Ocean modeling reveals that real-world alkalinity enhancement produces far smaller chemical perturbations than commonly tested in laboratory experiments.

Reporting by Helen Hill for MITgcm

Ocean alkalinity enhancement (OAE) is attracting growing attention as a carbon dioxide removal strategy because increasing seawater alkalinity enhances the ocean’s capacity to absorb and store atmospheric CO₂. Yet a major challenge for both researchers and regulators is determining whether experimental studies accurately represent the chemical conditions marine ecosystems would experience during real-world OAE deployments. Many laboratory and mesocosm experiments expose organisms to alkalinity perturbations exceeding 1,000 μmol kg⁻¹, but the extent to which such conditions occur outside controlled settings has remained poorly understood.

To address this question, Lennart Bach (Institute for Marine and Antarctic Studies, University of Tasmania) working with Mike Tyka (Google) and colleagues Bin Wang, and Katja Fennel (Department of Oceanography, Dalhousie University) combined a dilution framework with regional and global numerical simulations to quantify the magnitude, duration, and spatial extent of alkalinity perturbations arising from realistic OAE deployments. MITgcm was central to the analysis, providing a physically consistent representation of ocean circulation, mixing, and tracer transport across a range of spatial and temporal scales. The model enabled the authors to follow the fate of added alkalinity as it was dispersed by advection and turbulent mixing, allowing them to estimate the actual exposure histories experienced by marine organisms rather than simply the conditions at the point of release. The paper appears in the Journal of Geophysical Research: Oceans.

The simulations revealed a striking mismatch between commonly studied experimental conditions and plausible real-world exposures. Extremely large perturbations (>1,000 μmol kg⁻¹) were found to occur only immediately after discharge, persisting for minutes and occupying a vanishingly small fraction of the OAE-affected water volume. In contrast, alkalinity anomalies in the range of 1 to 100 μmol kg⁻¹ dominated the overwhelming majority of affected waters as dilution and mixing rapidly dispersed the added alkalinity. The study therefore suggests that environmental assessments focused primarily on very large perturbations may systematically overestimate ecological impacts if those perturbations are interpreted as representative of deployment-scale OAE.

Beyond highlighting a critical scaling issue, the work provides a quantitative framework for connecting laboratory experiments, field trials, and Earth-system-scale carbon removal assessments. By demonstrating how physical transport processes govern organism exposure to altered carbonate chemistry, the study helps establish more realistic targets for future experimental design and improves the scientific basis for evaluating the environmental risks and effectiveness of ocean alkalinity enhancement.

Want to find out more? Email Lennart 

Story image via Pexels

About the Researcher

Lennart T. Bach is a marine biogeochemist at the Institute for Marine and Antarctic Studies, University of Tasmania. His research explores ocean carbon cycling, ocean acidification, and marine carbon dioxide removal, with a focus on understanding the effectiveness and environmental consequences of ocean alkalinity enhancement.

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