Abstract

Mitigating global climate change will require one gigaton scale carbon dioxide removal (CDR) to complement rapid emission reductions. Oceans cover 71% of the Earth’s surface and have the potential to provide much of the required CDR. However, none of the proposed marine CDR methods (mCDRs) are sufficiently well understood to determine their real-world effectiveness and environmental side effects. Here, we argue that the use of natural mCDR analogs should become the third pillar of interconnection in the mCDR assessment as they bridge the gap between numerical simulations (i.e., large scale / reduced complexity ) and experimental studies (i.e., small scale / high complexity). Natural analogues of mCDR are freely available, can provide a wealth of data to inform mCDR, and do not require legal authorization or social license for their study. We propose four simple criteria for identifying particularly useful analogs: 1) large scale, 2) abruptness of disturbance, 3) availability of undisturbed control sites, and 4) reappearance. Based on these criteria, we highlight four examples: 1) equatorial upwelling as a natural analogue for artificial upwelling, 2) downstream from Kerguelen Island for oceanic iron fertilization, 3) seas Black and Caspian for the increase in alkalinity of the oceans, and 4) the Great Atlantic Sargassum Belt for reforestation of the oceans. These natural analogues provide a reality check for experimental evaluations and numerical modeling of mCDR. Ultimately, projections of the effectiveness and sustainability of the mCDR supported by observations of natural analogues will provide the real context for public debate and facilitate policy decisions on the implementation of the mCDR. We anticipate that a rigorous investigation of natural analogues will expedite the urgent evaluation of mCDR.