Solutions implemented to try to save the climate could asphyxiate the ocean
Article available in Open Access
A scientific study conducted by the GEOMAR Helmholtz Centre for Ocean Research and involving Marilaure Grégoire from the University of Liège, shows that certain methods aimed at improving the uptake of CO2by the oceans to tackle the climate crisis could considerably exacerbate their deoxygenation. Their potential impact on marine oxygen must therefore be systematically taken into account when assessing their relevance.
A
t a time when the fight against climate change is driving the exploration of ever more innovative solutions to capture carbon dioxide (CO₂), a new study reveals an unexpected and worrying consequence: some of these methods could greatly exacerbate the loss of oxygen in the oceans. Published in the scientific journal Environmental Research Letters, this study led by Professor Andreas Oschlies of the GEOMAR research centre (Germany) and involving members of the Global Ocean Oxygen Network (GO2NE) - including Marilaure Grégoire, Professor of Marine Systems Modelling at ULiège - shows that what seems beneficial for the climate is not necessarily harmless for the oceans.
Over the last few decades, the oceans have lost around 2% of their dissolved oxygen, mainly as a result of global warming," explains Andreas Oschlies. This phenomenon, known as deoxygenation, has deleterious effects on marine fauna and threatens the balance of ecosystems. At first sight, it would be logical to think that reducing CO2 emissions or removing this gas from the atmosphere could help restore the balance. But the results we have obtained seriously qualify this hypothesis."
Some methods more harmful than useful
Of the marine CO₂ removal techniques studied - known as mCDR (for marine Carbon Dioxide Removal) - the most problematic are those based on biological processes: ocean fertilisation, the cultivation and massive immersion of algae, or the artificial upwelling of nutrient-rich deep waters.
"These methods stimulate the growth of phytoplankton or macroalgae, which increases the absorption of CO₂," continues Marilaure Gregoire. But once the biomass is dead, its decomposition consumes large quantities of oxygen in the deep sea. The result: a worsening of deoxygenation, up to 44 times greater than the expected benefit in terms of oxygen gained by slowing warming!"
Safer alternatives
Conversely, geochemical approaches, such as enriching the alkalinity of the oceans by adding minerals, appear to have a negligible impact on oxygen. One positive exception among the biological solutions is the cultivation of harvested (rather than sunk) algae, which would increase oxygen levels in the ocean to the point of offsetting the losses accumulated over the last century. However, this process raises other questions, notably about the availability of nutrients and the effects on biodiversity.
As a recommendation, the researchers are calling for oxygen measurement to be systematically included in all experimental or operational mCDR projects. "The ocean is already under great pressure. Before we intervene on a large scale, we need to make sure that we don't exacerbate the situation", warns Professor Oschlies.
Scientific and political issues
This study comes at a time when governments such as Germany are relying increasingly on carbon elimination technologies to achieve carbon neutrality by 2050. But this research highlights the ecological dilemmas of these technological strategies. "Any ocean carbon capture solution will need to be assessed not only on its ability to remove CO₂, but also on its impact on marine oxygen, a vital resource for underwater life, concludes Marilaure Grégoire."
On 30 June 2025, UNESCO's Intergovernmental Commission is organising a science day on the ocean and solutions to combat climate change. Andreas Oschlies and Marilaure Grégoire will be presenting the results of this study and the scientific recommendations for implementing solutions to reoxygenate the oceans. Find out more
* mCDR (marine Carbon Dioxide Removal) is a method of actively removing CO₂ from the atmosphere using the ocean as a support or reservoir.
Scientific reference
Oschlies A., Slomp C. P., Altieri A. H., Gallo N. D., Gregoire M., Isensee K., Levin L. A., & Wu J., Potential impacts of marine carbon dioxide removal on ocean oxygen, June 2025, Environmental Research Letters. This publication is available in open access
