Calling global climate models into question, researchers find evidence that the ocean may have absorbed as much as 15% more CO2 than previously thought, requiring a re-think of future flux assessments.
Implications for science and policy are significant. The exchange of carbon dioxide (CO2) between the sea and air is a key part of the global carbon cycle and plays a critical role in buffering climate change.
As the ocean is a major absorber of CO2, accurate quantification of this sea-air CO2 flux is therefore vital for forecasting the future climate and developing effective climate-change mitigation strategies.
Published in Nature Communications journal, the study was led by Plymouth Marine Laboratory and GEOMAR Helmholtz Centre for Ocean Research Kiel, with a partner from Heriot-Watt University.
Asymmetric bubble effect
The CO2 story is complex. Sea-air CO2 fluxes vary regionally and seasonally between uptake and outgassing, meaning that in some areas the ocean is absorbing CO2 and in other areas, releasing it.
In turbulent areas where there is more wave action, uptake is generally greater due to bubbles of air getting engulfed by waves, which enables the seawater to ‘absorb’ the gas trapped within the bubble.
Traditionally, sea-air CO2 fluxes have been calculated using a blanket ‘symmetric’ equation that assumes the rate of gas transfer is dependent on the difference in CO2 concentration between the seawater and air, and regardless of whether the CO2 is actually being taken up or outgassed.
In recent years, however, questions have been raised as to whether an ‘asymmetric bubble effect’ had been overlooked — since submerged bubbles under pressure favour CO2 flux uptake over outgassing.
Whilst popular, this hypothesis had lacked the evidence necessary to win over the doubters, until now…
CO2 flux recalculated
In this first-of-its-kind study, some 4,082 hours of high-quality sea-air CO2 flux measurements were researched in total, collected over 17 ship cruises and covering a variety of ocean regions.
These were re-analysed to investigate if bubble-mediated transfer acts in a more asymmetric manner.
Using a novel ‘2-dimensional’ fitting method, the analysis shows clear field evidence of asymmetric bubble-mediated CO2 in the ship observations. When the team recalculated global sea-air CO2 fluxes (for 1991–2020) using the asymmetric formulation, they found that the global ocean’s CO2 uptake increased by about 15% (0.3-0.4 Pg C yr-1) compared with conventional, symmetric estimates.
The asymmetric increase in CO2 uptake is especially strong in regions with frequent high winds and wave breaking, such as in the Southern Ocean where stronger climate change impacts are already being seen.
Absorbing and buffering
These results suggest that the global ocean may have been absorbing more human-produced CO2 than previously thought, thereby further buffering climate change. This finding from the study has major implications for the understanding of the global carbon budget and climate change mitigation.
Going forward, the way CO2 is measured and reported must change, says Lead Author Dr Yuanxu Dong:
“This study challenges standard assumptions of the symmetric flux formula used in many carbon cycle and climate models. This means that many past estimates may be systematically biased and we urge that future CO2 flux assessments should adopt the asymmetric formula”.
Co-author and Chemical Oceanographer at Plymouth Marine Laboratory, Dr Ming-Xi Yang adds:
“Accounting for the asymmetry means that the ocean CO2 flux estimates diverge even more between what is computed from observations and what is estimated from global models.
“This suggests that there may be shortcomings in the global models and of course, we need those models to be as realistic as possible to make accurate future CO2 and climate projections.”
In terms of current and future research, the study team note that more outgassing measurements are needed, especially under high-wind and wave conditions that make them more challenging to obtain.
Climate, biodiversity, and pollution
A world leader in marine research, Plymouth Marine Laboratory (PML) is committed to delivery of impactful, cutting-edge environmental and social science in support of a healthy and sustainable ocean.
Under a banner of Science for Ocean Action, PML is focused on addressing the triple challenges of climate change, biodiversity loss and marine pollution. Its work harnesses the capabilities of marine observing systems, and involves use of AI, autonomy and advanced technologies for scientific research.
A former PhD student with Plymouth Marine Laboratory, Dr Yuanxu Dong is now a Humboldt fellowship postdoc at GEOMAR Helmholtz Centre for Ocean Research Kiel and Heidelberg University in Germany.
Further Reading:
- More about the lead study organisation from the UK Plymouth Marine Laboratory;
- More about GEOMAR Helmholtz Centre for Ocean Research Kiel, in Germany;
- Access the full paper on Asymmetric bubble-mediated gas transfer enhances global ocean CO2 uptake;
- Also on SustMeme, COP30: A house divided, if not defeated;
- Also on SustMeme, Plastic talks falter but tide to save oceans on rise;
- Also on SustMeme, AI helps digital twin scale coral reef restoration;
- Also on SustMeme, Giant ancient coral colony discovered in Red Sea;
- Also on SustMeme, Giant iceberg the size of Los Angeles breaks off Antarctica.
Check out the full archive of stories on the SustMeme Climate & Energy Channel, now available to Sponsor.
