Utilizing direct measurements of CO2 exchange between the atmosphere and the ocean, researchers discovered that the Southern Ocean takes in 25% more CO2 than earlier indirect estimates based on shipboard data.
The Southern Ocean is crucial in mitigating the impacts of human-caused CO2 emissions, playing a vital role in regulating the Earth's climate. Despite this, there are considerable uncertainties regarding the scale and variability of CO2 absorption in this region.
Historically, CO2 absorption has been estimated using data from the Surface Ocean CO2 Atlas (SOCAT) collected via research vessels and sail drones, profiling floats, and global ocean biogeochemistry models. These methods have yielded widely varying results.
The recent study employed an innovative technique known as eddy covariance, using flux systems mounted on ships' foremasts, to directly measure air-sea CO2 exchanges during seven research cruises.
Published in the journal Science Advances, the findings indicate that the Southern Ocean in summer is a substantial CO2 sink, contradicting weaker estimates from float data and models that the authors state "substantially underestimate" CO2 uptake.
The research team attributes these differences to temperature variations in the upper ocean and insufficient resolution in current models, which often average data over prolonged periods or large areas, failing to capture brief, intense CO2 uptake events.
Dr. Yuanxu Dong, lead author from UEA's Centre for Ocean and Atmospheric Sciences (COAS) and PML, currently at the GEOMAR Helmholtz Centre for Ocean Research Kiel, commented: "This is the first time a large number of direct air-sea CO2 flux observations have been used to assess existing flux products in the Southern Ocean. Our findings provide direct observational evidence that this ocean may take up more CO2 than previously recognized.
"Accurate quantification of the Southern Ocean CO2 sink is essential for the assessment of the Earth's climate. However, it is the most uncertain region regarding the estimate of its CO2 sink capacity.
"Our study reduces this uncertainty and improves the understanding of Southern Ocean CO2 uptake, and we recommend that future estimates should include temperature adjustments and higher resolution reconstruction and modelling."
The research team included scientists from the Alfred Wegener and Max Planck Institutes in Germany, the Flanders Marine Institute in Belgium, and the University of Hawai'i in the US. They examined inconsistencies in existing CO2 flux estimates and utilized eddy covariance observations to evaluate various data sets.
Data from the research cruises covered around 3300 hours, or approximately 175 days, of measurements during the Antarctic summers of 2019 and 2020, from November to April. Measurements were taken hourly, compared to roughly every 10 days from floats.
Dr. Mingxi Yang, co-author and Chemical Oceanographer at PML, stated: "The Southern Ocean is a key sink of CO2, but the magnitudes and the locations of this ocean uptake are uncertain. PML's autonomous and high frequency eddy covariance system has significantly increased the number of direct air-sea CO2 flux measurements in this region.
"This paper offers the first comparison between direct CO2 flux measurements and estimates from coarse data products and global models on a large spatial/temporal scale. It has helped validate these and shed light on ways to improve them."
Due to the difficulty of accessing the Southern Ocean during winter, shipboard measurements are often lacking for that season. The authors acknowledge their cruise data only covers the summer period but stress the importance of continued high-quality observations to improve air-sea CO2 flux estimates.
Future efforts may include expanding measurements to more ships and deploying additional buoys and sail drones, particularly in winter. Unattended platforms could also help fill seasonal data gaps.
Prof. Tom Bell, co-author and PML Ocean-Atmosphere Biogeochemist, added: "We have recently moved our flux system onto the new ice breaker, the RRS 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 essential for monitoring the current climate and forecasting future changes."
The researchers also note a significant decline in shipboard surface ocean CO2 measurements in recent years, due to the COVID pandemic and reduced funding. SOCAT datasets for the Southern Ocean dropped by 40% from 2017 to 2021.
Dr. Dorothee Bakker, of UEA's COAS and SOCAT chair, stated: "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."
The study received funding from the China Scholarship Council, the UK's Natural Environment Research Council (NERC), and the European Space Agency.
'Direct observational evidence of strong CO2 uptake in the Southern Ocean', Yuanxu Dong, Dorothee C E Bakker, Thomas G Bell, Mingxi Yang, Peter Landschutzer, Judith Hauck, Christian Rodenbeck, Vassilis Kitidis, Seth M Bushinsky, and Peter Liss, is published in Science Advances on July 24.
Research Report:Direct observational evidence of strong CO2 uptake in the Southern Ocean
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