Atmospheric CO2 removal by alkalinity enhancement in the North Sea

Increased ocean alkalinity reduces the activity of CO₂ in seawater and prompts an enhanced flux of CO₂ from the atmosphere into the ocean, thereby provides a promising means to reduce the atmospheric CO₂ concentrations. However, due to the high complexity of physical and biological processes in coastal waters, the possible effects of coastal alkalinity enhancement (AE) are unclear yet. We thereby aim to set up a model framework to simulate the carbon cycles in the North Sea, which further allows scenario studies to disentangle the efficiency of various forms of coastal AE measures as well as their side effects and ecosystem impacts on the Northwest European Shelf (NWES) system. In two scenarios, the same quota of alkalinity is added into two designated areas, the European coast and the middle North Sea along with the ship tracks, respectively. The alkalinity is distributed continuously and evenly into the two areas. Our results indicate that the North Sea is quickly adjusted to both AE deployments, given that the AE efficiency shows no significant trend since the second year of these deployments. Seasonally, AE is more efficient in wintertime. The efficiency reaches the lowest level in spring, implying that the ocean uptake of the atmospheric CO₂ is dominated by biological processes during this season. This modelling assessment will serve as a guide for coastal management and policy making that allows reconciling the application of Carbon Dioxide Removal (CDR) techniques with the maintenance of a good environmental status. It thereby offers an important yet unprecedented case study for a regional to local CDR deployment in the proximal coastal ocean of a temperate shelf sea.