New observations confirm the progressive acidification in the Mozambique Channel

Abstract

New observations obtained in 2021 and 2022 are presented and used to investigate the trend of the carbonate system (including pHT and aragonite saturation state, Ωar) in the southern sector of the Mozambique Channel. Using historical and new data in April–May we observed an acceleration of the acidification ranging from −0.012 decade−1 in 1963–1995 to −0.027 (±0.003) decade−1 in 1995–2022. Result from a neural network (FFNN) model for all seasons also suggests faster pH trend in recent decades, −0.011 decade−1 over 1985–1995 and −0.018 decade−1 over 1995–2022. In May 2022 we estimated Ωar of 3.49, about 0.3 lower than observed in May 1963 (Ωar = 3.86). The lowest Ωar value of 3.23 was evaluated from the FFNN model in September 2023 that corresponds to the hypothetical critical threshold value (3.25) for coral reefs. In 2025 a marine heat wave was observed in this region (sea surface temperature up to 30 °C) and data from a BGC-Argo float indicate that sea surface pH was the lowest in January 2025 (pHT = 7.95) whereas Ωar was the lowest in March 2025 (Ωar = 3.2). A projection of the CT concentrations based on observed anthropogenic CO2 in subsurface water and future anthropogenic CO2 emissions scenario, suggests that a risky level for corals (Ωar < 3) could be reached as soon as year 2034.