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.