Impacts of Simulated Coastal Ocean Alkalinity Enhancement on the Seasonal Cycle of the Air-Sea CO ₂ Flux and Surface Ocean pCO ₂ 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.