The Verification Challenge of Marine Carbon Dioxide Removal

It is increasingly obvious that, even when reaching net-zero emissions, removal of anthropogenic CO₂ from the atmosphere will be required. Some ocean-based removal technologies, while not proven for routine operation at scale, show promise. All of these rely on inducing a flux of CO₂ from the atmosphere into the ocean that is directly attributable to the removal intervention. Crucial for the economic viability of these technologies is the quantification of the cumulative net air-sea flux of CO₂ that an intervention can verifiably deliver. Because this flux is the difference between a realistic case with and a hypothetical case without intervention, it cannot be determined by observation alone-one must rely on a combination of informative observations and skillful models. Major uncertainties in the quantification of net CO₂ uptake include the removal of seawater with a dissolved inorganic carbon deficit from direct contact with the atmosphere and the inevitable rebalancing of carbon among Earth's mobile carbon pools.