Abstract. Fossil fuel use, cement manufacture and land-use changes are the primary sources of anthropogenic carbon dioxide (CO 2 ) to the atmosphere, with the ocean absorbing approximately 30 % (Sabine et al., 2004). Ocean uptake and chemical equilibration of anthropogenic CO 2 with seawater results in a gradual reduction in seawater pH and saturation states ( ) for calcium carbonate (CaCO 3 ) minerals in a process termed ocean acidification. Assessing the present and future impact of ocean acidification on marine ecosystems requires detection of the multi-decadal rate of change across ocean basins and at ocean time-series sites. Here, we show the longest continuous record of ocean CO 2 changes and ocean acidification in the North Atlantic subtropical gyre near Bermuda from 1983-2011. Dissolved inorganic carbon (DIC) and partial pressure of CO 2 (pCO 2 ) increased in surface seawater by ∼40 µmol kg −1 and ∼50 µatm (∼20 %), respectively. Increasing Revelle factor (β) values imply that the capacity of North Atlantic surface waters to absorb CO 2 has also diminished. As indicators of ocean acidification, seawater pH decreased by ∼0.05 (0.0017 yr −1 ) and values by ∼7-8 %. Such data provide critically needed multi-decadal information for assessing the North Atlantic Ocean CO 2 sink and the pH changes that determine marine ecosystem responses to ocean acidification.

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Fossil fuel use, cement manufacture and land-use changes are the primary sources of anthropogenic carbon dioxide (CO₂) to the atmosphere, with the ocean absorbing 30 %. Ocean uptake and chemical equilibration of anthropogenic CO₂with seawater results in a gradual reduction in seawater pH and saturation states (Ω) for calcium carbonate (CaCO₃) minerals in a process termed ocean acidification. Assessing the present and future impact of ocean acidification on marine ecosystems requires detection of the multi-decadal rate of change across ocean basins and at ocean time-series sites. Here, we show the longest continuous record of ocean CO₂ changes and ocean acidification in the North Atlantic subtropical gyre near Bermuda from 1983–2011. Dissolved inorganic carbon (DIC) and partial pressure of CO₂ (<i>p</i>CO₂) increased in surface seawater by ~40 μmol kg⁻¹ and ~50 μatm (~20 %), respectively. Increasing Revelle factor (<i>β</i>) values imply that the capacity of North Atlantic surface waters to absorb CO₂ has also diminished. As indicators of ocean acidification, seawater pH decreased by ~0.05 (0.0017 yr⁻¹) and Ω values by ~7–8 %. Such data provide critically needed multi-decadal information for assessing the North Atlantic Ocean CO₂sink and the pH changes that determine marine ecosystem responses to ocean acidification