| 29 Jun 2026
The role of temperature and salinity on ocean alkalinity enhancement performance
Abstract. Ocean alkalinity enhancement (OAE) is a promising carbon dioxide removal approach but its effectiveness is constrained by uncertainties in dissolution kinetics and carbonate precipitation under varying ocean conditions. Here, we systematically quantified the dissolution and net alkalinity delivery of three common OAE feedstocks, i.e., NaHCO3, Ca(OH)2, Mg(OH)2, across 16 temperature–salinity combinations (T = 4, 12, 20, 28 °C and S = 24, 29, 34, 38). Each treatment targeted an alkalinity increase of 500 µmol kg-1 and was monitored over 11 days to track changes in total alkalinity (TA) and dissolved inorganic carbon (DIC). Results showed that NaHCO3 dissolved rapidly and nearly completely under all conditions, reliably delivering the intended alkalinity. Ca(OH)2 delivered high net alkalinity in cold waters, but its effectiveness declined with increasing temperature and salinity. Net TA losses occurred at 28 °C and S ≥ 29, likely driven by secondary CaCO3 precipitation and particle passivation. Mg(OH)2 dissolved more slowly and exhibited strong salinity dependence: near–complete dissolution occurred at S ≤ 34 and lower temperatures, while higher salinity (S = 38) significantly inhibited dissolution, causing net TA losses at higher temperatures. The observed dissolution kinetics, including likely effects of secondary CaCO3 precipitation, were well described by a modified Noyes–Whitney function. A first application suggests that NaHCO3 is the most predictable feedstock across tested global conditions; Ca(OH)2 has more potential in cold environments but loses efficiency when temperature increases; and Mg(OH)2 is kinetically slower and susceptible to high–salinity inhibition. Experiment and global extrapolation imply that the carbon dioxide removal potential and efficiency are highly dependent on the combination of feedstock and environmental control.
Competing interests: JH is consulting for the Planeteers GmbH. All authors declare that they have no competing interests.
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