Efficiency of carbon dioxide removal by ocean alkalinity enhancement via enhanced weathering of mine tailings

Global mitigation commitments which aim to limit global warming to less than 2ºC require dramatic and rapid reductions in atmospheric carbon dioxide (CO₂) over the coming century. Carbon Dioxide Removal (CDR) technologies, whereby CO₂ is actively taken out of the atmosphere and “durably” stored terrestrially, geologically or in the ocean could be employed to help reduce or counter-balance CO₂ emissions to meet national net zero and net negative mitigation targets. Weathering processes would naturally draw atmospheric CO₂ down towards pre-industrial levels over hundreds of thousands of years. One such CDR approach involves accelerating the uptake of CO₂ through “enhanced weathering” (EW). CDR through EW of silicate minerals such as olivine or carbonate minerals can be achieved, for example, by spreading pulverized rocks on soils or employing mine tailings in specialized reactors to increase the weathering rate and hence carbon sequestration on decadal timescales. Here, we explore the efficiency of using mining waste to achieve CDR through EW. We exploit the results of a recent study by Bullock et al. (Science of the Total Environment, 2022) which produced one of the first comprehensive assessments of the global and country level suitability of mine tailings, accounting for reaction kinetics, and their potential for CO₂ drawdown. While the overall CDR from EW of mine tailings is relatively modest, such an approach may still help individual countries meet their net zero goals and it is useful to investigate the broader implications of the deployment of this approach. EW leads to the production of alkalinity and bicarbonate ions (through CDR). We use Bullock et al.’s estimates of the annual generation of these quantities over the coming century to force an ocean biogeochemical model to investigate the impact of the release of these constituents into the ocean on atmospheric CO₂ and ocean chemistry under various emission scenarios. In our simulations, alkalinity and dissolved inorganic carbon (DIC) are injected into the Exclusive Economic Zones of each suitable country or region as identified in Bullock et. al (2022). We examine in particular the competing effects of alkalinity (which increases CO₂ solubility) and outgassing of CO₂ (both due to the injection of DIC and reduction of atmospheric CO₂) on CDR and find that the latter can substantially reduce the efficiency of carbon dioxide removal (by as much as 25% in the lowest emission scenario).