We assess the long-term carbon cycle, climate and ocean acidification response of ocean alkalinity enhancement (OAE) using an emission-driven Earth system model across warming stabilization scenarios. OAE lowers atmospheric CO₂ and produces a scenario-independent linear cooling. Gross carbon capture efficiency remains high, while net efficiencies decline over time due to carbon-cycle feedbacks. Ocean acidification mitigation by OAE is dominated by CO₂ drawdown on long timescales.

Coastal seas play a key role in the global carbon cycle, yet what drives their carbonate chemistry remains poorly understood. We measured pH at high frequency for a year at the Wadden Sea-North Sea interface. Biological activity drives pH changes, while freshwater inflow shapes total alkalinity. Both processes influence dissolved inorganic carbon. The Wadden Sea acts as a net carbon dioxide source to the atmosphere. High-frequency monitoring is essential under a changing climate.