07-26-2024

Southern Ocean absorbs more CO2 than previously thought

Earth.com staff writer

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

The study, published in the journal Science Advances, used direct measurements of CO2 exchange between the air and sea, uncovering that the ocean around Antarctica takes in 25% more CO2 than earlier indirect estimates suggested.

Southern Ocean CO2 absorption

The Southern Ocean plays a critical role in absorbing CO2 produced by human activities, a vital process for regulating Earth’s climate. However, previous estimates of this CO2 absorption have been plagued by uncertainty, largely because they relied on indirect measurements from research ships, sail drones, profiling floats, and global ocean biogeochemistry models. These methods produced widely varying results.

To address this issue, the new study employed a technique called eddy covariance, which involves mounting flux systems on ships’ foremasts to directly measure air-sea CO2 fluxes. This method was applied during seven research cruises in the region, providing more accurate data.

The results indicate that the Southern Ocean is likely a stronger CO2 sink than previously thought, especially during the summer.

“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,” explained lead author Yuanxu Dong, a scientist at the GEOMAR Helmholtz Center for Ocean Research Kiel.

Underestimated CO2 uptake in Southern Ocean

Accurate measurement of the Southern Ocean’s CO2 absorption is crucial for assessing Earth’s climate. The study suggests that previous models underestimated the CO2 uptake due to factors like temperature variations in the upper ocean and insufficient resolution in data sampling.

Dong emphasizes the need for future estimates to include temperature adjustments and higher resolution reconstruction and modeling.

The research team, which 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, tackled inconsistencies in existing CO2 flux estimates.

The experts utilized the eddy covariance technique to assess these datasets, covering approximately 3300 hours of measurements during the Antarctic summer of 2019 and 2020. These measurements, taken hourly, provided a much clearer picture compared to the less frequent data from floats.

Significance of the study

Dr. Mingxi Yang, a co-author and Chemical Oceanographer at PML, highlighted the significance of these findings. “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.”

The study’s comparison between direct CO2 flux measurements and estimates from coarse data products and global models is a first on such a large spatial and temporal scale. It has helped validate these estimates and highlighted ways to improve them.

However, the study also points out a significant gap in winter data, which is challenging to collect due to the harsh conditions in the region. The authors suggest that expanding measurements to more ships and deploying additional buoys and sail drones, particularly during winter, could help fill this gap.

Professor Tom Bell, a co-author and PML Ocean-Atmosphere Biogeochemist, noted the importance of continued high-quality observations.

Forecasting future climate changes

“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 raised concerns about the decline in shipboard surface ocean CO2 measurements in recent years, partly due to the COVID-19 pandemic and reduced funding.

Dr. Dorothee Bakker, of UEA’s COAS and chair of SOCAT, stressed the need for sustained and expanded funding for these measurements to better understand and constrain Southern Ocean CO2 uptake and support global climate monitoring initiatives.

This groundbreaking research underscores the importance of the Southern Ocean in absorbing CO2 and the need for improved and continuous measurements to refine our understanding of its role in the global carbon cycle and climate regulation.