While the Pacific Ocean north of ~20°S has the lowest column inventory of anthropogenic carbon dioxide (CO2) outside of the Southern Ocean (Gruber et al. 2019), background dissolved inorganic carbon content is high as a result of respiration that occurs in the ocean’s interior and old “ages” of deep Pacific waters. Consequently, the northern Pacific Ocean, where deep water resurfaces, has naturally steep vertical gradients in buffering capacity (Egleston et al. 2010; Jiang et al. 2019), CO2 fugacity (fCO2; see section 3j), pH—a measure of acidity (Jiang et al. 2019), and calcium carbonate saturation states (Jiang et al. 2015) with many Pacific marine ecosystems vulnerable to ocean acidification impacts (Christian and Ono 2019). In combination with projected deoxygenation, sea surface temperature (SST) change, and a recent propensity for strong, lasting, large-scale marine heatwaves (Bopp et al. 2013; Sidebar 3.1), ocean acidification in the northern Pacific Ocean poses an accelerating threat to marine species and ecosystems that underpin economically important industries from fishing to tourism.