Carbon Dioxide Removal via Ocean Alkalinity Enhancement: Uneven Costs and Optimal Regions

Ocean alkalinity enhancement (OAE) could contribute gigatonne-scale atmospheric CO2 removal, but its feasibility hinges on poorly quantified techno-economic and physical limits. Here we map the global distribution of CO2 removal cost for ship-based OAE with hydrated lime by coupling country-specific lime production supply-chain, optimized ship routing, and spatially resolved carbonate-chemistry model accounting for secondary carbonate precipitation. We find that net CO2 removal spans $115–$500 per tCO2 globally. National cost differences are dominated by land production costs differences driven by national energy systems (e.g electricity and natural gas prices), whereas ocean regional contrasts —cheapest in subpolar and equatorial waters— reflect ocean physics and chemistry differences. We show that coastal carbonate secondary precipitation, Carbon Capture and Storage costs and availability, and existing shipping routes could spatially restrict near-term implementation, and highlight priority regions for monitoring and governance.