Ocean Alkalinity Enhancement efficiency in the North Pacific under influence of the Pacific Decadal Oscillation
Authors: van Langen Rosón Andrea , C. Franco Ana , Bernardello Raffaele , Schwinger Jörg , Keller David , Wey Hao-Wei 
Published: March 2024

Carbon Dioxide Removal (CDR) technologies are imperative for achieving net zero emissions, a crucial feat to meet the 2&#186; target set in the Paris Agreement. Ocean Alkalinity Enhancement (OAE) is a marine CDR technology that consists of increasing the Total Alkalinity (TA) of the ocean by depositing alkaline minerals to ocean surface waters. The increase in TA reduces the sea surface partial pressure of CO2 (pCO2), thereby enhancing oceanic CO2 uptake or reducing oceanic CO2 outgassing. Despite the potential of OAE to reduce atmospheric CO2 concentrations, the realistic implementation of OAE faces substantial impediments, including logistical feasibility and the lack of international ocean governance for its deployment in open waters. To address these obstacles and incentivize the development of a policy framework for OAE, we set forward optimal conditions that maximize the efficiency of OAE in the North Pacific Ocean, leveraging natural climatic variability induced by the Pacific Decadal Oscillation (PDO). The addition of TA at high Dissolved Inorganic Carbon (DIC) concentrations has the potential to induce a stronger decrease in pCO2 than at lower DIC concentrations. Therefore, natural temporal increases in surface DIC concentrations could potentially predispose the system for enhanced OAE efficiency. The PDO induces multi-decadal variations in the carbonate system, with the potential to influence the spatiotemporal variability in OAE efficiency. PDO phases have been shown to be predictable up to a decade ahead, thereby providing a practical indication for logistical planning of OAE deployment. We analyze the influence of the PDO on OAE efficiency in the North Pacific Ocean through four Earth System Model simulations under a high emission scenario (RCP8.5) spanning from 2020 to 2100. Using theoretical CO2 uptake efficiencies, as defined by Tyka et al. (2022) and Renforth and Henderson (2017), we describe how PDO states modulate variability in uptake efficiency via their control on DIC and TA concentrations. Subsequently, we analyze the realized uptake efficiency by contrasting oceanic air-water CO2 fluxes (FCO2) in simulations with continuous and homogenous global OAE deployment against simulations without CDR intervention per unit of added TA. Early results show regional differences in OAE efficiency rates during different PDO phases. During positive PDO phases, theoretical CO2 uptake efficiencies decrease in the Northeast Pacific while increasing in the central Western Pacific, corresponding to respectively lower and higher DIC concentrations. The inverse responses are observed during negative PDO phases. We discern differences between theoretical and realized CO2 uptake efficiencies, indicating the role of additional influential variables. Our study provides new insights into the impact of the PDO on OAE efficiency and the potential to optimize CDR strategies by aligning them with natural climatic variations.