Contrasting carbon dioxide removal potential and nutrient feedbacks of simulated ocean alkalinity enhancement and macroalgae afforestation

Alongside cuts to emissions, hundreds of gigatons of carbon dioxide removal (CDR) are likely to be required to limit global warming to below 1.5 °C or 2 °C this century. Ocean alkalinity enhancement (OAE) and macroalgae afforestation have received considerable attention within the portfolio of potential CDR options, but their efficacy and constraints remain uncertain. Here we compare the CDR potential and biogeochemical impacts of OAE and macroalgae afforestation in exclusive economic zones (EEZs) using a global high-resolution ocean biogeochemical model. Globally, our simulations indicate the CDR potential of OAE is more than seven times that of macroalgae afforestation for an equivalent mass of either dissolved olivine or harvested wet macroalgae biomass. This difference is predominately attributable to the respective alkalinity content of olivine and carbon content of wet macroalgae biomass. Accounting for potential nutrient impacts on phytoplankton production increases this disparity between the CDR efficiency of OAE and afforestation, and in both cases can result in regions of negative CDR. EEZs with higher CDR in response to OAE consistently exhibit higher CDR in response to macroalgae afforestation. However, nutrient feedbacks are shown to have different EEZ-specific impacts on phytoplankton net primary production. Our simulations indicate that ∼62% of the CDR flux associated with OAE occurs in the EEZ application regions, decreasing to ∼54% if olivine contains iron and silicate and ∼45% for macroalgae afforestation. This suggests that monitoring, reporting, and verification may be problematic for both techniques, as might the allocation of credits toward nationally determined contributions.