Marine carbon dioxide removal (mCDR) implementations can potentially remove 1–15 Gt CO2 yr−1, which encompasses the minimum requirement removal of CDR in 2050 to limit warming below 2 °C by 2100. Despite their potential, mCDR represents only a tiny fraction of currently deployed CDR methods. Their implementations require not only a comprehensive understanding of the marine processes involved, but also a robust monitoring for their effectiveness and potential environmental impacts. In this work, we surveyed the mCDR scientific literature to identify measurable indicators for quantitative information across different ecosystems and methods used for monitoring the carbon removed and environmental side-effects, and explore the main common challenges. We followed the preferred reporting items for systematic reviews and meta-analysis protocol PRISMA: the search queries were established, and the literature search and criteria for inclusion/exclusion were transparently defined. Our results reveal the lack of a direct linkage from the mCDR-induced rates of chemical and biological changes to the amount of carbon removed from the atmosphere and the magnitude of associated environmental side-effects. The heterogeneity of marine biogeochemical and ecological processes in time and space, together with the propagation of biogeochemical signals in an open system, represent the most common challenge reported for the monitoring approaches. Our results show that we are in need of indicators that provide reliable quantification of CO2 removal rates and information on environmental side-effects. Finally, based on our gap analysis, we provide monitoring recommendations, such as the use of common metrics and the baseline establishment. Currently, the lack of standardized indicators and monitoring procedures inhibits the verification, and hence, creates risks for further investment and prevents the entering of mCDRs in existing carbon certificate trading systems, hindering the long term growth of this sector.

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