A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
As atmospheric CO₂ rises, the ocean absorbs roughly 25–30% of anthropogenic CO₂ emissions. Dissolved CO₂ reacts with seawater to form carbonic acid, releasing hydrogen ions that lower pH — a process called ocean acidification. This shift reduces the availability of carbonate ions (CO₃²⁻), making it harder for calcifying organisms (corals, mollusks, echinoderms, pteropods) to build shells and skeletons from calcium carbonate minerals (aragonite and calcite). Polar waters are experiencing acidification fastest because cold water absorbs more CO₂.
Work through the chemical reactions: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻, and trace how rising H⁺ shifts the carbonate equilibrium. Compare the saturation horizon (depth below which CaCO₃ dissolves) under preindustrial and projected future conditions.
From your work on acid-base chemistry, you know that dissolving CO₂ in water produces carbonic acid (H₂CO₃), which dissociates to release hydrogen ions (H⁺) and lower pH. The ocean performs this reaction on a planetary scale. Seawater absorbs roughly a quarter of all CO₂ humans emit, and while this buffering has slowed atmospheric warming, it comes at a chemical cost. The absorbed CO₂ reacts with water through a chain you can trace step by step: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. The extra H⁺ ions then react with existing carbonate ions (CO₃²⁻) to form more bicarbonate: H⁺ + CO₃²⁻ → HCO₃⁻. The net result is more bicarbonate, more hydrogen ions, fewer carbonate ions, and a lower pH.
The term ocean acidification is sometimes misunderstood — it does not mean the ocean is becoming acidic in the everyday sense. Surface ocean pH has dropped from about 8.2 before industrialization to roughly 8.1 today, and projections suggest it could fall below 7.8 by 2100 under high-emission scenarios. The ocean remains basic, but that 0.1 unit drop represents a 26% increase in hydrogen ion concentration because the pH scale is logarithmic. What matters biologically is not the absolute pH but the direction and speed of change — marine organisms have evolved in a relatively stable chemical environment for millions of years.
The critical consequence is the reduction in carbonate ion concentration. Organisms that build shells and skeletons from calcium carbonate — corals, pteropods, oysters, sea urchins — need dissolved carbonate ions to construct their mineral structures. The saturation state (Ω) measures whether seawater has enough carbonate ions for CaCO₃ to remain stable: when Ω drops below 1, calcium carbonate dissolves faster than it forms. As acidification progresses, the saturation horizon — the depth below which carbonate minerals dissolve — rises closer to the surface, squeezing the habitable zone for calcifying organisms. Aragonite, the mineral form used by corals and pteropods, is more soluble than calcite, so aragonite-dependent organisms are affected first.
Geography matters enormously. Cold water absorbs more CO₂ than warm water (a gas solubility principle from your chemistry background), so polar and subpolar oceans are acidifying fastest. Arctic surface waters are already approaching aragonite undersaturation in some seasons. Upwelling zones along western coastlines bring naturally CO₂-rich deep water to the surface, creating acidification "hotspots" where shellfish fisheries are already experiencing larval die-offs. The combination of anthropogenic CO₂ and natural upwelling can push local conditions past biological thresholds decades ahead of the global average, making ocean acidification not just a future concern but a present one with measurable ecological and economic impacts.