Ocean carbon cycle feedbacks and the seasonal cycle of the carbonate system under ocean alkalinization

Ocean Alkalinization deliberately modifies the chemistry of the surface ocean to enhance the uptake of atmospheric CO₂. Here, we quantify, using idealized and scenario Earth system model (ESM) simulations, changes in carbon cycle feedbacks and in the seasonal cycle of the surface ocean carbonate system due to ocean alkalinization. We find that both, the sensitivity to changes in atmospheric CO₂ concentration (carbon-concentration feedback) as well as the sensitivity to temperature changes (carbon-climate feedback) are enhanced. While the temperature effect, which decreases ocean carbon uptake, remains small in our model, the carbon-concentration feedback enhances the uptake of carbon due to alkalinization by more than 20% compared to the carbon sequestration that alkalinity addition would facilitate at constant CO₂ levels. This effect depends on the trajectory of atmospheric CO₂ concentration, and leads to an increased loss of carbon from the ocean if net emissions become negative. The seasonal cycle of air-sea CO₂ fluxes is enhanced due to an increased buffer capacity in an alkalinized ocean. The seasonal cycle of H⁺-ion concentration is also enhanced, although it remains smaller than under preindustrial conditions. This, together with an increased seasonal cycle of the aragonite saturation state in some regions, has the potential to adversely affect ecosystem health.