Ozone hole

Also called: Ozone depletion · Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Class 11, Ch 11 "World Climate and Climate Change"

Meaning

The ozone hole is a region of the stratosphere where total ozone falls below 220 Dobson units (DU). It forms over Antarctica every southern spring (September–October). Chlorine and bromine released from man-made chemicals such as CFCs and halons cause it. It is not a real "hole", only an area of severe thinning.

It matters because a thinner ozone layer lets more harmful UV-B rays reach the ground. This hurts human health, crops and ocean life. The ozone hole is also the main proof that the Montreal Protocol works: the layer is now slowly healing.

Explanation

The ozone layer and how it is measured

• Ozone (O3): a molecule of three oxygen atoms. It is a minor gas, but life depends on it.
• Where it lies:
• About 90% of all atmospheric ozone is in the stratosphere (the calm, dry layer above the troposphere).

• Most of it is at about 15–35 km above the ground (NCERT Class 11 gives 10–50 km).

• How it forms:

• Strong sunlight splits an oxygen molecule (O2) into two single oxygen atoms (O).
• Each single atom joins an O2 molecule to make O3.

• Ozone absorbs UV light and breaks apart, then forms again. This cycle soaks up UV energy and heats the stratosphere, so temperature rises with height in this layer.

• What it blocks: it blocks UV-C almost fully and UV-B partly. This means fewer cases of skin cancer and cataracts (clouding of the eye lens), less damage to crops, and protection for plankton (tiny drifting plants and animals at the base of the ocean food chain).

• Dobson unit (DU): the unit for total column ozone, which means all the ozone in a column of air from the ground to the top of the atmosphere.
• 1 DU = a layer of pure ozone 0.01 mm thick at standard temperature and pressure (0°C, 1 atmosphere).
• The global average is about 300 DU. Squeezed to ground pressure, this would be a layer only about 3 mm thick.
• Ozone hole: an area where ozone falls below 220 DU.

The chemicals that destroy ozone

• Ozone-depleting substances (ODS): man-made chemicals that carry chlorine or bromine up to the stratosphere.

Chemical	Main use	Harmful element
CFCs (chlorofluorocarbons)	Refrigerators, ACs, aerosol sprays, foams	Chlorine
Halons	Fire extinguishers	Bromine
Carbon tetrachloride (CTC)	Solvent, cleaning agent	Chlorine
Methyl bromide	Fumigant for soil and stored grain	Bromine
HCFCs (hydrochlorofluorocarbons)	Stop-gap CFC substitutes in ACs and foams	Chlorine (weaker effect)

• Why they reach so high:
• These gases are very stable and do not break down in the lower air.
• They drift slowly up into the stratosphere.
• There, strong UV breaks them apart and frees chlorine and bromine.

How the hole forms, step by step

• Step 1: Polar winter traps the air
• A polar vortex (a ring of strong west winds circling the pole in winter) traps very cold air over Antarctica.

• Temperatures fall below about −78°C.

• Step 2: Polar stratospheric clouds form

• Polar stratospheric clouds (PSCs) are thin clouds of ice and nitric acid crystals. They form at about 15–25 km, and only in this extreme cold.

• Chemical reactions on their surfaces turn harmless "stored" chlorine into active chlorine that is ready to attack ozone.

• Step 3: Spring sunlight starts the damage

• The Sun returns in September. Its light releases free chlorine atoms.

• Catalytic cycle (the chlorine is not used up, so the reaction keeps repeating): Cl + O3 → ClO + O2. The ClO then reacts again and frees the Cl. One chlorine atom destroys ozone again and again.

• Step 4: The hole closes

• By November–December the vortex weakens.
• Warmer air that is rich in ozone mixes in, and the hole fills up.

Why the hole forms over Antarctica and not the Arctic

Factor	Antarctic	Arctic
Polar vortex	Colder, stronger, steadier	Weaker, often broken up
Polar stratospheric clouds	Widespread every winter	Fewer and short-lived
Result	A large hole every spring	Thinning only in some very cold years

• Reason:
• Antarctica is a continent ringed by open ocean. Few mountains disturb the winds, so the vortex stays closed.
• In the north, the mix of land, sea and mountain ranges (Rockies, Himalaya) creates air waves that break up the vortex.

Where it occurs

• Antarctica (South Pole): the ozone hole forms here every September–October.
• Discovery: reported in 1985 by British Antarctic Survey scientists Joseph Farman, Brian Gardiner and Jonathan Shanklin.
• 2025 season: the average area from 7 September to 13 October 2025 was about 18.71 million km². This made it the 5th smallest since 1992 [7][8].
• Its largest one-day size was 22.86 million km² on 9 September 2025. That is about 30% smaller than the record hole of 2006 (average 26.60 million km²) [7].
• In 2025 the hole broke up nearly three weeks earlier than the average of the past decade. A weaker-than-normal polar vortex in August 2025 kept temperatures higher [7].

• Since peaking around 2000, ozone-depleting substances in the Antarctic stratosphere have fallen by about one-third relative to pre-ozone-hole levels [7].

• Arctic (North Pole): there is no regular hole. Ozone thins only in some very cold years.

• Global treaties:
• Vienna Convention (1985): a framework treaty for research and cooperation, with no binding cuts.
• Montreal Protocol: adopted on 16 September 1987. It sets binding phase-out dates for ODS. Developed countries cut first, and developing countries (Article 5 parties) get more time. In 2009 it became the first UN treaty ratified by every country. About 99% of ozone-depleting substances controlled by the Protocol have now been phased out worldwide [3].

• Kigali Amendment: agreed in 2016 at Kigali, Rwanda, and in force from 1 January 2019. It phases down HFCs.

• India:

• India joined the Montreal Protocol in 1992. The Ozone Cell of the Ministry of Environment, Forest and Climate Change (MoEFCC) runs the programme [3].
• India phased out CFCs, carbon tetrachloride and halons by 2010 [3].
• HCFCs: India follows an HCFC Phase-out Management Plan (HPMP), and its roadmap was launched in 2009 [4]. The 19th Meeting of the Parties (2007) moved the HCFC phase-out 10 years earlier [4]. By 2025, India had cut HCFC production and consumption by 67.5% [4].
• India Cooling Action Plan (ICAP, 2019): a 20-year plan (2017–18 to 2037–38) to cut cooling demand and refrigerant use and to shift to climate-friendly refrigerants.

Don't confuse with

• Ground-level ozone: this is the same gas, but it sits near the ground in the troposphere. It forms when vehicle and factory fumes react in sunlight. It is a pollutant that harms lungs and crops and is part of urban smog. Stratospheric ozone is the "good" shield.
• Polar winter vs ozone hole season: the cold and the PSCs build up in winter, but ozone is destroyed in spring (September–October), because sunlight is needed to free the chlorine.
• HFCs vs CFCs/HCFCs: HFCs have no chlorine, so they do not deplete ozone. They are strong greenhouse gases, often thousands of times stronger than CO2, so the Kigali Amendment controls them for climate reasons. CFCs and HCFCs do deplete ozone.
• Vienna Convention vs Montreal Protocol: Vienna (1985) is only a framework with no binding cuts. Montreal (1987) sets binding phase-out schedules.

Prelims Hooks

• Ozone hole = total ozone below 220 DU. The global average is about 300 DU, and 1 DU = 0.01 mm of pure ozone at STP.
• Halons carry bromine, while CFCs carry chlorine. Polar stratospheric clouds form only below about −78°C.
• Chronology: Vienna Convention 1985 → Montreal Protocol 1987 → universal ratification 2009 → Kigali 2016 (in force 2019). World Ozone Day = 16 September.
• Recovery to 1980 ozone levels, from earliest to latest: rest of the world ~2040 → Arctic ~2045 → Antarctic ~2066 [5][6].
• India's Kigali path: baseline 2024–26, freeze 2028, then cuts of 10% (2032) → 20% (2037) → 30% (2042) → 85% (2047) [2].
• The 2025 Antarctic ozone hole was the 5th smallest since 1992. The largest on record was in 2006 [7].

Mains Points

• Causes and process (GS-I):
• Man-made CFCs and halons carry chlorine and bromine into the stratosphere.
• The very cold Antarctic polar vortex forms PSCs, which turn chlorine into its active form.

• Spring sunlight then starts the catalytic loss of ozone. This explains why the hole is seasonal, polar, and much larger in the south than in the north.

• Montreal Protocol as a model treaty (GS-III):

• It has universal ratification (2009), clear deadlines, extra time for developing countries and science-based reviews.
• About 99% of ODS have been phased out [3], and the layer is set to recover between about 2040 and 2066 [5].

• Its lessons apply to climate agreements. Experts warn that stratospheric aerosol injection (a geoengineering idea to spray reflective sulphate particles into the stratosphere) could deepen the Antarctic ozone hole and delay recovery [5].

• The ozone–climate link and India (GS-III):

• HFCs replaced CFCs and HCFCs. This fixed the ozone problem but created a climate problem.
• Full compliance with the Kigali Amendment could avoid 0.3–0.5°C of warming by 2100 [5].
• For India, ICAP 2019 and the Kigali steps (freeze in 2028, an 85% cut by 2047) [2] matter because AC demand is rising fast in hot cities.

Related concepts

• Ozone layer
• Dobson unit
• Polar stratospheric clouds
• Montreal Protocol
• Kigali Amendment
• Stratospheric aerosol injection

Read more

• The ozone layer, its depletion and the Montreal Protocol