Seasonal Variability in the Ocean Carbonate System under Ocean Alkalinity Enhancement

Limiting global warming to 1.5°C requires comprehensive strategies that combine robust reductions in emissions with carbon dioxide removal techniques, such as ocean alkalinity enhancement (OAE). Ensuring effective implementation of OAE requires a thorough framework for monitoring, reporting, and verification (MRV). However, the natural variability of partial pressure of carbon dioxide (pCO₂) in the ocean overshadows the changes anticipated from OAE (Ho et al. 2023), and seasonal variability is expected to escalate with ongoing warming (Gallego et al. 2018). This presents a challenge for MRV, and highlights the need to dissect the influences of both warming and OAE on seasonal carbonate chemistry, particularly in areas undergoing OAE. We examine how OAE alters the amplitude of these seasonal shifts compared to a control scenario with no OAE. Using an Earth System Model (FOCI; Matthes et al. 2020; Chien et al. 2022), We compare the impact on seasonal dynamics of CO₂ flux, pCO₂, pH, Alkalinity, and Dissolved Inorganic Carbon (DIC) when OAE is implemented in coastal areas vs open ocean and regions of upwelling vs regions of downwelling, with background emissions following either SSP126 or SSP370. In an example of uniformly and continuously deployed OAE on the European coastline, there is a reduction in the seasonal variance of ocean carbonate chemistry in comparison to the baseline, in both scenarios, for alkalinity, DIC, pH and fCO₂. The amplitude in the seasonal cycle of air-sea CO2 flux is greater when OAE is implemented (66% difference between baseline and OAE scenarios by 2100 following SSP126, 60% following SSP370). Outside of the region where OAE is implemented, there is minimal difference on the amplitude of seasonal fluctuations in CO₂ flux between the baseline and OAE scenarios, implying that in this case, the impacts of OAE are not far-reaching. This has important implications for MRV and national accounting strategies, with influx of CO₂(and therefore air-sea flux) being one way of providing the basis to calculate carbon credits for OAE deployment (Bach et al. 2023).

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Angeles Gallego et al. (2018). Drivers of future seasonal cycle changes in oceanic pCO2. Biogeosciences, 15(17), 5315–5327. https://doi.org/10.5194/bg-15-5315-2018

Matthes, K., et al. (2020). The Flexible Ocean and Climate Infrastructure version 1 (FOCI1): Mean state and variability. Geoscientific Model Development, 13(6), 2533–2568. https://doi.org/10.5194/gmd-13-2533-2020

Chien, C. te et al. (2022). FOCI-MOPS v1 - integration of marine biogeochemistry within the Flexible Ocean and Climate Infrastructure version 1 (FOCI 1) Earth system model. Geoscientific Model Development, 15(15), 5987–6024. https://doi.org/10.5194/gmd-15-5987-2022

Bach, L. T. et al. (2023). Toward a consensus framework to evaluate air–sea CO2 equilibration for marine CO2 removal. Limnology And Oceanography Letters, 8(5), 685–691. https://doi.org/10.1002/lol2.10330