Long‐Term Trends of Direct and Indirect Anthropogenic Effects on Changes in Ocean pH

Abstract

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 (pHobs) time series. All Ei‐anth in the ocean interior under 150 m cause a pH decrease of 0.0012 ± 0.0003 per year superimposed on an 18‐year oscillation. Subtracting the pH decrease by Ei‐anth from the pHobs trend for the same region yields a net linear pH decreasing trend of 0.0021 ± 0.0003 per year by Ed‐anth for the whole ocean, indicating that pH decreases by Ei‐anth account for ~40% of total ocean acidification.

Quasi‐Real‐Time and High‐Resolution Spatiotemporal Distribution of Ocean Anthropogenic CO2

“…Argo floats equipped with pH sensors in the subarctic North Pacific will inevitably be deployed in the near future. Since the pH sensors give us the high‐resolution spatiotemporal distribution of ocean pH, we will easily use the sensor pH and our parameterized pH to separate the direct‐ and indirect‐anthropogenic effect on ocean acidification in the future (Watanabe et al, ).…”

Long-term ocean acidification trends in coastal waters around Japan

“…Pacific open ocean over the past 50 years 11 . Therefore, we attempted to evaluate whether these changes are also characterized by decadal oscillations in coastal areas.…”

The Southern Ocean with the largest uptake of anthropogenic nitrogen into the ocean interior

“…Heretofore, the separation of these two terms of N obs was challenging. A method to estimate the variation in the external term of the observed ocean carbon species across different arbitrary years was proposed recently 10 . This method could be extended to distinguish N in and N ex (see Supplementary Text S4).…”

“…Recently, a new method capable of estimating the change in anthropogenic CO 2 impact on the ocean interior across decadal time intervals using parameterization techniques was proposed 10 , which makes it possible to…”