Environmental impacts and carbon capture potential of ocean alkalinity enhancement

Abstract Minimizing anthropogenic climate disruption in the coming century will likely require both deep and rapid cuts to greenhouse gas emissions and carbon dioxide removal (CDR) from Earth’s atmosphere. Ocean alkalinity enhancement — the modification of surface ocean chemistry to drive marine uptake of atmospheric CO2 — is seen as a potentially significant component of ocean-based CDR portfolios. However, there has been limited mechanistic exploration of the large-scale CDR potential of ocean alkalinity enhancement, potential bottlenecks in alkalinity release, and the biophysical impacts of alkaline mineral feedstocks on marine ecology and the ocean’s biological carbon pump. Here we use a hierarchy of models to evaluate the CDR potential and environmental impacts of ocean alkalinity enhancement using mineral feedstocks. We find that natural alkalinity sources lead to very low CDR efficiency while strongly perturbing food quality and fecal pellet production by marine zooplankton. Artificial alkalinity sources are potentially capable of significant CDR with reduced environmental impact, but their deployment at scale faces major challenges associated with substrate limitation and process CO2 emissions during feedstock production. Taken together, our results highlight distinct challenges for ocean alkalinity enhancement as a CDR strategy and indicate that mineral-based ocean alkalinity enhancement should be pursued with caution.