The Southern Ocean surrounding Antarctica makes up less than a third of the global ocean surface, yet scientists believe it “plays an outsized role in the climate system,” according to a 2021 journal article in Geophysical Research Letters. Water and currents from other oceans meet here, allowing for heat and carbon transfer as this body of water connects the ocean and atmosphere.
Scientists have long believed the Southern Ocean to be a crucial carbon sink, but recent data is raising questions about whether the Southern Ocean may not actually be quite as much of a carbon sink as they once thought. To learn more and broaden their understanding, they are using new technology to collect additional data – especially during the seldom-surveyed winter season.
“The Southern Ocean is a really interesting place,” says oceanographer Adrienne Sutton, who works for NOAA’s Pacific Marine Environmental Laboratory. “It covers only 30% of the global ocean surface but it represents as much as 75% of heat uptake and 40% of carbon uptake for the globe. So it plays an outsized role in climate, and that’s why we’re really interested in collecting more data there.”
Since the world and its systems are so interconnected, learning about the Southern Ocean’s carbon uptake is important in calculating global carbon estimates.
“As we try to manage carbon we need to know the natural sinks,” says Bronte Tilbrook, senior principal research scientist with Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere in Australia. “About half the carbon is currently taken up by natural sinks both on land and in the ocean.” He points out that land sinks are very complex and difficult to quantify, so the ocean sink’s atmospheric measurements are needed. “In the ocean we have data that will allow us to get better ideas of how much carbon is going into the land, how much is going into the ocean, and what’s remaining in the atmosphere from emissions,” Tilbrook says. “The ocean data is really critical for understanding the global carbon budget.”
Challenging winter conditions complicate data gathering efforts
The waters off Antarctica are known for their brutal conditions, with ferocious waves and storms, and long, cold, dark winters when ships can become entrapped in ice. It all makes for a challenging place to get to, let alone conduct research. While some ships travel in the region during summer and retrieve data, winter measurements are scarce. To tell the complete story of the ocean, scientists need to know what is happening year-round in its vast waters.
“Measuring the Southern Ocean in the winter can be so challenging that we have major gaps in our understanding because we’re just missing that season in huge swaths of the Southern Ocean, and so that was really our mission to collect data there,” Sutton says.
To safely collect data from the Southern Ocean, a company called Saildrone developed a 23-foot-long, wind driven and solar powered uncrewed surface vehicle (USV) that can be controlled remotely. It has sensors and instruments to obtain a wide variety of data, and also has an array of cameras. Built specifically to handle the conditions of the Southern Ocean, it has already proven its durability, even surviving a collision with an iceberg. It is just one of the company’s USVs, which are used for a variety of scientific projects ranging from collecting hurricane data to mapping the ocean floor.
In early 2019, scientists launched a Saildrone USV from Bluff, New Zealand, and it covered 13,670 miles before being recovered off Bluff’s coast 196 days later. The journey was the first autonomous circumnavigation of Antarctica.
Along the way, it collected data every hour. Scientists have now analyzed some of that data, and Sutton, Tilbrook, and University of South Florida oceanography professor Nancy Williams published their findings about carbon flux in Geophysical Research Letters. They noted the importance of expanding a network of observations and of using a variety of tools such as USVs like the Saildrone, floats, ships, and other devices.
New insights on varying year-to-year Southern Ocean CO2 exchange
Before Saildrone, Argo floats collected biogeochemical measurements from 2014 to 2017. When analyzing this data, scientists found the Southern Ocean’s carbon sink may be much smaller than previously believed. A 2018 paper by Alison Gray et al. in Geophysical Research Letters found the floats recorded “significantly stronger outgassing” than was found through previous ship-based estimates.
“[It was] a big discrepancy,” Sutton says. “There was a lot of debate in the community on ‘is that because 2014-2017 were anomalous years?’ And they were kind of strange years compared to the Southern Ocean mean annual average,” she says, noting some scientists’ concerns that there could also be an issue with the float sensors. They learned 2014 and 2015 were fairly anomalous years, off gassing much more CO2 than what they believed was typical. However, she says 2019, when Saildrone circumnavigated the continent, represented what they believe to be a more representative year.
“The scientific community is still grappling with trying to figure out what is the true year-to-year variability in CO2 exchange in the Southern Ocean,” Sutton says. “Because what we’re finding with these new technologies between floats and Saildrone is that there is huge interannual variability that we are not capturing now with our traditional measurements.”
Gathering more data, and making sure this data includes measurements from all times of year, is an important step. The 2018 report authors had written that “Our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year-round biogeochemical observations in the Southern Ocean.”
Better understanding the future by better knowing the present
As scientists learn more about the Southern Ocean’s carbon uptake, they can also begin to formulate answers to other questions. For example, if the Southern Ocean is a smaller carbon sink than previously believed, where is the carbon going instead? These findings could indicate larger sinks in other parts of the ocean or even a change in the land sink.
The best way to plan for the future is to learn more about what’s going on currently. In 2022, two Saildrone USVs will be deployed to the Southern Ocean to gather more data, and Williams and Sutton plan to analyze that new data with a focus on how eddies impact the air-sea CO2 exchange. Tilbrook is also applying for refunding for Australia’s Integrated Marine Observing System to collect measurements beyond 2022 from Australian ships. While ship-based measurements are still vital to research, the researchers expect new technology like Saildrone to be a huge step toward learning more about the Southern Ocean and the global carbon cycle.
Tilbrook says traditional measurements, such as those recorded by ships, are still needed, but devices like Argo floats and Saildrones provide benefits because “they don’t require us to send people into hostile environments all the time.”
“We can now manage things better,” he says, “and we can get more data for less money, and it’s really a game changer, the technology developments that are happening due to the visions of a few people.”