Impacts of Simulated Coastal Ocean Alkalinity Enhancement on the Seasonal Cycle of CO2 Air-Sea Gas Exchange and ocean pCO2 in European Waters under Low and High Emission Scenarios

To reach climate neutrality, emission reduction must be complemented by carbon dioxide removal technologies aiming to sequester atmospheric CO₂ and store it in permanent natural reservoirs. The ocean, which already sequesters roughly a quarter of all anthropogenic CO₂ emissions annually, can play a crucial role in this effort. By storing carbon in forms that are not readily exchanged with the atmosphere, it acts as a vast and long-lasting carbon reservoir on a human-relevant timeframe. 

This potential has spurred growing interest in the development and deployment of ocean-based carbon dioxide removal technologies. One potentially scalable method is ocean alkalinity enhancement (OAE), which is performed by applying alkaline mineral rocks or solutions at the ocean surface to lower its CO₂ partial pressure (pCO₂) and accelerate CO₂ sequestration and storage as bicarbonate and carbonate ions. 

For large-scale application, it is crucial to understand the potential earth system feedbacks generated by alkalinity addition, considering both the space and time dimension. Spatially, coastal alkalinity addition was investigated, as it is more feasible from a political and logistical standpoint. Temporally, as seasonality is a fundamental component of the ocean net annual CO₂ uptake, attention was given to the changes to the seasonal CO₂ flux and ocean pCO₂ cycle. Additionally, different background climate scenarios were considered to assess whether varying levels of warming influence seasonal variations of OAE-induced ocean uptake. 

OAE was performed at the European coastline using an earth system model in emission-driven mode, with a low and a high climate change forcing (SSP1-2.6 and SSP3-7.0, respectively, following CMIP6 guidelines). No-OAE simulations were performed as baseline reference including climate change forcing. Alkalinity was applied continuously in the form of calcium hydroxide (Ca(OH)₂) at the first ocean layer. Between 2025 and 2035, the alkalinity flux was increased linearly until the equivalent of 1Gt yr-1 (equal to 27 Tmol yr-1) was reached, then maintained constant until the year 2100. 

Results found that: a) with alkalinity addition, the ocean CO₂ seasonal cycle is dampened due to the decreased sensitivity of an alkalinised ocean to CO₂ fluctuations; b) the CO₂ seasonal flux into the ocean is amplified given the larger pCO₂ imbalance at the air-sea interface; c) while the ocean pCO₂ seasonal amplitude reduction is stronger under low warming, the CO₂ flux seasonal amplification is stronger in the high warming pathway. 

This project has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement no. 101056939 (RESCUE)