Decline in surface ocean pH at the ALOHA station and global annual mean surface seawater pH (Copernicus Marine Service)

ALOHA station: A decline in pH corresponds to an increase in the acidity of seawater. Data originate from the ALOHA station pH time series in the central North Pacific, one of the world's longest continuous ocean acidification monitoring records (adapted from Dore, J.E., et al., 2009, 'Physical and biogeochemical modulation of ocean acidification in the central North Pacific', Proceedings of the National Academy of Sciences of the United States of America 106:12235-12240). Changes observed at ALOHA are similar to those observed over a shorter time frame in European seas. In the figure, "In situ measurement (Aloha station)" corresponds to pH measured directly in seawater samples, while "Calculated (Aloha station)" corresponds to pH calculated from dissolved inorganic carbon and total alkalinity measurements at in situ temperature.

CMEMS: Global mean surface ocean pH from the Copernicus Marine Service, based on a reconstruction approach combining in situ observations, satellite-derived data and empirical relationships. The indicator is available annually from 1985 to 2024. Trend and uncertainty are calculated as the slope and residual standard deviation of a linear least-squares regression.