Detecting anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean

Fossil fuel use, cement manufacture and land-use changes are the primary sources of anthropogenic carbon dioxide (CO<sub>2</sub>) to the atmosphere, with the ocean absorbing approximately 30% (Sabine et al., 2004). Ocean uptake and chemical equilibration of anthropogenic CO<sub>2</sub> with seawater results in a gradual reduction in seawater pH and saturation states (Ω) for calcium carbonate (CaCO<sub>3</sub>) 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<sub>2</sub> changes and ocean acidification in the North Atlantic subtropical gyre near Bermuda from 1983–2011. Dissolved inorganic carbon (DIC) and partial pressure of CO<sub>2</sub> (<i>p</i>CO<sub>2</sub>) increased in surface seawater by ~40 μmol kg<sup>−1</sup> and ~50 μatm (~20%), respectively. Increasing Revelle factor (<i>β</i>) values imply that the capacity of North Atlantic surface waters to absorb CO<sub>2</sub> has also diminished. As indicators of ocean acidification, seawater pH decreased by ~0.05 (0.0017 yr<sup>−1</sup>) and ω values by ~7–8%. Such data provide critically needed multi-decadal information for assessing the North Atlantic Ocean CO<sub>2</sub> sink and the pH changes that determine marine ecosystem responses to ocean acidification