The 25 year record of ocean carbon measurements at Station ALOHA document that the partial pressure of  (p) in the mixed layer is increasing at a rate slightly greater than the trend observed in the atmosphere. This has been accompanied by progressive decreases in seawater pH.  Although the effect of anthropogenic  is evidenced by long-term decreases in seawater pH throughout the upper 600 m, the rate of acidification at Station ALOHA varies with depth.  For example, in the upper mesopelagic waters (~160–310 m) pH is decreasing at nearly twice the rate observed in the surface waters. Such depth-dependent differences in acidification are due to a combination of regional differences in time-varying climate signatures, mixing, and changes in biological activity.