An empirical projection of ocean acidification in southwestern Japan over the 21st century

Most of Japan's coral reefs are distributed in the Ryukyu Islands, in the southwestern part of Japan. Since they support biodiversity in the tropical and subtropical seas and are vulnerable to ocean acidification as well as ocean heat waves and pollution, projecting acidification over multidecadal or longer periods of time is a great interest. Currently, the majority of long-term acidification projections are based on Earth System Models (ESMs), and the validation of these projections relies on intercomparisons among ESMs. This study evaluated the multi-decadal trends in total dissolved inorganic carbon (DIC) concentrations around the Ryukyu Islands over the past 25 years from 1995 to 2019. A multiple linear regression using temperature, salinity and time parameters as explanatory variables was applied to evaluate the salinity-normalized dissolved inorganic carbon (nDIC) concentrations. The coefficient of time (+1.15 ± 0.03 μmol kg⁻¹ yr⁻¹) was not significantly different from the rise of nDIC that was calculated from the growth rate of atmospheric CO₂ concentrations during the same period. Assuming that nDIC in this region will continue to increase at a rate that tracks the expected growth rate of atmospheric CO₂ concentrations, we projected future trends of pH and aragonite saturation state (Ω_A) under scenarios RCP4.5 and RCP8.5. The empirical projection of acidification by the end of the 21st century was generally consistent with projections based on ESMs. At present, global corals are generally distributed in waters with Ω_A > 3.0. According to the empirical projection under the RCP8.5 scenario, Ω_A around Okinawa Island would fall below 3.0 in winter in the 2030s and throughout the year in the 2060s.