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 CO 2 . 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 CO 2 , with surface seawater pH continuing to fall as atmospheric CO 2 rises.