| Summary: | The oceans are acidifying in response to the oceanic uptake of anthropogenic CO2 from the atmosphere, yet the global-scale progression of this acidification has been poorly documented so far by observations. Here, we fill this gap and use an observation-based product, OceanSODA-ETHZ, to determine the trends and drivers of the surface ocean aragonite saturation state (Ωar) and pH over the last four decades (1982-2021). In the global mean, Ωar and pH declined at rates of -0.071 ± 0.001 decade-1 and -0.0170 ± 0.0001 decade-1, respectively. These trends are driven primarily by the increase in the surface ocean concentration of dissolved inorganic carbon (DIC) in response to the uptake of anthropogenic CO2 but moderated by changes in natural DIC. Surface warming enhances the decrease in pH, accounting for ∼15% of the global trend. Substantial ENSO-driven interannual variability is superimposed on these trends, with Ωar showing greater variability than pH.
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