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Ozone Layer and Depletion | ISC Class 11 Geography

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GeographyISC Class 11 · Composition and Structure of the Atmosphere

The Ozone Layer and Ozone Depletion (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Ozone (O3) forms in the stratosphere, mainly between about 15 and 35 km, and absorbs harmful ultraviolet radiation. Chlorofluorocarbons and other substances release chlorine and bromine that destroy ozone, most severely over Antarctica in spring. The Montreal Protocol (1987) phased them out and the Kigali Amendment (2016) phases down HFCs.

Understand The Ozone Layer and Ozone Depletion

Ozone layer. The part of the stratosphere, mainly 15 to 35 km, where ozone is concentrated. Ultraviolet rays split O2 into atoms which combine with O2 to form O3. It absorbs most harmful UV-B and UV-C, protecting living things from skin cancer, cataracts and damage to plants and plankton, and its absorbed energy warms the stratosphere. Ozone is measured in Dobson Units; an ozone hole is an area where total ozone falls below about 220 Dobson Units.

Causes of depletion. Chlorofluorocarbons (old refrigerators, air conditioners, aerosol sprays, foam-making), halons (fire extinguishers), carbon tetrachloride and methyl chloroform (solvents), nitrous oxide from fertilisers and nitrogen oxides from high-flying supersonic aircraft. These stable gases rise to the stratosphere where UV breaks them down, releasing chlorine and bromine. One chlorine atom acts as a catalyst and destroys thousands of ozone molecules: Cl + O3 gives ClO + O2, then ClO + O gives Cl + O2.

Antarctica. Very cold polar stratospheric clouds in the polar winter speed these reactions when sunlight returns in spring (September to November); a hole was reported in the mid-1980s.

Effects. More UV: skin cancer, cataracts, weaker immunity; harm to crops, phytoplankton and the marine food chain; damage to plastics.

Action. Montreal Protocol (1987) phased out CFCs and other ozone-depleting substances; its Kigali Amendment (2016) phases down HFCs, which are greenhouse gases. India is a party. The layer is slowly recovering; 16 September is World Ozone Day.

The Ozone Layer and Ozone Depletion: key facts

Ozone formation
O2 split by UV | O + O2 gives O3
Catalytic destruction
Cl + O3 gives ClO + O2 | ClO + O gives Cl + O2
Chlorine is recycled.
Key dates
  • Montreal Protocol 1987
  • Kigali Amendment 2016
  • World Ozone Day 16 September
Ozone hole
Below about 220 Dobson Units | Antarctica, September to November

How to answer The Ozone Layer and Ozone Depletion questions

Questions are 3 to 4 marks: causes, effects and international action; sums use Dobson Units.

  1. 1State where ozone lies and its role.
  2. 2List causes and the reaction cycle.
  3. 3Explain Antarctic depletion with polar stratospheric clouds.
  4. 4Close with the Montreal Protocol and Kigali Amendment.

The Ozone Layer and Ozone Depletion in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Falling ozone reading

Suppose a station records 300 Dobson Units normally and a 28 per cent fall in spring.

Is it an ozone hole by the chapter's definition?

Try it first, then show the solutionSolution
  1. Fall = 0.28 x 300 = 84 DU; new value = 300 - 84 = 216 DU.
  2. 216 is below about 220 DU, so it qualifies as a hole.

Answer: 216 DU; below 220, so yes.

Memory hook: Below 220 DU is a hole.

Sun cream logic

Suppose a village sees more sunburn complaints after a spring dip in ozone.

Link ozone loss to sunburn.

Try it first, then show the solutionSolution
  1. Less ozone means more UV reaches the ground.
  2. More UV causes skin damage.

Answer: Less ozone lets more UV through.

Memory hook: Thin shield, more burn.

Tricks to remember The Ozone Layer and Ozone Depletion

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • One Cl, thousands of O3

    One chlorine atom destroys thousands of ozone molecules.

    Use it when: Catalyst role.

  • 1987, 2016

    Montreal 1987 phases out; Kigali 2016 phases down HFCs.

    Use it when: Agreements.

The Ozone Layer and Ozone Depletion: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Agreement · 1 mark · MCQBoard level

Which treaty of 1987 committed countries to phase out ozone-depleting substances?
(a) Kyoto Protocol
(b) Montreal Protocol
(c) Paris Agreement
(d) Ramsar Convention

Try it first, then show the solutionSolution
  1. 1987.

Answer: (b) Montreal Protocol.

Example 2: Protective role · 2 marks · Short answerBoard level

State two reasons why the ozone layer is called a protective shield.

Try it first, then show the solutionSolution
  1. It absorbs most harmful UV-B and UV-C radiation.
  2. It protects living things from skin cancer and cataracts and protects plants and plankton.

Answer: It absorbs harmful ultraviolet radiation, protecting health and ecosystems.

Example 3: Dobson sums · 4 marks · Long answerHarder board level

Normal ozone over a region is 320 DU. After depletion it is 40 per cent lower. Find the new value and say whether it is a hole by the 220 DU criterion. Then find the percentage fall that would give exactly 220 DU.

Try it first, then show the solutionSolution
  1. Fall = 0.40 x 320 = 128 DU; new value = 320 - 128 = 192 DU.
  2. 192 is below 220, so it is a hole.
  3. Fall to 220 = 320 - 220 = 100 DU.
  4. Percentage = 100 / 320 x 100 = 31.25 per cent.

Answer: 192 DU (a hole); 31.25 per cent fall gives 220 DU.

Example 4: Chlorine cycle · 4 marks · Long answerHarder board level

Explain why chlorine from CFCs destroys so much ozone and show the reactions.

Try it first, then show the solutionSolution
  1. CFCs reach the stratosphere, where UV releases chlorine atoms.
  2. Cl + O3 gives ClO + O2.
  3. ClO + O gives Cl + O2, freeing the chlorine again.
  4. So one chlorine atom acts as a catalyst and can destroy thousands of ozone molecules.

Answer: Chlorine is regenerated in the cycle, so it acts as a catalyst for destroying ozone.

Example 5: Why Antarctica · 5 marks · Case-basedHOTS / competency

Case: Most CFCs were released in the northern hemisphere, yet the hole appears over Antarctica. (a) Name the season. (b) Explain the role of polar stratospheric clouds. (c) Name the agreements that reduced the problem.

Try it first, then show the solutionSolution
  1. (a) Spring, September to November.
  2. (b) Very cold clouds in the polar winter speed up the chlorine reactions when sunlight returns.
  3. (c) The Montreal Protocol (1987) and its Kigali Amendment (2016).

Answer: (a) Spring (b) cold clouds speed the reactions (c) Montreal and Kigali.

Where students lose marks in The Ozone Layer and Ozone Depletion

The slips examiners see most often, and what to write instead.

  • Mistake: Confusing ozone depletion with global warming

    Greenhouse effect is written for ozone loss.

    Do this: Ozone loss increases UV; greenhouse gases trap heat.

    Usually costs: 2 marks.

  • Mistake: Writing 220 as percentage

    Dobson Units misread.

    Wrong working

    220 per cent

    Right working

    About 220 Dobson Units

    Do this: The hole criterion is below about 220 Dobson Units.

    Usually costs: 1 mark.

  • Mistake: Missing year of protocol

    Only the name is given.

    Do this: Montreal Protocol 1987; Kigali 2016.

    Usually costs: 1 mark.

Exam tips

  • Write the two chlorine reactions.
  • Give both agreements with years.
  • Mention the Antarctic spring.
  • Say World Ozone Day is 16 September.

Practice questions on The Ozone Layer and Ozone Depletion

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 1 mark · MCQBoard level

    The ozone layer lies mainly in the:
    (a) troposphere
    (b) stratosphere
    (c) mesosphere
    (d) exosphere

    Hint

    15 to 35 km.

    Answer

    (b) Stratosphere.

  2. Q2 · 3 marks · Short answerHarder board level

    Name three ozone-depleting substances and their sources.

    Hint

    CFCs, halons, solvents.

    Answer

    CFCs from old refrigerators, air conditioners, sprays and foam; halons from fire extinguishers; carbon tetrachloride or methyl chloroform from solvents.

  3. Q3 · 3 marks · Short answerHarder board level

    Normal ozone is 280 DU and falls to 224 DU. Find the percentage fall and say if it is a hole.

    Hint

    Fall over original.

    Answer

    Fall = 280 - 224 = 56 DU; 56/280 x 100 = 20 per cent. 224 is above about 220 DU, so it is not yet a hole.

  4. Q4 · 4 marks · Long answerHarder board level

    Explain two effects of ozone depletion on humans and one on the marine food chain.

    Hint

    UV exposure.

    Answer

    More UV reaches the ground, raising skin cancer and cataracts and weakening immunity; it harms phytoplankton, reducing the base of the marine food chain.

  5. Q5 · 4 marks · Case-basedHOTS / competency

    Case: A report says the layer is recovering but slowly, and HFCs are still being phased down. (a) Why does recovery take decades? (b) Why target HFCs? (c) Name the amendment.

    Hint

    Stable gases persist; HFCs warm the climate.

    Answer

    (a) The stable gases already in the atmosphere persist for a long time. (b) HFCs are greenhouse gases. (c) Kigali Amendment, 2016.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.Unit for ozone?Show answer

    Dobson Unit.

  2. 2.Ozone layer height?Show answer

    About 15 to 35 km.

  3. 3.Which gas is the catalyst?Show answer

    Chlorine.

  4. 4.Agreement phasing down HFCs?Show answer

    Kigali Amendment, 2016.

  5. 5.World Ozone Day?Show answer

    16 September.

The Ozone Layer and Ozone Depletion: frequently asked questions

Does ozone depletion cause global warming?

No, it is mainly a UV problem; some substances involved are also greenhouse gases.

Why does ozone warm the stratosphere?

It absorbs ultraviolet energy.

Is India in the Montreal Protocol?

Yes, India is a party.