Global warming has led to a significant loss of oxygen in the ocean, with approximately two percent of the ocean's oxygen inventory disappearing over recent decades. This loss has had serious ecological consequences. While climate mitigation measures are expected to help counteract oxygen decline, a new study suggests that many proposed marine carbon dioxide removal (mCDR) methods, especially those based on biological processes, could actually worsen oxygen loss in the ocean.

Prof. Dr. Andreas Oschlies, the lead author of the study and head of the Biogeochemical Modelling research division at GEOMAR, along with an international team from the UNESCO Global Ocean Oxygen Network (GO2NE), conducted a comprehensive assessment using global model simulations. They analyzed the direct impacts of various mCDR approaches on ocean oxygen and their indirect effects through climate mitigation. The results have been published in Environmental Research Letters.

Ocean Fertilization and Seaweed Sinking Among Critical Approaches

The study highlights several biotic mCDR methods as particularly critical, including ocean fertilization, large-scale macroalgae farming with subsequent sinking of biomass, and artificial upwelling of nutrient-rich deep water. These methods involve increasing photosynthetic biomass production, followed by decomposition in the ocean interior, which consumes oxygen at levels comparable to the current rate of global deoxygenation caused by ocean warming.

Oschlies emphasizes that methods increasing biomass production in the ocean, leading to oxygen-consuming decomposition, cannot be considered harmless climate solutions. Model simulations indicate that such approaches could result in a decrease in dissolved oxygen four to 40 times greater than the oxygen gain expected from reduced global warming.

In contrast, geochemical mCDR approaches that do not involve nutrient input, such as ocean alkalinity enhancement through the addition of alkaline substances based on limestone, appear to have minimal effects on ocean oxygen levels, similar to simply reducing CO2 emissions.

Among all methods studied, only large-scale macroalgae farming with biomass harvesting showed an overall increase in oceanic oxygen levels. In this case, no additional oxygen is consumed within the marine environment, and the removal of nutrients limits oxygen consumption elsewhere. Model results suggest that if implemented at a sufficient scale, this approach could even reverse past oxygen losses, providing up to ten times more oxygen than has been lost due to climate change within a century. However, the removal of nutrients could negatively impact biological productivity in the ocean.

Call for Systematic Monitoring of Ocean Oxygen

Based on these findings, the authors recommend the mandatory inclusion of oxygen measurements in all future mCDR research and deployment efforts. They stress the importance of ensuring that large-scale measures do not further threaten marine environmental conditions that marine life relies on.

Background: Carbon Dioxide Removal as Part of Climate Strategy

Despite ambitious climate policies, Germany is projected to emit 10 to 20 percent of today's greenhouse gas levels in three decades, continuing to drive global warming. Carbon dioxide removal (CDR) is being considered to help achieve net-zero emissions. The ocean plays a crucial role in the global carbon cycle due to its natural CO2 uptake and storage capacity. Marine Carbon Dioxide Removal (mCDR) approaches aim to accelerate these natural processes to increase the ocean's carbon uptake capacity.