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Anthropogenic atmospheric CO2 uptake by the ocean is causing a global ocean pH decline, resulting in direct anthropogenic ocean acidification (Ed-anth). However, indirect anthropogenic effects of changes in biological and physical processes in the ocean also influence ocean pH (Ei-anth). Therefore, distinguishing between Ed-anth and Ei-anth is essential for predicting the progress of overall ocean acidification. We demonstrate long-term trends of Ed-anth and Ei-anth in the subarctic North Pacific using a parameterization technique with an observed pH (pH(obs)) time series. All Ei-anth in the ocean interior under 150m cause a pH decrease of 0.00120.0003 per year superimposed on an 18-year oscillation. Subtracting the pH decrease by Ei-anth from the pH(obs) trend for the same region yields a net linear pH decreasing trend of 0.00210.0003 per year by Ed-anth for the whole ocean, indicating that pH decreases by Ei-anth account for similar to 40% of total ocean acidification.

Plain Language Summary Human activities have increased atmospheric CO2 by similar to 1.4 times in the last two centuries, and this increase has exacerbated ocean acidification. However, natural processes also influence ocean acidification. Therefore, distinguishing between human activity-derived and natural process-derived CO2 effects on the ocean is essential for predicting the progress of overall ocean acidification. We demonstrate long-term trends of ocean acidification, caused by these effects, in the subarctic North Pacific using a statistical technique with observed ocean carbonate species data. Subtracting the trend by the natural process-derived CO2 effect from the observed total ocean acidification yields a net decline of ocean pH caused by the human activity-derived CO2 effect for the entire ocean. According to this calculation, we noted that the natural process-derived CO2 effect on total ocean acidification is similar to 40%.