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There is increasing discussion of implementing marine CDR (mCDR) approaches, which range from methods based on natural processes to more industrial techniques (NASEM, 2022). Here, we focus on open-​ocean mCDR approaches, including alkalinization (i.e., adding alkaline substances, such as olivine or lime, to seawater to enhance the ocean’s chemical uptake of CO2 from the atmosphere) and nutrient fertilization (i.e., adding a nutrient that limits phytoplankton photosynthesis, such as iron, to surface waters to enhance the photosynthetic uptake of DIC), which aim to enhance DIC sequestration resulting from increased CO2 influx from the atmosphere.

There is a growing body of literature on various aspects of mCDR approaches. Published mCDR studies have addressed the appropriateness of implementation, testing the efficiency of sequestering CO2 and/or assessing detrimental ecological effects (laboratory/mesocosm studies, field trials), and identifying potential deployment sites. Such pilot studies are precursors to possible future mCDR deployments (NASEM, 2022), which should only occur in cases where the pilot studies indicate that mCDR would not unduly disrupt marine ecosystems. In contrast, this paper addresses the situation where mCDRs are to be deployed at scales commensurate with the target of removing gigatons of atmospheric carbon.

Here, considering information from satellites and autonomous platforms combined with artificial intelligence (AI) and models (Figure 1), we describe a future operational monitoring system for the detection, attribution, and determination of side effects of open-​ocean mCDR deployments. We mainly address the monitoring challenge described in NASEM (2022), based upon the current and expected readiness of observational platforms and sensors. This approach ensures that the proposed monitoring system would be tractable and deployable. The assessment of future mCDR deployments will include three components, together referred to as MRV: measurement or monitoring (M) as described in this study, reporting (R) of the resulting data to a certified authority, and verification (V) by this authority, using data and models, that any deployment is successful at increasing CO2 influx from the atmosphere and enhancing its sequestration in the ocean. Successful verification of removal and sequestration will result in certification of the mCDR. The last two MRV components are mentioned in the last section of the study.