A new study warns that some marine carbon dioxide removal (mCDR) strategies could unintentionally worsen oxygen depletion in the world’s oceans — posing a serious threat to marine life.
Published in Environmental Research Letters, the research led by Prof. Dr. Andreas Oschlies from the GEOMAR Helmholtz Center in Germany emphasizes that while mCDR methods aim to combat climate change, they may come at a hidden ecological cost.
Oceans have already lost about 2% of their oxygen content over the last several decades, a trend linked to rising global temperatures. As climate change continues, further oxygen loss is expected, putting additional stress on marine ecosystems, and the new study suggests some CDR methods may actually intensify this issue.
Using global climate models, Oschlies and his team analyzed how various mCDR techniques — especially those that boost ocean biomass — affect oxygen levels. Their findings raise red flags about certain biological approaches.
Techniques like ocean fertilization, artificial upwelling, and large-scale seaweed farming are designed to stimulate the growth of photosynthetic organisms that absorb CO2.
However, once this biomass dies and decomposes, the process consumes large amounts of oxygen, potentially worsening existing low-oxygen zones. According to the study, the oxygen loss from some of these methods could exceed their climate benefits by up to 40 times.
In contrast, geochemical solutions — such as enhancing ocean alkalinity by adding limestone-based substances — appear to have little effect on oxygen levels.
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These methods don’t rely on nutrient stimulation or biomass production, making them less likely to disrupt marine oxygen balance, however their long-term impact still needs thorough evaluation.
One approach that showed potential in the study was macroalgae farming with active biomass removal. When seaweed is harvested rather than left to decompose in the ocean, it may help recover oxygen lost due to climate change.
Model projections indicate this technique could replenish up to 10 times the amount of oxygen lost over a century. However, the researchers also caution that large-scale implementation could impact ocean ecosystems in other ways and must be assessed carefully.
The study underscores a crucial point: carbon removal strategies should be scrutinized not just for their climate benefits but also for their ecological side effects.
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