Accounting for carbon sinks and sources around the world is critical for scientists and policy makers looking to quantify Earth’s carbon budget and stave off the most catastrophic effects of climate change. The Southern Ocean absorbs roughly 40% of the 2.6 petagrams of anthropogenic carbon that dissolves into the global ocean each year, according to current estimates, making it one of the planet’s most vital sinks. But it’s also undersampled.

Researchers typically quantify the movement of carbon dioxide between the atmosphere and the sea by measuring the partial pressure of carbon dioxide in ocean surface waters with instruments on ships or moorings. These shipboard observations are accurate but sparse, with most data collected in the Northern Hemisphere in summer.

To get a more complete estimate of Southern Ocean carbon exchange, Bushinsky et al. combined shipboard measurements with data collected by biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. The team used two independent mapping techniques to create global maps of carbon dioxide partial pressures in surface waters and combined these with National Oceanic and Atmospheric Administration measurements of the atmospheric partial pressure of carbon dioxide to calculate the monthly air–sea flux. The addition of the float observations led the team to downsize the estimate of the Southern Ocean as a carbon sink. Whereas shipboard data alone indicated a mean annual flux of 1.14 petagrams of carbon into the Southern Ocean, combining the data sets revealed that the sink was a third less strong over the 2015–2017 period, with a mean flux of 0.75 petagram per year into the ocean.

Prior work has indicated that float-derived estimates of carbon dioxide partial pressures might be biased slightly high. The team tested the impact of this potential bias and found that it was not enough to account for the full reduction in the Southern Ocean’s carbon sink estimate. The study thus suggests that accurate measurements of the Southern Ocean carbon sink require researchers to use both float- and ship-based measures.

The authors also note that estimates of human-induced carbon dioxide emissions and how much of that is hanging around in the atmosphere are fairly robust, so if the Southern Ocean is taking up less carbon dioxide than previously thought, the excess carbon must be going elsewhere. The team identified other areas on land and at sea where carbon flux estimates might need to be updated. (Global Biogeochemical Cycles, https://doi.org/10.1029/2019GB006176, 2019)

—Kate Wheeling, Freelance Writer