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https://doi.org/10.5194/egusphere-2025-3469
https://doi.org/10.5194/egusphere-2025-3469
29 Jul 2025
 | 29 Jul 2025
Status: this preprint is open for discussion and under review for Biogeosciences (BG).

New observations confirm the progressive acidification in the Mozambique Channel

Nicolas Metzl, Claire Lo Monaco, Aline Tribollet, Jean-François Ternon, Frédéric Chevallier, and Marion Gehlen

Abstract. New observations obtained in 2021 and 2022 are presented and used to investigate the trend of the carbonate system (including pH 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 TS.decade-1 in 1963–1995 to -0.027 (±0.003) TS.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 TS.decade-1 over 1985–1995 and -0.018 TS.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 low in January 2025 (pH = 7.95) whereas War was low in Mach 2025 (Ωar = 3.2). A projection of the CT concentrations based on observed anthropogenic CO2 in subsurface water and emissions scenario, suggests that a risky level for corals (Ωar < 3) could be reached as soon as year 2034.

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Nicolas Metzl, Claire Lo Monaco, Aline Tribollet, Jean-François Ternon, Frédéric Chevallier, and Marion Gehlen

Status: open (until 13 Sep 2025)

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Nicolas Metzl, Claire Lo Monaco, Aline Tribollet, Jean-François Ternon, Frédéric Chevallier, and Marion Gehlen

Data sets

SOCAT D. C. E. Bakker et al. https://doi.org/10.25921/9wpn-th28

SNAPO-CO2 N. Metzl et al. https://doi.org/10.17882/102337

Nicolas Metzl, Claire Lo Monaco, Aline Tribollet, Jean-François Ternon, Frédéric Chevallier, and Marion Gehlen

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Short summary
In the Mozambique Channel, observed acceleration of the ocean acidification in the recent decades is mainly driven by anthropogenic CO2 uptake. In this region the aragonite saturation state reached 3.2 in 2025 and could be as low as 3 in the next 10 years with potential impact on marine ecosystem including corals reefs areas.
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