Changes in variability under projected warming alter ocean acidity extremes

The uptake of anthropogenic CO₂ by the ocean causes ocean acidification, i.e. an increase in ocean acidity ([H⁺]) and a decrease in aragonite saturation state (Ω_A). On top of this secular change, recent studies suggest that the seasonal cycles of [H⁺] and Ω_A will strongly change over the 21^st century because of the nonlinearity in carbonate chemistry, potentially leading to more frequent and intense ocean acidity extreme events. Here, we use daily output from ensemble simulations of a fully coupled Earth system model (GFDL-ESM2M) run with the RCP8.5 and RCP2.6 scenarios to quantify the impact of variability changes in [H⁺] and Ω_A on changes in extreme event characteristics such as frequency, duration, intensity, and volume. Using the preindustrial 99^th (for H⁺) and 1^st (Ω_A) percentile, we isolate the impact of variability changes by subtracting the ensemble-mean shifts from the simulations. The model projects large changes for all [H⁺] extreme event characteristics at the global scale. The number of days with extreme [H⁺] conditions for surface waters quintuples by the end of the 21^st century compared to present day under the RCP8.5 scenario, and the duration and maximal intensity of individual events doubles and triples, respectively. At subsurface, similar but slightly smaller trends are simulated. The volume of individual extreme events in the upper 200m of the water column is projected to quadruple over the 21^st century. Increases in [H⁺] seasonality and to a lower extent daily variability dominate changes in surface [H⁺] extremes, whereas changes in interannual and seasonal variability are more important at depth. In contrast to [H⁺] extremes, the occurrence of low Ω_A extreme events is generally projected to decrease. We also analyzed how marine heatwaves may alter [H⁺] and thereby potentially lead to compound ocean acidity extreme and marine heatwave events. Increases in [H⁺] extremes will increase the risk of severe impacts on marine organisms that are commonly adapted to more stable environments.