The equations showing CO2 reacting with water look like they generate more, not less carbonate. How does ocean acidification decrease the amount of carbonate ions in seawater?
This is a common point of confusion, because step-by-step equilibrium equations describing the carbonate system in seawater do not capture the dynamic chemical environment of seawater. There are several reactions that can occur between carbon dioxide (CO2), water (H2O), carbonic acid (H2CO3), bicarbonate ion (HCO3-), and carbonate ion (CO32-). One of the possible reactions does create carbonate ions and lowers pH: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32- However, at the current ocean pH level, another reaction also occurs that consumes carbonate ions and does not change pH: CO2 + H2O + CO32- ↔ 2HCO3- The second equation describes the reaction that occurs most often in the modern oceans, but the first reaction also occurs, so the resulting overall change is a decrease in carbonate and a decrease in pH. — Christopher L.
