Understanding carbon sequestration in the Southern Ocean
The Southern Ocean, which encircles Antarctica, absorbs more carbon dioxide than any other body of water in the world. Therefore, despite its relatively small size, it is critical for mitigating climate change. “Colder waters are able to dissolve more carbon dioxide than warmer waters,” explains Bieito Fernández Castro, lecturer in Physical Oceanography at the University of Southampton and SO-CUP project coordinator. The Subantarctic Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIW) are two key water flows that form in the Southern Ocean during winter. They are responsible for almost all the carbon sequestration in the southern latitudes, and help to slow down climate change. The EU-funded SO-CUP project, undertaken with the support of the Marie Skłodowska-Curie Actions programme, sought to further understand their formation and carbon sequestration rates – something that will help scientists predict how the global climate will evolve in the coming decades. “The scarcity of data in this remote, harsh region during winter represents a critical bottleneck for the understanding of the current state and future evolution of the Southern Ocean carbon sink,” says Fernández Castro.
Taking advantage of new oceanographic data
Since 2014, hundreds of autonomous profiling floats equipped with novel biogeochemical sensors have been added into the Southern Ocean under the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) programme. The SO-CUP team used this unprecedented data source on the Southern Ocean carbon cycle to investigate the dynamic processes underpinning SAMW and AAIW. The researchers combined float data analyses with the output from a three-dimensional ocean model, which also used float data to improve its simulations. “The float data is extremely novel as it allows us to directly observe the ocean carbon system in the remote Southern Ocean year-round,” Fernández Castro notes.
Overturning scientific understanding
The project had two key results, which challenge the traditional view of the Southern Ocean carbon sink. The first was the discovery of subtropical water sources key to the formation and carbon concentrations of SAMW, which contradicts previous thinking that it originated from the warming of northward flowing Antarctic waters. “Our results show that subtropical waters can constitute up to 75 % of the SAMW volume,” says Fernández Castro. The second was that while carbon concentrations in the Southern Ocean water masses are primarily determined by their temperature, the transfer of other waters around Antarctica into the SAMW and AAIW is principally carried out by marine algae taking up carbon dioxide. “This places biological processes at the forefront,” notes Fernández Castro. “These findings will transform the way in which the scientific community thinks of the Southern Ocean carbon sink and will eventually help improve its representation in climate models.”
Diving deeper into the Southern Ocean
Many important questions arose from the SO-CUP project, which the team hopes to address in the future. This includes investigating how changes in SAMW observed during the last few decades are reflected by the relative variations of their sources, what processes are driving those variations and how they impact the ocean carbon sink. “I would also like to investigate the role of AAIW further,” Fernández Castro adds. “Although often linked to SAMW, my analysis made clear that these water masses are radically different in their properties and in the way they reach the ocean interior.”
Keywords
SO-CUP, southern, ocean, carbon, sink, sequestration, climate, change
