Compiled by the editorial desk with reference to the study published in Environmental Research Letters and statements from the lead author, Andreas Oschlies.

As global temperatures continue to climb and emissions remain at record highs, scientists are investigating large-scale interventions to mitigate climate change. Among the most discussed is marine carbon dioxide removal (mCDR), which leverages the ocean's natural capacity as a carbon sink. However, a new study published in Environmental Research Letters cautions that some of these methods could inadvertently worsen ocean deoxygenation, threatening marine life.

The research, led by Andreas Oschlies of the GEOMAR Helmholtz Center for Ocean Research Kiel, examined the potential side effects of various mCDR approaches. The team's models indicate that biotic methods—those that stimulate biological productivity—could reduce dissolved oxygen levels by 4 to 40 times more than the oxygen gains expected from reduced global warming. This finding challenges the assumption that ocean-based solutions are inherently beneficial.

Why Oxygen Loss Matters

Ocean deoxygenation is already a pressing issue. According to a 2018 study, the ocean has lost nearly 2% of its total dissolved oxygen over the past fifty years—approximately 77 billion metric tons. Warmer waters hold less oxygen, and climate change has accelerated the formation of dead zones, areas where oxygen levels are so low that marine life cannot survive. These zones can span thousands of square miles, forcing species to flee or perish.

The new research highlights that certain mCDR techniques, such as ocean fertilization, could exacerbate this problem. Ocean fertilization involves adding nutrients to the sea to boost phytoplankton growth, which in theory absorbs CO2. However, when these organisms die and sink, bacteria decompose them, consuming additional oxygen in the process. Oschlies noted that methods increasing biomass production and subsequent decomposition cannot be considered harmless climate solutions.

Abiotic Methods Show Promise

Not all mCDR approaches are problematic. The study found that abiotic methods, such as adding limestone or other minerals to seawater to convert CO2 into stable molecules, have minimal impact on oxygen levels. These techniques avoid the biological decomposition cycle that drives oxygen depletion. The researchers emphasize that future research should prioritize assessing the oxygen footprint of any proposed intervention.

Oschlies stressed the importance of a holistic view: "What helps the climate is not automatically good for the ocean." He added that the ocean is already under significant pressure from climate change, and large-scale interventions must not further threaten marine environments. The study calls for a cautious, evidence-based approach to mCDR, ensuring that solutions do not create new problems.

As the world seeks urgent climate action, this research serves as a critical reminder that technological fixes must be evaluated for their full environmental impact. While the ocean offers a potential ally in carbon removal, its delicate balance must be preserved.