Concept

Ocean acidification

The decrease in seawater pH caused by absorption of atmospheric CO₂ — sometimes called 'climate change's evil twin.' Surface ocean pH has dropped from ~8.2 (pre-industrial) to ~8.1 (today); a 30% increase in hydrogen-ion concentration. Projected continued acidification through the 21st century threatens shell-forming organisms (corals, oysters, mussels, pteropods, foraminifera, coccolithophores) whose calcification becomes harder under more acidic conditions. The base of marine food webs depends on these organisms; ocean acidification therefore has cascading consequences for marine ecosystems and the human communities that depend on them.

The chemistry

CO₂ dissolved in seawater forms carbonic acid:

CO₂ + H₂O ↔ H₂CO₃ (carbonic acid)
H₂CO₃ ↔ H⁺ + HCO₃⁻ (bicarbonate)
HCO₃⁻ ↔ H⁺ + CO₃²⁻ (carbonate)

The increased H⁺ (hydrogen ion) concentration is acidification — a lower pH. Additionally, the increased H⁺ shifts the equilibrium against carbonate (CO₃²⁻), which is what shell-forming organisms use to build their shells.

Magnitude

Pre-industrial ocean pH — ~8.2
Today — ~8.1
Projected 2100 (under high-emission scenarios) — ~7.8-7.9
Hydrogen ion concentration increase to date — ~30%
Projected by 2100 — ~150% over pre-industrial

The pH scale is logarithmic: 0.1 pH change = 30% change in H⁺ concentration.

Effects on organisms

Shell-formers (calcifying organisms):

Corals — calcium-carbonate skeletons; reduced growth and weaker structure
Oysters, mussels, clams — shellfish industries directly affected
Pteropods — pelagic snails; foundational food in cold-water food webs; thin shells particularly vulnerable
Foraminifera, coccolithophores — single-celled plankton with calcium-carbonate shells; foundational marine primary producers; crucial for ocean carbon cycling
Echinoderms — sea stars, sea urchins, sand dollars
Crustaceans — crabs, lobsters; effects vary

Other effects:

Behavior changes — fish behavior altered by pH; some species lose predator-detection abilities
Reproduction — many marine organisms have pH-sensitive reproductive stages
Coral reef collapse — combined with warming, projected widespread coral die-off

Documented impacts

Pacific Northwest oyster industry collapse (2007-2010) — among the first major economic impacts; ocean-acidified water entered hatcheries; larvae couldn’t form shells; industry restructured around buffered intake water
Coral bleaching and decline — multiple causes including warming and acidification; widespread global coral decline
Pteropod shell dissolution — observed in Southern Ocean
Larval-shellfish mortality — documented across multiple species

Why this matters

Ocean acidification affects:

The base of marine food webs (foundational plankton)
Major commercial fisheries (shellfish, reef-fish)
Coral reefs and their associated economies (~$30B+ annual value)
Carbon-cycle dynamics (calcium-carbonate-forming organisms also remove carbon long-term)
Biogeochemical cycles broadly

The chemistry is unambiguous; the timeframes are decades-to-centuries; the effects are already documented and accelerating.

Sources

IPCC AR6 Working Group I — ocean acidification
The Ocean Acidification Time Bomb, multiple peer-reviewed publications
NOAA Ocean Acidification Program documentation

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