Four Decades of Trends and Drivers of Global Surface Ocean Acidification

Abstract The oceans are acidifying in response to the oceanic uptake of anthropogenic carbon dioxide (CO 2 ) 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 updated version of the in situ and satellite observation‐based product OceanSODA‐ETHZ to determine the trends and drivers of the surface ocean aragonite saturation state (Ω ar ) and pH = –log(H + ) (total scale) over the last four decades (1982–2021). In the global mean, Ω ar and pH declined at rates of −0.071 ± 0.006 decade −1 and −0.0166 ± 0.0010 decade −1 , respectively, with the errors of the trends largely reflecting the uncertainties in the reconstructed pH and Ω ar fields. These global mean trends are driven primarily by the increase in surface ocean concentration of dissolved inorganic carbon (DIC) in response to the uptake of anthropogenic CO 2 , but moderated by changes in natural DIC. Surface warming enhances the decrease in pH, accounting for ∼15% of the global trend. The long‐term trends vary substantially across regions and also differ distinctly between pH and Ω ar . The highest trends in pH are found in the high latitudes, while Ω ar decreases the fastest in the low latitudes. These regional differences are primarily a consequence of regional differences in the ability of the surface ocean to take up and buffer the anthropogenic CO 2 . Substantial El Niño‐driven interannual variability is superimposed on these trends, with Ω ar showing greater variability than pH, resulting in substantially longer time of emergence for Ω ar .