Ocean chemistry

What is Ocean Acidification?

Ocean acidification is a change in seawater chemistry linked to additional atmospheric CO₂. When CO₂ dissolves in seawater, it increases hydrogen ions (H⁺), which lowers pH and shifts inorganic carbon from carbonate (CO₃²⁻) toward bicarbonate (HCO₃⁻).

pH reflects H⁺ on a log scale

Carbonate system: CO₂ / H₂CO₃ / HCO₃⁻ / CO₃²⁻

Acidification ≠ “turning acidic”

1) CO₂ enters seawater

CO₂ moves between air and sea. Wind and waves can speed up this exchange at the surface.

2) H⁺ increases → pH decreases

Dissolved CO₂ forms carbonic acid, which partially dissociates and increases H⁺. That change lowers pH.

3) Carbonate shifts toward bicarbonate

Higher H⁺ pushes the carbonate system away from CO₃²⁻ and toward HCO₃⁻ at equilibrium.

Learn Alongside Experts

Ocean acidification is a chemistry shift: CO₂ uptake changes the balance of dissolved inorganic carbon.

Definition

Ocean acidification is an increase in seawater H⁺ (hydrogen ions) driven by uptake of atmospheric CO₂. This lowers pH and changes how inorganic carbon is distributed among CO₂(aq), H₂CO₃, HCO₃⁻, and CO₃²⁻.

What pH tells you

pH is a log scale, so even a small shift can correspond to a meaningful change in H⁺. Ocean acidification can occur even when seawater remains above pH 7 (still basic).

pH = −log₁₀[H⁺]
Lower pH ⟹ higher [H⁺]
“Acidification” describes direction (lower pH), not necessarily crossing pH 7.

Core idea: Ocean acidification is a predictable equilibrium shift in the carbonate system. The “why it matters” story comes next—this page is the foundation.

Why it matters Solutions

The key reactions

Read this as a chain: CO₂ enters → carbonic acid forms → H⁺ increases → the carbonate balance shifts. You don’t need to memorize every line—focus on what changes and in which direction.

CO₂ + H₂O ⇌ H₂CO₃ CO₂ hydrates to carbonic acid (often grouped with CO₂(aq) in seawater chemistry)

H₂CO₃ ⇌ H⁺ + HCO₃⁻ carbonic acid dissociates, increasing H⁺ and producing bicarbonate

HCO₃⁻ ⇌ H⁺ + CO₃²⁻ more H⁺ shifts equilibrium away from carbonate (CO₃²⁻)

What “carbonate system” means in practice

Seawater stores dissolved inorganic carbon mostly as bicarbonate, with smaller fractions as dissolved CO₂/carbonic acid and carbonate. When CO₂ increases, the system re-partitions: HCO₃⁻ rises and CO₃²⁻ declines.

DIC (dissolved inorganic carbon) = CO₂(aq) + H₂CO₃ + HCO₃⁻ + CO₃²⁻
Ocean acidification changes the relative proportions of these species at equilibrium.
Those shifts are the definition; biological and economic impacts are downstream.

Common misconceptions

Not “the ocean turning into acid”

Acidification means moving toward lower pH, not necessarily dropping below pH 7.

Not only about pH

pH is one indicator. The central chemistry is the carbonate-system shift.

Not a one-step reaction

It involves multiple equilibria, so conditions like mixing and temperature can influence the outcome.

Quick self-check: Try summarizing ocean acidification in one sentence using “CO₂,” “H⁺,” and “carbonate system.”