Revisiting the Additionality and Durability of Carbon Uptake in Large-Scale Ocean Iron Fertilization

Climate stabilization pathways limiting global warming to 1.5-2\(\:℃\) targets require emission reductions with additional carbon dioxide removal (CDR) of ~ 2–3 PgC year ^− 1 . Here, we assess the long-term additionality and durability of carbon uptake from large-scale ocean iron fertilization (OIF) using an Earth System Model under emissions-driven scenarios. Our simulations suggest that sustained century-scale fertilization in the southern sector of the Southern Ocean (SSO) yields a moderate contribution (~ 30PgC, or 0.3 PgC/yr). In other iron-limited regions, the additional carbon uptake induced by fertilization is largely offset (70–100%) by the non-fertilized regions. Terminating OIF after 30 years, the ocean retains 50% of the additional carbon in the SSO, while retention becomes negligible in other regions. Global CDR rates are 2\(\:-\)7 times lower than prior idealized estimates. OIF-induced CDR is constrained by multiple biogeochemical processes: local and remote nutrient depletion, re-entrainment of previously sequestered carbon, incomplete macronutrient drawdown, reduced phytoplankton carbon to phosphorus ratios. Incomplete macronutrient drawdown represented the largest difference from earlier studies, reflecting uptake controls by light, top-down controls, and rates of macronutrient resupply. Further insight into CDR potential requires application of our standardized design across modeling institutions, and targeted analysis of top-down and bottom-up controls on nutrient drawdown.