On the Efficiency and Durability of Purposefully Sinking Seaweed Biomass as a Marine Carbon Dioxide Removal Strategy

Abstract Large‐scale farming and purposeful sinking of seaweed has been suggested as a marine Carbon Dioxide Removal (mCDR) strategy. Farmed seaweed uptakes dissolved inorganic carbon (DIC) from the mixed layer, resulting in a CO 2 deficit that causes an influx of atmospheric CO 2 into the surface ocean. The carbon‐rich seaweed is then harvested and conveyed to depth where it is either eventually remineralized back to DIC or incorporated into the sediment. To explore abiotic drivers of seaweed mCDR efficiency and durability, we simulate the advection and mixing of a DIC perturbation through the ocean with a steady‐state global ocean circulation model that includes an interacting atmosphere with realistic air‐sea gas exchange. We find that initially many locations in the global ocean can be nearly as efficient as direct air capture (DAC) and are temporarily durable (sequestered for >100 years) if biomass is conveyed rapidly (≥1,000 m d −1 ) to a depth (≥2,000 m) before it is eventually ventilated back to the atmosphere. Durability timescales and efficiencies are significantly reduced for slower vertical conveyance (