Highlights
- Global aragonite saturation state declined at −0.0068 ± 0.00013 yr−1 from 1985 to 2023.
- Equatorial Pacific shows fastest acidification with Ωₐᵣ declines of −0.012 yr−1.
- Novel trend-based K-means clustering identifies emerging coral reef risk zones.
- Strong pCO₂-Ωₐᵣ correlations (ρ < −0.9) in tropical upwelling regions monthly.
- Framework supports SDG 14.3 and Global Biodiversity Framework implementation.
Abstract
Ocean acidification driven by anthropogenic CO₂ uptake poses a critical threat to coral reef ecosystems. Using global surface ocean carbonate data from 1985 to 2023, we provide a high-resolution, observation-based assessment of long-term trends in pCO₂, pH, and aragonite saturation state (Ωₐᵣ). Our results show a robust global decline in Ωₐᵣ (−0.0068 ± 0.00013 yr−1), with the most pronounced losses in the equatorial Pacific and Southern Hemisphere. Monthly correlations reveal strong inverse pCO₂–Ωₐᵣ relationships (ρ < −0.9) and positive pH–Ωₐᵣ correlations (ρ > 0.9) in tropical upwelling zones, highlighting spatially persistent acidification stress. A key innovation of this study is the use of a trend-based K-means clustering framework that classifies ocean regions into high, moderate, and low impact categories based on Ωₐᵣ decline rates. Unlike conventional assessments relying on absolute Ωₐᵣ thresholds, this approach identifies regions that remain suitable today but are deteriorating rapidly, thus at risk of crossing biological thresholds in the near future. High-impact zones, including the Coral Triangle and eastern tropical Pacific, show Ωₐᵣ declines of −0.010 to −0.012 yr−1 and have already reached levels near 2.4–3.0, suggesting growing stress on reef calcification. By leveraging high-resolution observational data rather than model projections, this approach reduces uncertainty and offers a scalable tool for anticipating ecological vulnerability under ongoing acidification. The findings underscore the urgent need for conservation in tropical high-impact zones and for sustained monitoring in lower-risk regions. This work provides a science-based framework to support spatially targeted reef management and informs global policy priorities including SDG 14.3 and the Global Biodiversity Framework.
Chen W. B., Chen H., Hsiao S. C., Liang T. Y., Shih H. J. & Chang C. H., 2026. A trend-based ecological indicator framework for spatially classifying ocean acidification risk to global coral reefs. Ecological Indicators 182: 114545. doi: 10.1016/j.ecolind.2025.114545. Article.
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