Revisiting the additionality and durability of carbon uptake in large-scale ocean iron fertilization

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Abstract
Climate stabilization pathways limiting global warming to 1.5° to 2°C require rapid emission reductions alongside additional carbon dioxide removal (CDR) of ∼2 to 3 PgC year −1 . Our simulations show that sustained century-scale ocean iron fertilization (OIF) in the southern sector of the Southern Ocean (SSO) provides a contribution of ∼30 PgC, equivalent to ∼0.3 PgC year −1 . This uptake nearly matches the additional carbon uptake from global fertilization. In other iron-limited regions, fertilization-induced uptake is largely offset (70 to 100%) by anomalous outgassing in nonfertilized regions. After terminating OIF following 30 years of fertilization, the ocean retains ∼50% of the additional carbon in the SSO, whereas retention is negligible elsewhere. These results identify the SSO as the most favorable region for durable carbon removal through large-scale OIF, particularly in terms of additionality and durability, although its carbon uptake is smaller than earlier estimates. OIF-induced CDR is constrained by local and remote nutrient depletion, carbon reentrainment, incomplete macronutrient drawdown, and reduced phytoplankton carbon-to-phosphorus ratios. Incomplete macronutrient drawdown reflects the uptake controls by light, top-down controls, and rates of macronutrient resupply. Further insight into OIF potential will be enabled by standardized multimodel comparisons and targeted analysis of nutrient drawdown controls.
Funding Information
  • National Oceanic and Atmospheric Administration (NA23OAR4320198)

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