AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/9/9-7)
Unit 9 · Topic 9.7
9.7 Ocean Acidification
The ocean absorbs a large share of the CO₂ people release. That CO₂ forms carbonic acid, lowering seawater pH, a process called ocean acidification. Lower pH also leaves less carbonate in the water, making it harder for corals, shellfish and some plankton to build calcium carbonate shells and skeletons.
Key terms
- ocean acidification
- carbonic acid
- pH
- calcium carbonate
- coral reef
The chemistry
The ocean has absorbed roughly a quarter to a third of the CO₂ humans have released since the Industrial Revolution. That slows climate change, but it changes the water's chemistry.
When CO₂ dissolves in seawater, it reacts with water to form carbonic acid: CO₂ + H₂O → H₂CO₃. Carbonic acid releases hydrogen ions: H₂CO₃ → H⁺ + HCO₃⁻ (bicarbonate). More hydrogen ions means lower pH, so the water is more acidic.
Many of those extra hydrogen ions then combine with carbonate ions already in seawater: H⁺ + CO₃²⁻ → HCO₃⁻. That uses up carbonate (CO₃²⁻), the building block that shell-making organisms need.
How much has pH changed?
Surface ocean pH has fallen from about 8.2 before the Industrial Revolution to about 8.1 today. That sounds small, but the pH scale is logarithmic. A drop of 0.1 means the hydrogen ion concentration has increased by about 26 percent (10 to the 0.1 power is about 1.26). Under high emission scenarios, pH could fall to about 7.8 by 2100.
Seawater is still basic (above pH 7). 'Acidification' means it is moving toward the acidic end of the scale, not that it has become an acid.
Effects on marine life
- Corals: with less carbonate available, coral skeletons grow more slowly and are weaker, so reefs build up more slowly and erode more easily. Combined with bleaching from warming, this is a double threat.
- Shellfish: oysters, clams, mussels and sea urchins have trouble forming shells, especially as larvae. Oyster hatcheries in the US Pacific Northwest saw larvae die in large numbers in the late 2000s when acidic deep water welled up along the coast.
- Plankton: some tiny plankton build calcium carbonate shells, such as pteropods (sea butterflies) and coccolithophores. They are at the base of many food webs, so harm to them can ripple up to fish, seabirds and whales.
- In more acidic water, existing shells and skeletons can even begin to dissolve.
Solutions
The main solution is reducing CO₂ emissions, since the ocean takes up CO₂ in proportion to how much is in the air. Local steps can help ecosystems cope: protecting and restoring seagrass meadows and kelp forests, which absorb CO₂ through photosynthesis and can locally raise pH; cutting nutrient runoff, which worsens local acidification when algal blooms decompose; and monitoring water chemistry at shellfish farms so they can adjust.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
How big is a 0.1 pH drop?
Surface ocean pH has dropped from 8.2 to 8.1. (a) By what factor has the hydrogen ion concentration increased? Express as a percent increase. (b) If pH falls to 7.8, by what factor will hydrogen ion concentration have increased compared with 8.2?
Show the solutionHide the solution
- Step 1: Each 1-unit drop in pH multiplies hydrogen ion concentration by 10. For other drops, the factor is 10 raised to the size of the drop.
- Step 2: (a) Drop = 8.2 − 8.1 = 0.1. Factor = 10⁰·¹ ≈ 1.26, an increase of about 26%.
- Step 3: (b) Drop = 8.2 − 7.8 = 0.4. Factor = 10⁰·⁴ ≈ 2.51, so about 2.5 times the original hydrogen ion concentration.
Answer: (a) About 1.26 times, a 26% increase. (b) About 2.5 times.
- Example 2
Trap: acidification does not mean the ocean is acidic
A student says, 'Ocean acidification means the ocean's pH is now below 7.' Correct the student and explain why the change still matters.
Show the solutionHide the solution
- Step 1: Ocean pH is about 8.1, so the ocean is still slightly basic.
- Step 2: Acidification describes the direction of change: pH is decreasing, toward acidic.
- Step 3: It matters because even a slightly lower pH reduces the carbonate available for shells and skeletons, and organisms like oyster larvae are sensitive to small changes.
Answer: The ocean is still basic (about pH 8.1); acidification means its pH is falling, which reduces carbonate and harms shell-building organisms.
Common mistakes
- Saying the ocean has become acidic. It is still basic; its pH is falling.
- Leaving out the carbonate step. The key harm is that extra H⁺ ties up carbonate ions that organisms need for calcium carbonate.
- Treating a 0.1 pH change as trivial. On a logarithmic scale it is about a 26% increase in hydrogen ions.
On the exam
- Expect to write or explain the chain: CO₂ dissolves → carbonic acid → more H⁺ → lower pH and less carbonate → harder to build shells.
- pH calculations may ask how many times more acidic one value is than another; use powers of 10.
Connected topics
Videos
Check yourself
5 questions on 9.7 Ocean Acidification. Pick an answer to see if you got it, and why.
Which organisms are most directly threatened by ocean acidification?
Which human activity contributes most directly to ocean acidification?
| Year | Atmospheric CO₂ (ppm) | Average surface seawater pH |
|---|---|---|
| 1990 | 354 | 8.11 |
| 2000 | 370 | 8.09 |
| 2010 | 390 | 8.07 |
| 2020 | 414 | 8.05 |
Hypothetical data from an open-ocean monitoring station
Which statement best explains the trend in surface seawater pH?
By about what percentage did the hydrogen ion concentration of the surface seawater increase from 1990 to 2020?
Which statement best explains why falling pH makes it harder for oysters and corals to build shells and skeletons?
0 of 5 answered