Abstract
The ocean’s chemistry is changing due to the uptake of anthropogenic carbon dioxide (CO2). This phenomenon, commonly referred to as “Ocean Acidification”, is endangering coral reefs and the broader marine ecosystems. In this study, we combine a recent observational seawater CO2 data product, i.e., the 6th version of the Surface Ocean CO2 Atlas (1991–2018, ~23 million observations), with temporal trends at individual locations of the global ocean from a robust Earth System Model to provide a high-resolution regionally varying view of global surface ocean pH and the Revelle Factor. The climatology extends from the pre-Industrial era (1750 C.E.) to the end of this century under historical atmospheric CO2 concentrations (pre-2005) and the Representative Concentrations Pathways (post-2005) of the Intergovernmental Panel on Climate Change (IPCC)’s 5th Assessment Report. By linking the modeled pH trends to the observed modern pH distribution, the climatology benefits from recent improvements in both model design and observational data coverage, and is likely to provide improved regional OA trajectories than the model output could alone, therefore, will help guide the regional OA adaptation strategies. We show that air-sea CO2 disequilibrium is the dominant mode of spatial variability for surface pH, and discuss why pH and calcium carbonate mineral saturation states, two important metrics for OA, show contrasting spatial variability.
Highlights
The influences of the two temperature processes on surface ocean pH were quantified by examining seawater at the average global sea surface temperature (SST), salinity, dissolved inorganic carbon (DIC), and Total alkalinity (TA) of 18.35 °C, 34.87, 2020 μmol kg−1, 2306 μmol kg–1 respectively[26]
Seawater CO2 chemistry data needed for the pH and Revelle Factor calculations were extracted from the 6th version of the Surface Ocean CO2 Atlas (SOCATv6, 1991–2018, ~23 million observations)[23]
Silicate and phosphate values for all SOCATv6 stations were extracted from the gridded Global Ocean Data Analysis Project version 2 (GLODAPv2) climatologies[32]
Summary
The influences of the two temperature processes on surface ocean pH (chemical speciation vs. gas exchange) were quantified by examining seawater at the average global SST, salinity, DIC, and TA of 18.35 °C, 34.87, 2020 μmol kg−1, 2306 μmol kg–1 respectively[26]. The impact of the first process, i.e., the temperature dependence of the chemical speciation of seawater CO2 chemistry species, was isolated by assuming constant TA and DIC and varying temperatures of [0 °C:5 °C:30 °C], reflecting variations from the poles to the equator. Hydrogen ion (H+) generation caused by the dissociation of HCO3− can be estimated from the change of carbonate ion concentration ([CO32−]). H+ generation caused by the dissociation of H2O can be estimated from the change of hydroxide ion concentration ([OH−]). H+ consumption caused by borate can be estimated from the change of borate alkalinity [B(OH)4−], and that caused by its reaction with HCO3− can be estimated from the change of aqueous carbon dioxide concentration ([CO2*])
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