Illuminating deep-sea considerations and experimental approaches for mCDR proposals

As society recognizes the urgency of reducing atmospheric CO2 levels, industries and nations are increasingly considering marine carbon dioxide removal (mCDR) in their climate mitigation portfolios. The deep sea (defined as depths below 200 m) is the storage site for removed carbon for most mCDR technologies [1, 2] because, here, carbon is out of contact with the atmosphere on societally relevant timescales (>100 years). However, the deep sea is often treated as a ‘black box’ without sufficient consideration given to deep-sea ecological processes and ecosystem services that may be impacted by mCDR activities (e.g. [2, 3]). The often held ‘out of sight, out of mind’ relationship with the deep sea has previously been used to justify disposal of radioactive, military, and chemical waste in the deep sea [4]. These activities were assumed harmless due to the large and sparsely inhabited nature of the deep sea, and expectations that waste would be permanently removed and that negative impacts would remain in the deep sea and not impact coastal areas or socioeconomic activities.