Ocean Acidification
The ocean has absorbed about 30% of anthropogenic CO₂ emissions, slowing atmospheric warming but acidifying surface waters in the process. Average ocean pH has fallen from ~8.2 (pre-industrial) to ~8.1 today — a 30% increase in hydrogen ion concentration on the logarithmic pH scale. The chemistry is irreversible on human timescales.
Key insights
The chemistry is unambiguous
Atmospheric CO₂ dissolves into seawater, forming carbonic acid (H₂CO₃) which dissociates into H⁺ and HCO₃⁻. Higher H⁺ concentration = lower pH = more acidic. This chemistry has been measured continuously at the Hawaii Ocean Time-series (HOT) since 1988 and at parallel stations worldwide. The trend is consistent: surface pH falls ~0.02 units per decade in step with rising atmospheric CO₂.
Calcifying organisms are the canary
Corals, oysters, clams, pteropods and many planktonic species build skeletons from calcium carbonate. As ocean pH falls, the saturation state of aragonite (the form they use) declines, making shell-building harder and dissolution of existing structures more likely. Aragonite saturation in the Southern Ocean is projected to drop below 1.0 (the dissolution threshold) by ~2050, affecting the Antarctic food web.
Timescales are very long
If anthropogenic CO₂ emissions stopped today, atmospheric CO₂ would slowly fall as the deep ocean and weathering absorbed the excess. Surface ocean pH would recover over centuries to millennia. The deep ocean acidification, lagged by hundreds of years, is essentially locked in. Unlike many climate effects, ocean acidification is independent of geoengineering — solar dimming would slow warming but not change ocean chemistry.
Surface ocean pH at the Hawaii Ocean Time-series 1988–2024
pH (total scale)
Key Finding: Long-term decline of ~0.02 pH units per decade — among the most precisely measured climate signals.
Surface ocean CO₂ partial pressure (pCO₂) 1988–2024
μatm, station ALOHA
Key Finding: Surface pCO₂ has risen in step with atmospheric CO₂. The difference between the two — air-sea ΔpCO₂ — drives the ocean uptake rate.
Methodology & caveats
The pH scale is logarithmic
A pH change from 8.2 to 8.1 represents a 26% increase in H⁺ ion concentration (10^0.1 - 1 ≈ 0.26). Small numerical changes on the pH scale represent large chemical changes. The frequently-quoted '30% increase' refers to H⁺ concentration, not pH units.
Carbonate system measurement
Ocean carbonate chemistry is fully described by any two of: pH, total alkalinity (TA), dissolved inorganic carbon (DIC), and CO₂ partial pressure. Time-series stations measure DIC and TA most precisely; pH is derived. Worldwide, the GLODAPv2 data product synthesizes ship-based measurements and Argo float surface pH measurements.
Regional variation
Polar oceans absorb more CO₂ per unit area because cold water holds more dissolved gas. The Arctic and Southern Ocean are acidifying fastest. Upwelling regions (US Pacific coast, Peruvian coast) periodically bring corrosive deep water to the surface — Pacific oyster hatcheries in Oregon experienced larval mortality crises in 2007–08 traced to upwelling pH events.