Ocean acidification

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The change in seawater carbonate chemistry caused by the ocean absorbing a large share of the carbon dioxide emitted to the atmosphere — roughly a quarter to a third of anthropogenic emissions. Dissolved carbon dioxide forms carbonic acid, which dissociates and releases hydrogen ions; those ions react with carbonate ions to form bicarbonate. Two things therefore happen at once: pH falls, and the concentration of the carbonate ion that corals need to build aragonite falls with it. Surface ocean pH has declined by roughly 0.1 units since the pre-industrial period, which on a logarithmic scale is an increase in hydrogen ion concentration of about thirty per cent. For a reef the operative quantity is the aragonite saturation state: as it drops, calcification costs the coral more energy and net reef accretion slows even where no coral has died. Acidification and bleaching share a cause and share nothing else — different timescale, different mechanism, different tissue. A reef can be entirely unbleached and still be losing its carbonate budget.