Challenges of efficiency calculations for OAE in the Earth System Model FOCI

The largest driver of future climate will be emissions of greenhouse gases and aerosols from human activity. With ongoing climate warming, the task of reducing global emissions is becoming increasingly pressing. To reach net zero emissions, strong emission reduction is needed, however there are emissions that will be hard to avoid. These need to be compensated for with new technologies. Ocean Alkalinity Enhancement (OAE) is a method for ocean-based Carbon Dioxide Removal (CDR) and describes the sequestration of CO₂ from the atmosphere through the deliberate increase of ocean alkalinity. There are multiple definitions for the carbon uptake efficiency of OAE. In this study, we run experiments with the FOCI Earth system model with alkalinity additions at eight different sections of the European coast. Based on these experiments and a reference simulation without alkalinity enhancement, we use three different metrics to calculate the CO₂ uptake efficiency based on a) alkalinity and ocean carbon inventories, b) carbon fluxes through the ocean surface, and c) changes to the global atmospheric CO₂ concentration. We highlight challenges and benefits of each method and put it into context of efficiency evaluation for future OAE application studies.