Direct observations of basin-wide acidification of the North Pacific Ocean

Global ocean acidification is a prominent, inexorable change associated with rising levels of atmospheric CO2. Here we present the first basin‐wide direct observations of recently declining pH, along with estimates of anthropogenic and non‐anthropogenic contributions to that signal. Along 152°W in the North Pacific Ocean (22–56°N), pH changes between 1991 and 2006 were essentially zero below about 800 m depth. However, in the upper 500 m, significant pH changes, as large as −0.06, were observed. Anthropogenic and non‐anthropogenic contributions over the upper 800 m are estimated to be of similar magnitude. In the surface mixed layer (depths to ∼100 m), the extent of pH change is consistent with that expected under conditions of seawater/atmosphere equilibration, with an average rate of change of −0.0017/yr. Future mixed layer changes can be expected to closely mirror changes in atmospheric CO2, with surface seawater pH continuing to fall as atmospheric CO2 rises.