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The ocean may have absorbed significantly more carbon dioxide (CO2) than previously calculated. A new study by the GEOMAR Helmholtz Centre for Ocean Research Kiel and the Plymouth Marine Laboratory shows that the exchange of gases between air and sea is not symmetric, and that the global ocean has taken up around 15 per cent more CO2 than suggested by conventional estimates. In windy regions, air bubbles entrained by breaking waves substantially enhance the uptake of CO2. The results are based on extensive direct measurements from the ocean and have now been published in the journal Nature Communications.
The exchange of carbon dioxide between the ocean and the atmosphere is a key component of the global carbon cycle. Acting as a vast buffer, the ocean absorbs a substantial share of human-made CO2 emissions, thereby slowing the pace of climate change.
How effective this buffering is, depends on how efficiently CO2 is transferred between air and water. Until now, most calculations assumed that this exchange is symmetric – meaning that CO2 enters and leaves the ocean at the same rate.
“With our study, we fundamentally challenge this assumption, known as the symmetric flux formulation,” says lead author Dr Yuanxu Dong, a Humboldt Fellow at the GEOMAR Helmholtz Centre for Ocean Research Kiel and Heidelberg University. Because this assumption is widely used in carbon cycle and climate models, many previous estimates may be systematically biased.
When waves swallow air
In regions with strong winds and heavy wave action, breaking waves entrain air bubbles into the water. These bubbles act like tiny transport capsules: under increased pressure below the surface, CO2 dissolves particularly efficiently into the ocean. Put simply, the gas is not just exchanged, but actively “pushed” into the water.
This bubble-mediated gas transfer favours CO2 uptake much more strongly than outgassing – an asymmetric effect that had previously been hypothesised but never directly demonstrated using field data.
First direct evidence from observations
In the study, an international research team led by Plymouth Marine Laboratory and GEOMAR Helmholtz Centre for Ocean Research Kiel, in collaboration with the Heriot-Watt University, analysed a total of 4,082 hours of high-quality measurements of air–sea CO2 flux measurements. The data were collected during 17 research cruises across a wide range of ocean regions.
Using a newly developed two-dimensional analysis method, the researchers were able for the first time to demonstrate directly from observational data that air–sea gas exchange is indeed asymmetric. Based on this approach, the team recalculated global air–sea CO2 fluxes for the period from 1991 to 2020. The result: on average, the global ocean absorbed around 0.3 to 0.4 petagrams more carbon per year – about 15 per cent more than previous estimates.
Particularly strong effect in the Southern Ocean
The effect varies regionally. The additional CO2 uptake is especially pronounced in regions with frequent strong winds and breaking waves, such as the Southern Ocean – an area where some of the most severe impacts of climate change are already being observed.
Seasonal differences also play a role: during winter, when storms are more frequent, the asymmetric effect becomes even stronger. Overall, the revised calculations significantly increase the fraction of the ocean surface that acts as a net sink for CO2.
Implications for climate models and the global carbon budget
The findings suggest that the ocean’s role as a CO2 sink has so far been underestimated. At the same time, the gap between observation-based estimates and the results of many global climate models becomes even more apparent.
“We therefore strongly advocate that future CO2 flux assessments should adopt the asymmetric formulation,” says Yuanxu Dong. Co-author Dr Ming-Xi Yang, chemical oceanographer at the Plymouth Marine Laboratory, adds: “Accounting for this asymmetry means that ocean CO2 flux estimates derived from observations diverge even further from those produced by global models. This points to shortcomings in the models – and these models need to be as realistic as possible in order to make reliable future CO2 and climate projections.”
Open questions and future research needs
Despite the robustness of the results, the researchers emphasise that uncertainties remain. In particular, measurements of CO2 outgassing under extreme wind and wave conditions are still scarce, as they are technically challenging to obtain. Additional data are needed to further constrain the asymmetric effect and to better integrate it into global models.
What is already clear, however, is that the ocean plays an even more important role in the climate system than previously assumed – and that seemingly small processes, such as air bubbles in breaking waves, can have global consequences.
Dong, Y., Yang, M., Bell, T.G. et al., Woolf, D.K., Marandino C.A. Asymmetric bubble-mediated gas transfer enhances global ocean CO2 uptake. Nat Commun 16, 10595 (2025)
https://doi.org/10.1038/s41467-025-66652-5
https://www.geomar.de/n10128 – images for download
https://pml.ac.uk/news/how-bubbles-may-speed-up-co2-uptake-by-the-ocean/ Original press release on the Plymouth Marine Laboratory website
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