# Explain how rising atmospheric CO₂ concentrations affect the pH of the ocean and outline the consequences of this change for organisms with calcium carbonate structures.

> IB DP Biology Higher Level (2023 syllabus) — D4.3 Climate change · Explain · 4 marks

## Mark scheme (4 marks)

1. CO₂ dissolves in seawater to form carbonic acid (H₂CO₃)
2. Carbonic acid dissociates, releasing hydrogen ions (H⁺), lowering seawater pH — a process termed ocean acidification
3. Excess H⁺ ions react with carbonate ions (CO₃²⁻), reducing carbonate ion concentration available for calcification
4. Organisms such as corals, molluscs, and echinoderms struggle to form or maintain calcium carbonate (CaCO₃) shells/skeletons, leading to weaker structures, reduced growth, or dissolution of existing structures

## Key terms

- [calcium carbonate](https://www.gradenine.co.uk/glossary/calcium-carbonate)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-rising-atmospheric-co-concentrations-5eabb7b1) · Published by Druglandscape Ltd.