Accelerated Ocean acidification (1985–2022) threatens tropical coral reefs and highlights biogeochemical refugia for marine conservation

Ocean acidification (OA) poses a growing threat to tropical coral reef ecosystems, yet the spatiotemporal dynamics of acidification and their implications for conservation remain undercharacterized. Here, we present a comprehensive global assessment of surface ocean carbonate chemistry from 1985 to 2022, using an observation-constrained, neural-network–based CMEMS dataset. We quantify long-term trends, detect structural accelerations, and identify regions of extreme and stable carbonate conditions based on pH, aragonite saturation (Ωₐᵣ), and calcite saturation (Ω ca ). Piecewise regression analysis reveals significant accelerations in the decline of all three parameters, with breakpoints in the late 1990s (Ωₐᵣ, Ω ca ) and post-2009 (pH). Spatial trend and Z -score analyses (Z ≤ −2.0) indicate that the Coral Triangle and adjacent Indo-Pacific regions have experienced the most intense acidification exposure, both in trend magnitude and extreme event frequency. Concurrently, we identify chemically stable zones, defined by Ωₐᵣ ≥ 3.0 and Ω ca ≥ 1.5, that persist in approximately 12 % of tropical ocean surface waters from 2015 to 2022. Critically, these stable zones show substantial overlap with major coral reef provinces, yet protection gaps remain. In Indonesia, the Philippines, and Papua New Guinea, less than 35 % of coral reef area falls within marine protected areas (MPAs), despite high stable zone prevalence. Our findings highlight an urgent opportunity to integrate carbonate chemistry stability into marine spatial planning. Designating MPAs within these refugial zones offers a climate-informed strategy to sustain coral ecosystem resilience under intensifying anthropogenic CO₂ forcing. • Ocean acidification rose post-2009 in pH, Ωₐᵣ, Ωca across all oceans. • Coral Triangle faced frequent acidification extremes from 2015 to 2022. • Stable carbonate zones (Ωₐᵣ ≥ 3.0, Ωca ≥ 1.5) span 12 % of tropics. • Coral nations lack protection despite stable zone overlap. • Chemistry-based refugia aid climate-smart MPA planning.