New research led by the University of East Anglia (UEA) and Plymouth Marine Laboratory (PML) – in collaboration with several partners including VLIZ – has found that the Southern Ocean absorbs more carbon dioxide (CO2) than previously thought. Using direct measurements of CO2 exchange or fluxes between air and sea, the scientists found that the ocean around Antarctica absorbs 25% more CO2 than previous indirect estimates based on data from ships, sailing drones, floats and global model calculations suggested. The results appeared in the renowned journal Science Advances.
The Southern Ocean plays a major role in absorbing CO2 emitted by human activities, a process vital for controlling the Earth's climate. However, there are big uncertainties in the magnitude and variability in this flux. Until now it has been estimated using measurements, from ships, sail drones, data from profiling floats deployed in the ocean, and global ocean biogeochemistry models.
Peter Landschützer, scientific director at the Flanders Marine Institute (VLIZ) and co-author of the paper, says: “All these approaches so far showed a large variation in estimating the CO2 flux. A long-standing problem – the kinetic transfer of a gas across the air-sea interface – still causes considerable uncertainty in our estimation of carbon sinks worldwide. In the new study, we used the so-called ‘eddy covariance technique’, in which flux systems are mounted on the foremast of ships, and directly measure the CO2 flux between air and sea.”
During seven research cruises in the Antarctic summer, about 3,300 hours or almost 175 days of measurements were collected in an area with highly dynamic frontal zones. Every hour a measurement took place, providing a much finer resolution compared to every 10 days for measurements with floats, for example. Lead author of the publication Dr Yuanxu Dong, working at Centre for Ocean and Atmospheric Sciences (UEA-COAS), Plymouth Marine Laboratory in the UK and the GEOMAR Helmholtz Centre for Ocean Research in Germany, said, "This is the first time we have been able to use such a large number of direct air-sea CO2 flux observations to assess existing flux estimates in the Southern Ocean."
The results – published in Science Advances – show that the Southern Ocean may take on a much stronger role as a carbon sink (CO2 sink) during the summer months (from November to April) than previously assumed based on the much weaker estimates from floats and model simulations.
Yuanxu Dong led an international research team that also included scientists from the Alfred Wegener Institute and Max Planck Institute from Germany, the Flanders Marine Institute (VLIZ) from Belgium and the University of Hawai'i from the US. They investigated inconsistencies in existing CO2 flux estimates and then used eddy covariance flux observations to assess the different existing datasets. This included using a two-step neural network data extrapolation method developed by VLIZ research director Peter Landschützer.
“One of the most interesting findings of this study is the realisation that the time scales, at which we traditionally measure the partial pressure of CO2 at the sea surface to then calculate air-sea CO2 flux, definitely do matter for the interpretation of the ocean as a carbon sink,” says Peter Landschützer. “We may have to re-think how we want to combine measurements with different time scales in regional air-sea CO2 flux reconstructions. This is particularly important if we combine measurements from different platforms, such as those from ships, floats, robots and other unmanned platforms.” A combining of data from various observation platforms will continue to be necessary in the future. During the winter period, the Southern Ocean is a very inaccessible area and it remains difficult to collect direct measurements using the eddy covariance technique.
The researchers in the team do warn of the drastic decline in the amount of shipboard surface ocean CO2 measurements in recent years, caused partly by the COVID pandemic, but also by sharply reduced research funding. The number of datasets annually delivered to the Surface Ocean CO2 Atlas (SOCAT) decreased by 35% from 2017 to 2021 – and by 40% for the Southern Ocean. COAS researcher Dorothee Bakker, also chair of SOCAT warns, “There is a real need for sustained and expanded funding of surface ocean CO2 measurements and their SOCAT synthesis, in order to constrain Southern Ocean CO2 uptake, to support the World Meteorological Organization’s Global Greenhouse Gas Watch monitoring initiative and to inform climate policy.”
‘Direct observational evidence of strong CO2 uptake in the Southern Ocean’, Yuanxu Dong, Dorothee C E Bakker, Thomas G Bell, Mingxi Yang, Peter Landschützer, Judith Hauck, Christian Rödenbeck, Vassilis Kitidis, Seth M Bushinsky, and Peter Liss, is published in Science Advances on July 24.
