Simulated Earth system response to acid downwelling as a form of ocean alkalinity enhancement

Abstract ‘Acid downwelling’ (AD) is a proposed marine carbon dioxide removal (CDR) method, which describes the idea of electrochemically splitting open ocean surface water into an alkaline solution to remain at the surface ocean and cause additional ocean CO 2 uptake, and into an acidic solution that is pumped down into the deep ocean for disposal via vertical pipes. In this study, we simulate idealized large-scale AD in an Earth system model of intermediate complexity with different acid injection depths and downwelling intensities and find a maximum marine CDR (mCDR) potential for continuous AD (0.25 Pmol yr −1 ) of 320 Pg C until the end of the millennium under an extended RCP 4.5 CO 2 emissions scenario. However, the acidity temporarily stored at depth resurfaces primarily around the Southern Ocean via ocean circulation and causes regional CO 2 outgassing. Furthermore, too intense downwelling of warm surface water leads to an increase in ocean interior temperatures causing further Earth system feedbacks and accelerates the re-emergence of downwelled acidity to the surface. However, the extent to which this re-emergence causes CO 2 outgassing into the atmosphere is emissions scenario dependent. This study highlights that large-scale ocean circulation, the investigated time frame, and the future CO 2 emission scenario all need to be considered in order to determine the mCDR potential of AD.