How CO₂ makes the sea more acidic
The ocean takes in about 25% of the CO₂ we emit. In water, CO₂ reacts with water to make carbonic acid, which gives up hydrogen ions (H⁺):
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
More H⁺ means lower pH. Since the 1800s, the average pH of surface sea water has fallen from about 8.2 to about 8.1. The sea is still slightly basic (above 7), so the word is "acidification": it is moving towards acid, not becoming acid.
pH is a log scale. A fall of 0.1 means about 26% more H⁺. A fall of 0.4 (from 8.2 to 7.8) means about 2.5 times the H⁺.
Why shells and coral suffer
Shells and coral skeletons are made of calcium carbonate (CaCO₃). Animals build them from calcium ions and carbonate ions (CO₃²⁻) in sea water.
Extra H⁺ ions react with carbonate: H⁺ + CO₃²⁻ → HCO₃⁻. This leaves less carbonate to build shells. Building takes more energy. Shells and skeletons get thinner, and in very acidic water they can dissolve.
Young animals such as baby oysters and tiny sea snails called pteropods are hit first. Pteropods are food for fish, so the harm spreads up the food chain.
Coral reefs and their ecosystems
Reefs cover less than 1% of the sea floor but give a home to about a quarter of all sea species. They protect coasts from waves, give fish for food, and bring tourism.
Reef corals are tiny animals (polyps) that live with colourful algae called zooxanthellae inside their bodies. The algae make food by photosynthesis and give coral its colour. The coral gives the algae a safe home. They are partners.
Coral bleaching: a different cause
When sea water stays about 1 to 2 °C above the normal summer high for weeks, corals are stressed. They push out their algae partners. Without the algae the clear coral body shows the white skeleton: bleaching.
A bleached coral is not dead yet, but it is starving. If the water cools in time, algae can return and the coral recovers. If the heat lasts, the coral dies and algae and seaweed cover it.
Acidification and bleaching are different: acidification (CO₂ in water) weakens skeleton-building; bleaching (heat) removes the colourful partners. Both come from the same cause (extra CO₂ in the air, which also warms the planet) and they hurt reefs together. Other stresses such as pollution, mud and overfishing make it worse.
What can we do?
- Cut CO₂ emissions. This is the only fix for the root cause.
- Reduce local stress: stop sewage and farm runoff, avoid overfishing and harmful tourism.
- Protect reefs in marine protected areas.
- Restore reefs by growing and replanting coral. Scientists also study heat-tolerant corals.
- Monitor sea pH and temperature so that shellfish farms can plan.
Try it: see CO₂ make an acid
Fill a glass with water and add a few drops of red cabbage juice (or universal indicator). Blow gently through a straw into the water for a minute (never suck in). The colour moves towards the acid side, because the CO₂ in your breath made carbonic acid. Next, in the 3D, slide the CO₂ control and watch the pH label and the H⁺ ions.
Key formulas and definitions
- CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- H⁺ + CO₃²⁻ → HCO₃⁻ (carbonate is used up)
- Shell / coral material: calcium carbonate, CaCO₃
- pH = −log₁₀[H⁺]; a fall of 1 pH unit = 10 times more H⁺
- Change in [H⁺] = 10^(fall in pH). Fall of 0.1 → × 1.26; fall of 0.4 → × 2.5
Worked examples
1. Surface sea pH fell from 8.2 to 8.1. By what factor did H⁺ increase?
Fall = 0.1. Factor = 10^0.1 ≈ 1.26. H⁺ rose by about 26%.
2. Sea water goes from pH 8.2 to pH 7.2. How many times more H⁺ is there?
Fall = 1.0. Factor = 10^1 = 10 times more H⁺.
3. Write the reaction that shows how extra H⁺ uses up carbonate. Why is that bad for a clam?
H⁺ + CO₃²⁻ → HCO₃⁻. The clam builds its shell from carbonate, so less carbonate means a thinner, weaker shell and more energy to build it.
4. A reef warms from 28 °C to 31 °C for many weeks and the coral turns white. Is this acidification? Explain.
No. It is bleaching caused by heat. The coral threw out its algae partners. Acidification is about pH and carbonate, not temperature.
5. Why can the ocean be called "more acidic" even when its pH is above 7?
pH is falling, so H⁺ is rising and the water is moving towards the acidic side. It is still slightly basic overall.
6. pH goes from 8.2 to 7.8 by the end of the century in a high-CO₂ future. Find the factor by which H⁺ rises.
Fall = 0.4. Factor = 10^0.4 ≈ 2.5, so H⁺ becomes about 2.5 times larger.
Common mistakes
- Saying the sea is now an acid. Its pH is about 8.1, still above 7. The word is acidification (a move towards acid).
- Mixing up bleaching and acidification. Bleaching = heat stress, white coral. Acidification = CO₂ and pH, weaker shells.
- Forgetting that pH is a log scale. A fall of 1 means 10 times more H⁺, not just a bit more.
- Thinking bleached coral is always dead. It can recover if the water cools in time.