Impacts of Simulated Coastal Ocean Alkalinity Enhancement on the Seasonal Cycle of the Air-Sea CO₂ Flux and Surface Ocean pCO₂ in European Waters under a Low- and a High-Emission Scenario

Abstract. One potentially scalable method to remove CO₂ from the air is ocean alkalinity enhancement (OAE), which works to lower surface ocean partial pressure (pCO₂) and accelerate CO₂ sequestration and durable storage. This study explores how OAE might affect the seasonal carbon cycle, which plays a key role in the ocean’s annual CO₂ uptake. By analysing earth system model simulations of OAE implemented continuously at the European coastline until 2100 under low and high climate forcing, it was found that: when carbon cycle seasonality is temperature-driven, a) OAE enhances CO₂ uptake in winter, when it is naturally strongest, and it reduces ocean pCO₂ in summer, when it is naturally highest; b) higher CO₂ emissions increase the sensitivity of the seasonal carbon cycle; c) a region with a shallow bathymetry and well-mixed waters may be ideal for implementing OAE due to fast air-sea CO₂ equilibration.