Southern Ocean absorbs 25% more carbon dioxide than previously thought

The flux system collecting data for this study is currently in use on a new research vessel, the RRSS Sir David Attenborough. The picture shows the carbon dioxide flux system used by the RRSS Sir David Attenborough during its research voyage in Antarctica in 2024. Photo credit: Tom Bell/PML

New research led by the University of East Anglia (UEA) and Plymouth Marine Laboratory (PML) has found that the Southern Ocean absorbs more carbon dioxide (CO2) than previously thought.

By directly measuring the exchange, or flux, of carbon dioxide between the air and ocean, scientists found that the ocean around Antarctica absorbs 25% more carbon dioxide than previously estimated indirectly based on shipboard data.

The Southern Ocean plays an important role in absorbing carbon dioxide emitted by human activities, a process critical to controlling Earth's climate. However, there are large uncertainties in the magnitude and variation of this flux.

To date, it has been estimated using shipboard measurements such as data collected from research vessels and unmanned sailing vessels for the Surface Ocean Carbon Dioxide Atlas (SOCAT), data obtained from profiling buoys deployed in the ocean, and global ocean biogeochemical models. These different methods lead to large differences in estimation results.

The new study used a new technique called eddy covariance - a flux system mounted on the ship's foremast - to directly measure air-ocean carbon dioxide fluxes during seven expedition voyages in the region.

The findings, published in the journal Science Advances, suggest that the Southern Ocean is likely a strong carbon dioxide sink in summer, casting doubt on much weaker estimates based on planktonic data and model simulations, which the authors say "significantly underestimate" the observed amount of carbon dioxide uptake.

The authors suggest that this difference can be explained by taking into account temperature changes in the upper ocean and limited resolution, such as too long averaging time scales or too large sampling intervals.

Implications of new findings for climate models

Lead author Dr. Yuanxu Dong from the UEA Center for Oceanic and Atmospheric Sciences (COAS) and PML currently works at the GEOMAR Helmholtz Center for Marine Research in Kiel, which is funded by a Humboldt Foundation scholarship. He said: "This is the first time a large number of direct air-ocean CO2 flux observations have been used to assess existing flux products in the Southern Ocean. Our results provide direct observational evidence that the Southern Ocean may be absorbing more CO2 than previously recognized.

"Accurate quantification of the Southern Ocean CO2 sink is critical for assessing Earth's climate. However, the Southern Ocean is the region with the most uncertainty in estimates of its CO2 sink capacity. Our study reduces this uncertainty and improves understanding of Southern Ocean CO2 uptake, and we recommend that future estimates should include temperature adjustments and higher-resolution reconstructions and modeling."

The research team, which also included scientists from the Alfred Wegener Institute and the Max Planck Institute in Germany, the Flanders Institute for Oceanography in Belgium, and the University of Hawaii in the United States, investigated inconsistencies in existing CO2 flux estimates and then used eddy covariance flux observations to evaluate different data sets.

The cruise data covered approximately 3,300 hours (approximately 175 days) of measurements in the Antarctic summer of 2019 and 2020, defined in the study as November to April of the following year, and the measurement area is a highly dynamic frontal zone. Measurements are taken every hour, compared to approximately every 10 days for buoy measurements.

"The Southern Ocean is an important carbon dioxide sink, but the magnitude and location of the ocean's uptake of carbon dioxide is uncertain," said study co-author Dr. Mingxi Yang, a chemical oceanographer at PML. "PML's autonomous high-frequency eddy covariance system has significantly increased the number of direct air-ocean carbon dioxide flux measurements in the region. This paper is the first to compare direct measurements of carbon dioxide flux over large spatial and temporal scales with estimates from coarse data products and global models. It helps validate these data and sheds light on ways to improve them."

Challenges and opportunities for expanding CO2 flux observations

The lack of winter data is a common problem for boats because access to the area is difficult at that time, and buoys can partially solve this problem. The authors acknowledge that their cruise data only cover part of the Southern Ocean during summer, but they say continued efforts to produce high-quality observations are critical to improving estimates of air-sea carbon dioxide fluxes.

This could include expanding surveys to more vessels and further deploying buoys and unmanned sailing vessels, particularly during the winter. More observations during the winter through unmanned platforms will also help fill seasonal data gaps.

Professor Tom Bell, co-author of the paper and a marine-atmospheric biogeochemist at PML, added: "We recently moved the flux system to the new icebreaker RRSS Sir David Attenborough and collected the first set of flux measurements during a research cruise in the Weddell Sea earlier this year. We aim to continue this valuable work over the coming years, which is critical for monitoring current climate and predicting future changes."

Conclusion and call to action

Researchers also warn that the number of shipborne surface ocean carbon dioxide measurements has dropped sharply in recent years, partly due to the COVID pandemic but also related to reduced funding. For example, the number of annual datasets for SOCAT decreased by 35% from 2017 to 2021, and for the Southern Ocean by 40%.

Dr. Dorothee Bakker from the Center for Atmospheric Studies (COAS) at the University of Energy, UK, is chair of SOCAT: "To limit the uptake of carbon dioxide in the Southern Ocean, support the World Meteorological Organization's Global Greenhouse Gas Observation monitoring program, and inform climate policy, we really need continued and expanded funding for surface ocean carbon dioxide measurements and their SOCAT synthesis."

Compiled from /ScitechDaily