Trends and drivers in global surface ocean pH over the past 3 decades

Abstract. We report global long-term trends in surface ocean pH using a new pH data set computed by combining fCO₂ observations from the Surface Ocean CO₂ Atlas (SOCAT) version 2 with surface alkalinity estimates based on temperature and salinity. Trends were determined over the periods 1981–2011 and 1991–2011 for a set of 17 biomes using a weighted linear least squares method. We observe significant decreases in surface ocean pH in ~70% of all biomes and a mean rate of decrease of 0.0018 ± 0.0004 yr⁻¹ for 1991–2011. We are not able to calculate a global trend for 1981–2011 because too few biomes have enough data for this. In half the biomes, the rate of change is commensurate with the trends expected based on the assumption that the surface ocean pH change is only driven by the surface ocean CO₂ chemistry remaining in a transient equilibrium with the increase in atmospheric CO₂. In the remaining biomes, deviations from such equilibrium may reflect that the trend of surface ocean fCO₂ is not equal to that of the atmosphere, most notably in the equatorial Pacific Ocean, or may reflect changes in the oceanic buffer (Revelle) factor. We conclude that well-planned and long-term sustained observational networks are key to reliably document the ongoing and future changes in ocean carbon chemistry due to anthropogenic forcing.