The ozone layer, its depletion and the Montreal Protocol

Detail

What the ozone layer is and where it lies

• Ozone (O3): a molecule made of three oxygen atoms. It is a minor gas, but it matters a great deal for life.
• Ozone layer: the part of the stratosphere (the layer above the troposphere, where the air is calm and dry) that holds most ozone.
• About 90% of all atmospheric ozone is in the stratosphere.

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

• How ozone forms in the stratosphere:

• Strong sunlight splits an oxygen molecule (O2) into two single oxygen atoms (O).
• Each single atom joins another O2 molecule → O3 (ozone).
• Ozone absorbs UV light and breaks apart again. This constant cycle soaks up UV energy and heats the stratosphere. This is why temperature rises with height in the stratosphere.

Why the ozone layer matters

• It absorbs harmful ultraviolet (UV) radiation, mainly UV-B and UV-C. UV-C is almost fully blocked. UV-B is partly blocked.
• Protection given:
• Humans: fewer cases of skin cancer and cataracts (clouding of the eye lens).
• Crops: less damage to plant growth.
• Oceans: protects plankton (tiny drifting plants and animals at the base of the marine food chain).

Good ozone vs bad ozone

Feature	Stratospheric ozone ("good")	Ground-level ozone ("bad")
Height	About 15–35 km (stratosphere)	Near the ground (troposphere)
Source	Natural: sunlight splits O2	Vehicle and factory fumes reacting in sunlight
Effect	Shields life from UV-B and UV-C	A pollutant. It harms lungs and crops and is part of urban smog

Measuring ozone: the Dobson unit

• Dobson unit (DU): the unit for total column ozone (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).

• Global average ≈ 300 DU. If all the ozone above a place were squeezed to ground pressure, it would be only about 3 mm thick.

• Ozone hole: an area where total ozone falls below 220 DU. It is not a real "hole". It is a region of severe thinning.

The Antarctic ozone hole

• When and where: over Antarctica every southern spring (September–October).
• Discovery: reported in 1985 by the British Antarctic Survey scientists Joseph Farman, Brian Gardiner and Jonathan Shanklin.
• Recent size (2025 season):
• 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].

What destroys ozone: the chemicals

• Ozone-depleting substances (ODS): man-made chemicals that carry chlorine or bromine into 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)	Interim CFC substitutes in ACs and foams	Chlorine (weaker effect)

• These gases are very stable. They do not break down in the lower air. They drift slowly up to the stratosphere, where strong UV breaks them and frees chlorine and bromine.

How the hole forms: step by step

• Step 1: Polar winter isolates 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): thin clouds of ice and nitric acid crystals. They form at about 15–25 km only in this extreme cold.

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

• Step 3: Spring sunlight starts the destruction

• When the Sun returns in September, its light releases free chlorine atoms.

• Catalytic cycle (the chlorine is not used up, so it repeats): Cl + O3 → ClO + O2, and then ClO reacts again to free Cl. One chlorine atom destroys ozone again and again.

• Step 4: The hole closes

• By November–December, the vortex weakens and warmer, ozone-rich air mixes in. The hole fills up.

Why the hole is over Antarctica and not the Arctic

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

• The Antarctic continent is ringed by open ocean. Few mountains disturb the winds, so the vortex stays closed. In the north, land, sea and mountain ranges (Rockies, Himalaya) create waves that disturb the vortex.

The global response: from Vienna to Kigali

• Vienna Convention for the Protection of the Ozone Layer (1985): a framework treaty. It set up research and cooperation but had no binding cuts.
• Montreal Protocol on Substances that Deplete the Ozone Layer:
• Adopted on 16 September 1987. This date is now World Ozone Day (International Day for the Preservation of the Ozone Layer).
• It sets binding phase-out schedules for ODS. Developed countries cut first. Developing countries (Article 5 parties) get more time.
• In 2009 it became the first UN treaty ratified by every country (universal ratification).

• About 99% of ozone-depleting substances controlled by the Protocol have now been phased out worldwide [3].

• Recovery forecast (WMO/UNEP Scientific Assessment of Ozone Depletion 2022):

Region	Return to 1980 ozone values
Antarctic	About 2066
Arctic	About 2045
Rest of the world	About 2040

• These dates assume current policies stay in place [5][6].

The Kigali Amendment: from ozone to climate

• The problem: CFCs and HCFCs were replaced by HFCs (hydrofluorocarbons). HFCs have no chlorine, so they do not harm ozone. But they are very strong greenhouse gases, often thousands of times stronger than CO2.
• Kigali Amendment: agreed in 2016 at Kigali, Rwanda, and in force from 1 January 2019. It phases down (gradually reduces, does not ban) HFCs.
• Full compliance is expected to avoid 0.3–0.5°C of global warming by 2100 [5].
• So the Montreal Protocol now protects both the ozone layer and the climate.

India and the ozone treaties

• Joining: India joined the Montreal Protocol in 1992. The Ozone Cell of the Ministry of Environment, Forest and Climate Change (MoEFCC) runs India's programme [3].
• Phase-outs completed: CFCs, carbon tetrachloride and halons were all phased out by 2010 [3].
• HCFCs:
• India follows an HCFC Phase-out Management Plan (HPMP). A roadmap was launched in 2009 with industry and research institutions [4].
• The 19th Meeting of the Parties (2007) moved the HCFC phase-out schedule 10 years earlier [4].

• By 2025, India had cut HCFC production and consumption by 67.5% [4].

• Kigali Amendment and India:

• The Union Cabinet approved ratification in August 2021 [2].
• India is in the group with a 2024–26 baseline and a freeze in 2028.
• Cuts come in four steps from 2032: 10% by 2032, 20% by 2037, 30% by 2042 and 85% by 2047 [2].

• A national HFC phase-down strategy was to be drawn up with industry by 2023 [2].

• India Cooling Action Plan (ICAP, 2019): a 20-year plan (2017–18 to 2037–38) to cut cooling demand and refrigerant use and to move to climate-friendly refrigerants. It supports the Kigali targets.

Beyond NCERT: stratospheric aerosol injection

• Stratospheric aerosol injection (SAI): a proposed geoengineering idea (deliberate large-scale change of Earth's climate). Planes would spray reflective sulphate particles into the stratosphere.
• The model it copies: big volcanic eruptions (e.g. Mount Pinatubo, Philippines, 1991) put sulphate into the stratosphere and cool the Earth for a year or two.
• Risks:
• Ozone: the 2022 Ozone Assessment found that SAI could deepen the Antarctic ozone hole and delay ozone recovery [5].
• Monsoon: less sunlight could weaken land–sea heating contrasts and disturb monsoon rainfall, including over South Asia.
• Termination shock: if spraying stops suddenly, the warming it was hiding returns very quickly.
• Governance: it is untested at scale. There is no agreement on who would decide the "global thermostat" (link: climate-change).

Prelims Hooks

• About 90% of ozone lies in the stratosphere, mostly at 15–35 km. Ground-level ozone is a pollutant, not a shield.
• 1 Dobson unit = 0.01 mm of pure ozone at STP. Global average ≈ 300 DU. Ozone hole = below 220 DU.
• Ozone hole forms over Antarctica in September–October (southern spring), not in the polar winter, because sunlight is needed to free the chlorine.
• Polar stratospheric clouds form below about −78°C. Halons carry bromine, while CFCs carry chlorine.
• Chronology: Vienna Convention 1985 → Montreal Protocol 1987 → universal ratification 2009 → Kigali 2016 (in force 2019).
• World Ozone Day = 16 September (the date the Montreal Protocol was adopted).
• HFCs do not deplete ozone but are strong greenhouse gases. This is why they are controlled by Kigali, a Montreal Protocol amendment, and not by the Kyoto Protocol alone.
• India's Kigali path: baseline 2024–26, freeze 2028, cuts of 10% (2032) → 20% (2037) → 30% (2042) → 85% (2047) [2].
• Recovery to 1980 levels: Rest of world ~2040 → Arctic ~2045 → Antarctic ~2066 (earliest to latest) [5].
• 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 → chlorine and bromine reach the stratosphere → very cold polar vortex forms PSCs → spring sunlight starts catalytic ozone loss. This explains why the hole is seasonal and polar, and bigger in the south than the north.
• Montreal Protocol as a model (GS-III): universal ratification (2009), clear deadlines, more time for developing countries and science-based reviews (2022 Assessment). Result: about 99% of ODS phased out [3] and recovery due by about 2040–2066 [5]. It is often called the most successful environmental treaty and offers lessons for climate agreements.
• Ozone–climate link (GS-III): CFC substitutes (HFCs) traded an ozone problem for a climate problem. Kigali fixes this and could avoid 0.3–0.5°C of warming by 2100 [5]. For India, ICAP 2019 and the Kigali steps (2028 freeze; 85% cut by 2047) [2] matter because AC demand is rising fast in hot cities.
• Emerging risk (GS-III): geoengineering through stratospheric aerosol injection could delay ozone recovery [5], disturb the monsoon and cause termination shock. A precautionary, multilateral governance approach is needed.