Investigating the Carbon Dioxide Removal (CDR) potential of Acid Downwelling.
Supervisors: Andreas Oschlies, Wolfgang Koeve (GEOMAR)
Reaching anthropogenic net-zero carbon emissions by mid-century requires great effort towards reduction of CO2 emissions and finding long-term solutions to deal with residual CO2 emissions. One proposed marine carbon dioxide removal (mCDR) method is that of Acid Downwelling (AD), where surface ocean water is treated electrochemically to generate an alkaline solution which is then released back into the surface ocean,where it enhances the CO2 uptake capacity of seawater from the atmosphere, thereby reducing atmospheric CO2. However,this electrochemical process also generates an acidic solution as a by product, and AD pumps down this acidity into the interior ocean via artificial downwelling as ameans to store and, subsequently, neutralize the acidity via dissolution of sedimentary CaCO3. AD has already been simulated in an Earth System model in an idealized manner at different depth levels (between1000-3500m).  However, the artificial downwelling component of AD has not been explicitly modelled in these  simulations as the proposed downwelling intensity of 0.015 Sv was considered to be insignificant. Here, we attempt to simulate AD while explicitly modelling artificial downwelling alongwith the changes in alkalinity (increase at the surface and an decrease at depth to represent acidity) and then determine its mCDR potential. AD is modelled using the UVic ESCM version 2.9.in an oL and configuration at three different pipe depths (1000 m, 2000 m and 3000 m) under an R CP4.5 CO2 emissions forcing from the year 2025 to 2100.We also conduct extended experiments until year 2300 with AD stopped at 2100.Our results confirm that a downwelling intensity of 0.015 Sv when modelled,does not cause anysignificant  Earth System feedbacks and them CDR potential of AD is consistent with previous studies. However, we argue that a point source increase of 0.25 P molyr−1 alkalinity at the surface and an equivalent decrease of alkalinity at depth causes extreme changes in local pH and is likely to alter local ecosystems. We suggest that dilute streams can be generated via an increase in the downwelling intensity, however, the increase in downwelling intensity is accompanied by an increase in unwanted Earth Systemfeed backs.