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The current unprecedented rate of rising atmospheric carbon dioxide (CO2) has contributed to a marked decrease in pH of the surface ocean and has the potential to threaten marine ecosystems. However, the spatial and temporal variability of ocean acidification is poorly known due to the lack of long-term instrumental seawater pH records. Accurate prediction of the impacts of ocean acidification on marine biota is also limited by a lack of knowledge on various regional controllers on the variation of seawater pH over decadal to centennial time-scales. Here, we present geochemical proxies of boron and carbon isotopic compositions (δ11B and δ13C) in annual and 5-year resolution in the skeletons of four Porites corals from the southern Great Barrier Reef (GBR) over the past 170 years. The coral δ11B-pH proxy record of reef water shows a 0.21±0.10 pH unit decline for the period 1835-2008. This is accompanied …