Southern Ocean absorbs less CO₂ than previously thought

By acting as a heat and carbon sink, the Southern Ocean is an important force in regulating Earth’s climate. Yet observational studies of the icy waters are scarce, particularly in winter, so climate scientists have limited data to explain the ocean’s precise role. A new analysis of three years’ worth of carbon dioxide flux estimates from robotic buoys, like the one pictured above, suggests that the Southern Ocean is only a weak sink, with a sizable area near Antarctica that actually releases CO₂.

Water in the upper 1000 m of the ocean absorbs CO₂ from the atmosphere. At the same time, upwelling of carbon-rich water from the ocean’s depths brings dissolved inorganic carbon to the surface, where it can escape into the atmosphere. The rate at which the sea–air carbon transfer occurs depends on atmospheric CO₂ concentrations and the strength of global ocean circulation (see Physics Today, April 2017, page 21 ).

In 2014 researchers began deploying floats equipped with biogeochemical sensors to measure the Southern Ocean’s pH and other properties as part of the SOCCOM project , directed by Jorge Sarmiento at Princeton University. The floats drift with the currents and dive to 2 km depth, taking measurements every 5 to 10 days and transmitting them to shore via satellite. Sarmiento, Alison Gray at the University of Washington, and colleagues used pH data gathered by 35 floats between 2014 and 2017 to calculate the amount of CO₂ dissolved in waters extending up to 3500 km from Antarctica. Those calculations, along with measurements of atmospheric CO₂, enabled the team to determine whether the ocean was a net carbon source or sink.

The researchers found that a large region of the Southern Ocean near Antarctica’s sea ice released 0.36 petagrams (PgC, one billion metric tons) of carbon per year. Most of that outgassing occurred during winter months. (For comparison, global fossil-fuel burning in 2016 released 9.9 PgC .) Measurements from four other regions suggested that overall, the Southern Ocean is a weak sink that absorbs 0.08 PgC/year. Climate models tend to calculate an order of magnitude larger absorption, around 0.9 PgC/year, which is consistent with estimates from ships traversing the region primarily in summer.

The researchers conclude that an unaccounted-for carbon sink must exist elsewhere to supplement the lower-than-expected contribution of the Southern Ocean. The study suggests that current understanding of source and sink distribution may need revision and highlights the need for sustained year-round observations. (A. R. Gray et al., Geophys. Res. Lett., in press, doi:10.1029/2018GL078013 .)