On the Efficiency and Durability of Ocean Iron Fertilization as a Carbon Dioxide Removal Solution

Ocean iron fertilization (OIF) is thought to be a promising CO2 removal (CDR) methodology that can contribute to net negative carbon emission goals. In principle, adding iron to iron-depleted regions of the ocean enhances ocean productivity and the export of organic matter to depth allowing atmospheric CO2 to invade into the ocean. However, if no carbon is buried on the seafloor, OIF carbon sequestration will be temporary. Furthermore, ocean mixing and subduction of the perturbed dissolved inorganic carbon (DIC) distribution and the relatively slow rates of air-sea gas exchange can alter the efficiency of OIF atmospheric CO2 uptake. To investigate these processes, we assess the efficiency and durability of OIF CDR using an ocean circulation model coupled to a well-mixed atmosphere. We find that for a typical export flux profile, OIF will be inefficient, with a global mean efficiency relative to direct air capture (DAC) of 24% after 20 years. The inefficiency is driven by ocean mixing and subduction of the DIC perturbation and slow rates of air-sea CO2 exchange. Deeper penetrating export flux profiles will increase OIF efficiency substantially and global mean efficiencies can approach ~50%. Nonetheless, OIF CDR is durable and most of the CO2 sequestered after 20 years will remain in the ocean for >100 years. Our work demonstrates that OIF efficiency and durability are determined by a combination of ocean circulation and mixing, the rate of gas exchange and the depth to which the carbon is exported before being remineralized.