Study Reveals Potential Risks of Climate Solutions on Ocean Health

A recent study from the Helmholtz Centre for Ocean Research Kiel (GEOMAR) has raised significant concerns regarding the impact of proposed climate solutions on ocean health, particularly regarding oxygen levels in the marine environment. The research, led by Professor Dr. Andreas Oschlies, highlights a troubling paradox: while global warming is responsible for a significant decrease in oceanic oxygen—approximately 2% of the ocean's oxygen inventory has been lost over recent decades—many marine carbon dioxide removal (mCDR) methods may exacerbate this issue instead of alleviating it.

The study, published in the journal Environmental Research Letters on June 14, 2025, employs idealized global model simulations to analyze both the direct impacts of various mCDR approaches on ocean oxygen and their indirect effects through climate mitigation strategies. Oschlies, who heads the Biogeochemical Modelling research division at GEOMAR, asserts, "What helps the climate is not automatically good for the ocean." This statement encapsulates the study's core findings, revealing that certain mCDR methods, especially those reliant on biological processes, could lead to further oxygen depletion in the ocean.

Among the methods scrutinized, ocean fertilization and large-scale macroalgae farming—where biomass is subsequently sunk—emerge as particularly problematic. These approaches aim to enhance photosynthetic production but result in significant oxygen consumption during the decomposition of organic matter in the ocean interior. This process could lead to a decrease in dissolved oxygen levels that is four to 40 times greater than the oxygen gain anticipated from reduced global warming.

Conversely, the study indicates that geochemical mCDR approaches, such as ocean alkalinity enhancement through the addition of alkaline substances, have minimal impacts on ocean oxygen levels and are comparable to merely reducing CO2 emissions. Notably, only large-scale macroalgae farming with biomass harvesting was found to result in an overall increase in oxygen levels in the ocean, although this method poses its own risks by limiting nutrients that are crucial for biological productivity.

Given these findings, the authors advocate for systematic monitoring of ocean oxygen levels in future mCDR research and deployment efforts. "The ocean is a complex system that is already under significant pressure," Oschlies warns. He emphasizes the necessity of ensuring that large-scale interventions do not further threaten marine ecosystems, which are vital for sustaining marine life.

The backdrop to this study is the pressing need for effective climate solutions as nations aim for net-zero emissions. Even with aggressive climate policies, Germany is projected to emit 10 to 20 percent of current greenhouse gas levels in 30 years, perpetuating the challenges of global warming. Marine carbon dioxide removal techniques are seen as pivotal in enhancing the ocean's capacity to absorb carbon, yet their implications for oxygen levels must be carefully considered.

In conclusion, the research underscores the complex interplay between climate solutions and ocean health, urging policymakers and researchers to prioritize comprehensive assessments of mCDR strategies to safeguard marine ecosystems while addressing climate change.