Enhancing the ocean's ability to absorb carbon dioxide (CO₂) is a strategy being explored to combat the climate crisis. However, certain methods may worsen the loss of oxygen in marine environments. It is crucial to consider the impact on marine oxygen levels when evaluating the suitability of these approaches.

An international team of researchers, led by Prof. Dr. Andreas Oschlies from the GEOMAR Helmholtz Center for Ocean Research Kiel, conducted a study to analyze the effects of marine carbon dioxide removal methods on global ocean oxygen levels. The findings, published in the journal Environmental Research Letters, reveal that some proposed methods could exacerbate oxygen loss in the ocean.

Ocean Deoxygenation and Climate Mitigation

The ocean has experienced a significant loss of oxygen due to global warming, with approximately 2% of its oxygen inventory depleted in recent decades. Climate mitigation measures are expected to help counteract this oxygen decline. However, the study suggests that many marine carbon dioxide removal (mCDR) methods, particularly those involving biological processes, could intensify oxygen loss in the ocean.

Prof. Dr. Oschlies emphasizes that what benefits the climate may not necessarily benefit the ocean. The research team used global model simulations to assess the direct and indirect impacts of various mCDR approaches on ocean oxygen levels and climate mitigation efforts.

Critical Approaches and Findings

The study identifies ocean fertilization, large-scale macroalgae farming with biomass sinking, and artificial upwelling of nutrient-rich deep water as particularly critical biotic mCDR methods. These methods involve enhancing biomass production through photosynthesis, followed by decomposition in the ocean interior, which consumes oxygen at rates comparable to current global deoxygenation caused by ocean warming.

Geochemical mCDR approaches, such as ocean alkalinity enhancement, show minimal effects on ocean oxygen levels compared to reducing CO₂ emissions. Large-scale macroalgae farming with biomass harvesting is the only method that resulted in an overall increase in oceanic oxygen levels, potentially reversing past oxygen losses if deployed at a sufficient scale.

Call for Monitoring and Climate Strategy

The study calls for systematic monitoring of ocean oxygen levels in all future mCDR research and deployment efforts. Prof. Dr. Oschlies stresses the importance of protecting marine environmental conditions while implementing large-scale measures to combat climate change.

As part of climate strategy, carbon dioxide removal (CDR) is being considered to achieve net-zero emissions. Marine carbon dioxide removal (mCDR) approaches aim to accelerate natural processes in the ocean to increase its carbon uptake capacity.

Source: Phys.org