Potential impacts of marine carbon dioxide removal on ocean oxygen

Global warming is the main cause for current ocean deoxygenation. Stopping further warming requires net-zero emissions. Achieving this will, in the foreseeable future, not be possible without the implementation of a portfolio of carbon dioxide removal (CDR) approaches. Given substantial uncertainties about the potential and durability of land-based approaches to deliver sufficient CDR, marine CDR options are receiving more and more interest. A deployment of marine CDR for mitigating global warming could therefore also be viewed as a measure for mitigating ocean deoxygenation if, and only if, the respective CDR measure itself does not lead to a larger oxygen loss than the reduction in atmospheric CO₂, and hence warming, would have caused.

We present the current state of knowledge regarding the potential impacts of various marine CDR options onto ocean oxygen, a key ocean state variable and an essential element for all higher forms of marine life. We show that particularly biotic approaches, such as ocean fertilization, macroalgae cultivation and organic matter dumping, can have significant impacts on ocean oxygen, whereas geochemical approaches may be applied in ways that could have only small additional impacts on dissolved oxygen beyond global warming. We suggest that marine oxygen should be considered in environmental impact assessments of marine CDR field experiments, that the change in oxygen levels should be accounted for in assessing the suitability of mCDR, and that oxygen should be measured prior to, during and after any potential deployment.