Key Idea: Most marks in topic 6.4 come from the Montreal Protocol and the recovery of the ozone layer. At HL, also learn halogen chemistry, the polar ozone hole, HFCs and air conditioning.
1. The Montreal Protocol (most marks)
- Signed in 1987; every UN member has ratified it.
- It phased out CFCs and other ozone-depleting substances (ODSs), with later dates for poorer countries.
- It worked: few producers, cheap alternatives, a fund to help, and rules tightened as science grew.
2. Depletion and recovery
- CFCs from fridges, aerosols and foams release chlorine, which destroys ozone.
- The Antarctic ozone hole grew fast in the 1980s and peaked at 26.6 million km² in 2006.
- Ozone should return to 1980 levels around 2066 over Antarctica: CFCs last decades.
3. Why it matters and HL ideas
- The ozone layer in the stratosphere absorbs most harmful UV radiation.
- More UVB causes skin cancer and cataracts, and cuts phytoplankton photosynthesis.
- At HL: halogen chemistry, the polar ozone hole in spring, HFCs and Kigali (2016), and air conditioning.
How it is examined:
| Question | How to score |
|---|---|
| [1] to [2] Describe ozone-hole data | Direction, peak year and values, with units. |
| [1] Name the cause of the decline | CFCs, or chlorine from ODSs. |
| [2] Outline the effects of ozone depletion | More UVB: skin cancer, cataracts, less phytoplankton growth. |
| [4] Explain how the data show the Protocol worked | Compare before and after 1987, and note the time lag. |
| [7] to [9] Evaluate or discuss the Montreal Protocol | Why it worked, its limits, a comparison with climate deals. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Ozone-depleting substance (ODS) | a chemical, such as a CFC, that destroys stratospheric ozone |
| CFC | a chlorofluorocarbon once used in fridges, aerosols and foams |
| Ozone hole | an area where ozone is below 220 Dobson units |
| Dobson unit (DU) | the unit for the amount of ozone in a column of air |
| UVB | ultraviolet radiation that damages DNA; ozone absorbs most of it |
| HFC | a CFC replacement that spares ozone but is a strong greenhouse gas |
| Polar stratospheric clouds | clouds below about -78 °C on which chlorine is released |
Last-minute check: answer, then tap to see.
Why does the Antarctic hole open in spring? Sunlight returns and splits chlorine stored on polar clouds.
How many ozone molecules can one chlorine atom destroy? Over 100,000: it is a catalyst.
Why were HFCs phased down at Kigali in 2016? They spare ozone but are potent greenhouse gases.
How does air conditioning harm the climate? High energy use and HFC leaks.
The Kigali Amendment (2016) added HFCs to the Montreal Protocol. Evaluate the replacement of CFCs with HFCs.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- For graph data, give the direction, the peak year and values with units.
- Say chlorine from CFCs destroys ozone; do not blame carbon dioxide.
- For the Protocol's success, compare the data before and after 1987.
- Explain why the Antarctic hole opens in spring: cold, clouds, then sunlight.