Using the Seasonal Cycle in the Ocean Carbonate System for Monitoring and Guiding OAE

One promising method of marine carbon dioxide removal is ocean alkalinity enhancement (OAE), which has the potential to sequester CO2 on timescales of up to hundreds of thousands of years (Oschlies et al. 2023). In order to quantify the potential contribution of OAE towards net zero targets, a comprehensive monitoring, reporting and verification (MRV) network is required. However, a significant challenge for MRV is the weak signal of OAE compared to natural variability of the marine carbonate system (Ho et al. 2023), including a seasonal cycle in pCO2 which is expected to amplify with ongoing climate change (Gallego et al. 2018). For this reason, in-situ observations must be complemented by modelling studies.

The seasonal cycle in the marine carbonate system has been shown to influence whether a region is a source or sink of CO2 (Fassbender et al. 2018), and has been shown to modulate detectability of alkalinity variations due to OAE (Wang et al. 2024). We use a coupled Earth system model with an ocean biogeochemistry component to explore the impact that OAE has on the seasonal cycle of the marine carbonate system and the implications this has for MRV. We simulate continuous alkalinity addition along the exposed coastlines of the North Sea/North Atlantic, and in the Chinese Exclusive Economic Zone. For each simulation, we quantify the change in the seasonal cycle in the region of alkalinity addition, the regions where the seasonal cycle is most strongly affected, and the time it takes for the signal of changing seasonality to emerge. Thus, we can consider whether the impact of OAE on seasonality in the carbonate system can be used to: enhance detectability of OAE; to define optimal timing of alkalinity addition; or to provide further guardrails for implementation based on the environmental or ecological impact of the changing seasonal cycle.