Ocean acidification—the steady decline in seawater pH as oceans absorb excess atmospheric CO₂—has reached alarming milestones in 2025. Reports from the Potsdam Institute and the Planetary Health Check confirm the seventh planetary boundary for ocean acidification has been crossed, with surface pH dropping ~0.1 units since industrialization (a 30-40% acidity increase). This pushes ecosystems beyond safe limits, particularly for calcifying organisms like corals, whose aragonite skeletons become stressed below saturation states (Ω_ar) of ~3 and dissolve below 1.
Thresholds and Projections
A December 2025 Communications Biology study using volcanic CO₂ vents as analogs shows progressive reef community shifts below Ω_ar ~3-4, with calcareous species declining and non-calcareous proliferating. Tropical reefs face the brunt: Coral Triangle regions experience intensified extremes. Projections under high-emission paths (SSP3-7.0/SSP5-8.5) forecast widespread undersaturation by mid-century, dissolving reefs and triggering food web cascades—pteropods, shellfish, and fish populations collapse, rippling to fisheries supporting billions.
Food Web Failure
Reefs support ~25% of marine species; dissolution erodes habitat, amplifying biodiversity loss. Cold-water corals and Arctic systems are acutely vulnerable. 2025 data shows refugia shrinking—only ~12% of tropical waters retain stable chemistry.
Mitigation via emissions cuts offers hope—low scenarios preserve some reefs—but inertia locks in decades of change.
The threshold is crossed; dissolution accelerates. Will we curb CO₂ enough to save the webs, or watch oceans unravel?