Sutton, an oceanographer at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle who has been working with Saildrone staff.

As the Southern Ocean transitions into winter, researchers hope to see whether the Saildrone will also pick up wintertime CO₂ belching. “There’s no one platform out there that can measure everything well,” Sutton says. “I’m interested in whether Saildrone can be part of that mix in the Southern Ocean.”

More changes

Getting today’s Southern Ocean carbon estimates right is crucial to forecasting. “The ultimate goal is to say what our climate is going to look like in 20 years or in 50 years,” Gray says.

How will the chemistry of the ocean change in the future, and how will that affect living organisms? Putting more CO₂ into the ocean makes it more acidic in ways that can harm marine critters. In March in Nature Climate Change, Lovenduski and colleagues estimated that the chemistry of the Southern Ocean could change so quickly that, by the end of this century, some parts could become toxic for small sea snails, an important part of the marine food web.

There are other big unknowns. How will changes in the Antarctic environment affect the Southern Ocean’s carbon uptake? As the Antarctic ice sheet melts (SN: 7/7/18, p. 6), for instance, it could send enormous pulses of freshwater into the Southern Ocean that could upset whatever is happening today.

The Southern Ocean is “still going to help us out,” absorbing at least part of humankind’s self-created climate mess, Landschützer says optimistically. “The only question is by how much.”

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This story appears in the June 8, 2019 Science News with the headline, “Southern Ocean — friend or foe? The waters surrounding Antarctica may not be absorbing as much carbon as expected.”