Global‐Scale Evaluation of Coastal Ocean Alkalinity Enhancement in a Fully Coupled Earth System Model

Abstract The Paris Agreement plans for “net‐zero” carbon dioxide (CO 2 ) emissions during the second half of the 21st century. However, reducing emissions from some sectors is challenging, and “net‐zero” permits carbon dioxide removal (CDR) activities. One CDR scheme is ocean alkalinity enhancement (OAE), which proposes dissolving basic minerals into seawater to increase its buffering capacity for CO 2 . While modeling studies have often investigated OAE at basin or global scale, some proposals focus on readily accessible coastal shelves, with TA added through the dissolution of seafloor olivine sands. Critically, by settling and dissolving sands on shallow seafloors, this retains the added TA in near‐surface waters in direct contact with atmospheric CO 2 . To investigate this, we add dissolved TA at a rate of ∼29 Teq y −1 to the global shelves (<100m) of an Earth system model (UKESM1) running a high emissions scenario. As UKESM1 is fully coupled, wider effects of OAE‐mediated increase in ocean CO 2 uptake –e.g. atmospheric xCO 2 , air temperature and marine pH– are fully quantified. Applying OAE from 2020 to 2100 decreases atmospheric xCO 2 ∼10 ppm, and increases air‐to‐sea CO 2 uptake ∼8%. In‐line with other studies, CO 2 uptake per unit of TA added occurs at a rate of ∼0.8 mol C (mol TA) −1 . Significantly for monitoring, advection of added TA results in ∼50% of CO 2 uptake occurring remotely from OAE operations, and the model also exhibits noticeable land carbon reservoir changes. While practical uncertainties and model representation caveats remain, this analysis estimates the effectiveness of this specific OAE scheme to assist with net‐zero planning.