The invasion of anthropogenicCO2 induces acidification of surface water masses, providing a fifth quantitative proof of its oceanic sequestration. AtmosphericCO2 dissolves in the ocean to form aqueousCO2, whose hydration leads to the formation of carbonic acid. This weak diacid exhibits two dissociations producing hydronium ions (H3O+), as well as bicarbonate ions (HCO3- ≈ 89 %) and carbonates (CO32- ≈ 11 %).
Seawater is a buffer solution with limited pH variations: at the surface, its value varies from 8 to 8.3 (or even 8.5 in cases of strong photosynthesis by phytoplankton), while at depth the pH drops to 7.4 between 1,000 and 2,000 m where bacteria remineralize organic matter. At the surface, plankton organisms also modify pH levels through the synthesis of organic matter, which consumes H3O+ ions, and the precipitation of calcium carbonate, which produces them by shifting acid-base balances. This biological synthesis therefore pumpsCO2 from the atmosphere into the ocean (the organic pump) and releasesCO2 from the ocean into the atmosphere (the " counter-pump " of limestone, i.e. solid carbonate).
Thousands of measurements of TCO2 and surface water alkalinity have enabled us to map geographical gradients in pH, with minima in the equatorial zone, and maxima at high latitudes and in highly productive coastal zones. Since the 1990s, direct pH measurements with seasonal resolution have been available for stations in Hawaii, Bermuda and the Canary Islands. All three time series show a similar progressive decrease, equivalent to around 0.02 pH units per decade. Thanks to the estimates of anthropogenicCO2 invasion described above, it is possible to estimate that mean pH has decreased by around 0.1 units since the beginning of the industrial era, which is in line with the extrapolation of the current pH decline.
