Study Reveals Marine Carbon Dioxide Removal Methods May Exacerbate Oxygen Loss

An international research team led by Professor Dr. Andreas Oschlies from the GEOMAR Helmholtz Center for Ocean Research Kiel has raised alarm over the potential negative impacts of marine carbon dioxide removal (mCDR) methods on ocean oxygen levels. Published on June 14, 2025, in the journal Environmental Research Letters, the study suggests that certain approaches aimed at enhancing the ocean's capacity to absorb carbon dioxide (CO₂) could significantly worsen ocean deoxygenation, a phenomenon that has seen approximately 2% of the ocean's oxygen inventory lost over the past decades due to global warming.

The significance of this study is underscored by the ongoing climate crisis, where the ocean plays a crucial role in the global carbon cycle. As a natural reservoir for carbon dioxide, the ocean's ability to sequester CO₂ is vital for mitigating climate change; however, the health of this ecosystem is increasingly compromised. The research posits that methods, particularly those based on biological processes such as ocean fertilization and macroalgae farming, could lead to oxygen depletion at levels 4 to 40 times greater than the oxygen gains expected from reduced global warming.

The methodology employed by the researchers involved idealized global model simulations to assess the direct and indirect impacts of various mCDR strategies on ocean oxygen levels. The study identifies ocean fertilization, large-scale macroalgae farming followed by biomass sinking, and artificial upwelling of nutrient-rich deep water as particularly concerning due to their reliance on enhancing photosynthetic biomass production. This process subsequently leads to oxygen-consuming decomposition, which could offset any climate benefits anticipated from these interventions.

In contrast, the research found that geochemical mCDR methods, such as ocean alkalinity enhancement through the addition of alkaline substances, have minimal adverse effects on ocean oxygen levels and are comparable to simply reducing CO₂ emissions. The only method that appeared to result in an overall increase in oceanic oxygen levels was large-scale macroalgae farming coupled with biomass harvesting, which, when performed appropriately, could potentially reverse past oxygen losses by providing up to ten times more oxygen than has been lost due to climate change within a century.

Given these findings, the researchers advocate for the systematic inclusion of oxygen monitoring in all future mCDR research and deployment efforts, emphasizing that any interventions made should not further threaten marine ecosystems already under pressure. This aligns with calls from various international organizations, including the UNESCO Global Ocean Oxygen Network (GO2NE), to ensure that marine life’s needs are prioritized in climate strategies.

The implications of these findings are profound, as they highlight the complex interplay between climate mitigation strategies and marine health, urging policymakers to carefully consider the ecological consequences of interventions designed to combat climate change. As the world grapples with the realities of climate change, the necessity for sustainable and environmentally sound approaches to carbon dioxide removal becomes increasingly critical. The study serves as a vital reminder that actions aimed at addressing one environmental issue may inadvertently exacerbate another, necessitating a more integrated approach to ocean and climate science.