Increasing atmospheric CO2 from man-made climate change is reducing surface ocean pH. Due to limited instrumental measurements and historical pH records in the world's oceans, seawater pH variability at the decadal and centennial scale remains largely unknown and requires documentation. Here we present evidence of striking secular trends of decreasing pH since the late nineteenth century with pronounced interannual to decadal–interdecadal pH variability in the South Pacific Ocean from 1689 to 2011 CE. High-amplitude oceanic pH changes, likely related to atmospheric CO2 uptake and seawater dissolved inorganic carbon fluctuations, reveal a coupled relationship to sea surface temperature variations and highlight the marked influence of El Niño/Southern Oscillation and Interdecadal Pacific Oscillation. We suggest changing surface winds strength and zonal advection processes as the main drivers responsible for regional pH variability up to 1881 CE, followed by the prominent role of anthropogenic CO2 in accelerating the process of ocean acidification.

First author affiliations:1. Institut de recherche pour le développement (IRD). Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN). Université Pierre et Marie Curie (UPMC).2. Centre Nationnal de la Recherche Scientifique (CNRS). Laboratoire des Sciences du Climat et de l'Environnement (LSCE).3. Institut Pierre-Simon Laplace (IPSL)4. Now at Leibniz Centre for Tropical Marine Research (ZMT).Primary funding for this research was supported by the Institute Pierre Simon Laplace (Labex L-IPSL) under the Work Packages 4 and 5, IMPACTS: Project 16 - Impacts of climate change (ocean acidification and warming) on corals, which is funded by The French National Research Agency, ANR (Grant no: ANR-10-LABX-0018).

Supplement to: Wu, Henry C; Dissard, Delphine; Douville, Eric; Blamart, Dominique; Bordier, Louise; Tribollet, Aline; Le Cornec, Florence; Pons-Branchu, Edwige; Dapoigny, Arnaud; Lazareth, Claire E (2018): Surface ocean pH variations since 1689 CE and recent ocean acidification in the tropical South Pacific. Nature Communications, 9(1), 2543