🌡️ Chapter 06 · Topic 03 · Climate Change

Ozone Layer Depletion

Stratospheric ozone, ozone-depleting substances (CFCs, HCFCs, halons), ozone hole over Antarctica, Vienna Convention, Montreal Protocol, Kigali Amendment, and ozone recovery — complete UPSC & PSC notes.

🛡️ Stratospheric Ozone — The Protective Shield

The ozone layer is a region of Earth’s stratosphere containing high concentrations of ozone (O₃). It is our planet’s natural sunscreen, absorbing most of the Sun’s harmful ultraviolet radiation.

Located in the stratosphere, approximately 15–35 km above Earth’s surface (peak concentration at ~25 km)
Absorbs UV-B (280–315 nm) and UV-C (100–280 nm) radiation — both harmful to life
UV-A (315–400 nm) is not absorbed by ozone — reaches Earth’s surface
Ozone concentration is measured in Dobson Units (DU) — 1 DU = 0.01 mm thickness of pure ozone at standard conditions
Normal ozone column: 300 DU; ozone hole defined as <220 DU
Despite being called a “layer,” ozone is spread thinly — if compressed to sea-level pressure, it would be only ~3 mm thick

📌 Tropospheric vs Stratospheric Ozone: Stratospheric ozone (15–35 km) = beneficial — protects from UV radiation. Tropospheric (ground-level) ozone = harmful — a secondary air pollutant formed from NOₓ + VOCs + sunlight; causes respiratory problems and crop damage. “Good ozone up high, bad ozone nearby” — a common exam distinction.

⚗️ Ozone Formation and Destruction — Chapman Cycle

The Chapman cycle (1930) describes the natural formation and destruction of stratospheric ozone:

Formation: O₂ + UV radiation → 2O (atomic oxygen); O + O₂ → O₃ (ozone)
Natural destruction: O₃ + UV → O₂ + O; O + O₃ → 2O₂
In the natural cycle, ozone formation and destruction are in balance
Human-made chemicals (CFCs etc.) disrupt this balance by accelerating ozone destruction

⭐ Key Point: The Chapman cycle shows that ozone is constantly being formed and destroyed naturally. The problem is that ozone-depleting substances (ODS) dramatically accelerate the destruction side, breaking the natural equilibrium. One chlorine atom from a CFC molecule can destroy 100,000 ozone molecules through a catalytic chain reaction.

☠️ Ozone-Depleting Substances (ODS)

Substance	Full Name	Main Uses	ODP	Status
CFCs	Chlorofluorocarbons (e.g., CFC-11, CFC-12)	Refrigerants, aerosol propellants, foam blowing agents	0.6–1.0	Phased out under Montreal Protocol
HCFCs	Hydrochlorofluorocarbons (e.g., HCFC-22)	Refrigerants (transitional replacement for CFCs)	0.02–0.11	Being phased out (transitional)
Halons	Bromofluorocarbons (e.g., Halon-1301)	Fire extinguishers	3–10	Phased out; highest ODP per molecule
Methyl Bromide	CH₃Br	Agricultural fumigant (soil sterilisation)	0.6	Phased out in developed countries
Carbon Tetrachloride	CCl₄	Solvent, fire extinguishers (historical)	1.1	Phased out
Methyl Chloroform	CH₃CCl₃	Industrial solvent	0.12	Phased out

📌 ODP (Ozone Depletion Potential): A measure of how much a substance depletes the ozone layer relative to CFC-11 (ODP = 1). Halons have the highest ODP per molecule (3–10). CFCs are the most important ODS due to their large quantities released. HFCs (used as CFC replacements) have zero ODP but high GWP — addressed by the Kigali Amendment.

🔬 Mechanism of Ozone Depletion by CFCs

CFCs are released at ground level (from refrigerators, aerosols, etc.) — they are very stable and do not react in the troposphere
CFCs slowly drift up to the stratosphere over 6–10 years
In the stratosphere, intense UV radiation breaks the CFC molecule: CFC → Cl (chlorine radical) + other fragments
The chlorine radical attacks ozone: Cl + O₃ → ClO + O₂
ClO reacts with atomic oxygen: ClO + O → Cl + O₂
The chlorine radical is regenerated — it is a catalyst (not consumed in the reaction)
One Cl atom can destroy 100,000 ozone molecules before being deactivated
Bromine (from halons) is even more effective at destroying ozone than chlorine

⭐ Nobel Prize: Mario Molina, F. Sherwood Rowland, and Paul Crutzen won the Nobel Prize in Chemistry (1995) for their work on the formation and decomposition of stratospheric ozone, including the discovery of the CFC-ozone depletion mechanism. Molina and Rowland first proposed the CFC-ozone link in 1974.

🕳️ Ozone Hole over Antarctica

The Antarctic ozone hole was discovered in 1985 by British scientists (Farman, Gardiner, Shanklin) from the British Antarctic Survey
It forms every year during the Antarctic spring (September–November)
Why Antarctica? Two special conditions:
- Polar Vortex — a strong circular wind pattern that isolates Antarctic air during winter, preventing mixing with ozone-rich air from lower latitudes
- Polar Stratospheric Clouds (PSCs) — form at very low temperatures (<−78°C) during Antarctic winter; provide surfaces for chemical reactions that activate chlorine from CFCs
In spring, sunlight returns → UV activates the chlorine → rapid ozone destruction → ozone hole forms
The hole has reached up to 29 million km² in size (larger than North America)
Ozone levels inside the hole can drop to <100 DU (normal ~300 DU)

📌 Arctic Ozone Hole: The Arctic also experiences ozone depletion, but less severe than Antarctica because the Arctic polar vortex is weaker and less stable. However, record Arctic ozone depletion occurred in 2020 — a 40% reduction over the Arctic. Climate change may actually worsen Arctic ozone depletion by cooling the stratosphere (even as the troposphere warms).

☀️ Effects of Ozone Depletion

Effect	Details
Skin cancer	UV-B causes DNA damage in skin cells → melanoma (most deadly) and non-melanoma skin cancers. 1% decrease in ozone → 2% increase in UV-B → 3% increase in skin cancer
Cataracts	UV-B damages the lens of the eye → cataracts (clouding of lens) → blindness. Leading cause of blindness globally
Immune suppression	UV-B suppresses the human immune system → increased susceptibility to infections and reduced vaccine effectiveness
Crop damage	UV-B damages plant DNA, reduces photosynthesis, stunts growth; affects soybean, wheat, rice yields
Marine ecosystem damage	UV-B penetrates ocean surface → damages phytoplankton (base of marine food chain) → disrupts entire marine ecosystem
Material degradation	UV-B degrades plastics, paints, rubber, and building materials faster

📜 International Treaties — Vienna Convention & Montreal Protocol

Vienna Convention (1985)

First international agreement to address ozone depletion
A framework convention — set the stage for action but did not mandate specific reductions
Established the principle of international cooperation on ozone protection
Entered into force: 1988; 197 parties (universal ratification)

Montreal Protocol (1987)

The most successful environmental treaty in history — often cited as the gold standard for international environmental cooperation
Mandated the phase-out of ozone-depleting substances (CFCs, halons, HCFCs, etc.)
197 parties — the only UN treaty to achieve universal ratification
Differentiated timelines: developed countries phased out CFCs by 1996; developing countries by 2010
Has prevented an estimated 2 million cases of skin cancer per year and protected 1.5 million lives annually
Also has significant climate co-benefits — ODS are also potent GHGs; Montreal Protocol has done more to slow climate change than the Kyoto Protocol
Secretariat: UNEP, Nairobi

⭐ Key Amendments to Montreal Protocol:

London Amendment (1990) — added more substances; accelerated phase-out
Copenhagen Amendment (1992) — accelerated CFC phase-out; added HCFCs and HBFCs
Beijing Amendment (1999) — added bromochloromethane
Kigali Amendment (2016) — phase-down of HFCs (see below)

Kigali Amendment (2016)

Adopted in Kigali, Rwanda in October 2016; entered into force January 2019
Targets HFCs (Hydrofluorocarbons) — used as replacements for CFCs; have zero ODP but very high GWP (hundreds to thousands)
HFCs are powerful greenhouse gases — their phase-down could prevent 0.5°C of warming by 2100
Developed countries: reduce HFCs by 85% by 2036 (baseline 2011–2013)
Developing countries (Group 1, including China): reduce by 80% by 2045
Developing countries (Group 2, including India, Pakistan, Gulf states): reduce by 85% by 2047
India ratified the Kigali Amendment in 2021

🌱 Ozone Layer Recovery

Thanks to the Montreal Protocol, atmospheric concentrations of most ODS are declining
The ozone layer is slowly recovering — but it takes decades because ODS persist in the atmosphere for many years
UNEP/WMO Scientific Assessment (2022): ozone layer expected to recover to 1980 levels by:
- Mid-latitudes: ~2040
- Arctic: ~2045
- Antarctica: ~2066
The Antarctic ozone hole is showing signs of recovery — but year-to-year variability is large
Concern: illegal CFC-11 emissions detected (2018–2019) — traced to eastern China; subsequently reduced

📌 India’s Commitments: India is a party to the Montreal Protocol and its amendments. India has successfully phased out CFCs and halons. India is phasing out HCFCs (HCFC-22 used in air conditioners). Under the Kigali Amendment, India will phase down HFCs by 85% by 2047. India’s HCFC Phase-out Management Plan (HPMP) is supported by the Multilateral Fund of the Montreal Protocol.

✅ Revision Checklist — Ozone Layer Depletion

✅ Ozone layer = stratosphere = 15–35 km altitude = absorbs UV-B and UV-C
✅ Dobson Units (DU) = measure of ozone column; normal ~300 DU; ozone hole <220 DU
✅ Chapman cycle = natural ozone formation and destruction = in balance without ODS
✅ CFCs = most important ODS = stable in troposphere = drift to stratosphere over 6–10 years
✅ 1 Cl atom from CFC = destroys 100,000 ozone molecules (catalytic)
✅ Halons = highest ODP per molecule (3–10) = used in fire extinguishers
✅ ODP = Ozone Depletion Potential = relative to CFC-11 (ODP=1)
✅ Antarctic ozone hole = discovered 1985 = forms in spring (Sep–Nov)
✅ Polar vortex + Polar Stratospheric Clouds = conditions for Antarctic ozone hole
✅ Effects: skin cancer, cataracts, immune suppression, crop damage, marine ecosystem damage
✅ 1% ozone decrease → 2% UV-B increase → 3% skin cancer increase
✅ Vienna Convention (1985) = framework; Montreal Protocol (1987) = binding phase-out
✅ Montreal Protocol = only UN treaty with universal ratification (197 parties)
✅ Most successful environmental treaty = prevented 2 million skin cancer cases/year
✅ Kigali Amendment (2016) = phase-down of HFCs = could prevent 0.5°C warming
✅ HFCs = zero ODP but high GWP = replaced CFCs = now being phased down
✅ India ratified Kigali Amendment in 2021; HFC phase-down by 85% by 2047
✅ Ozone recovery: mid-latitudes ~2040; Arctic ~2045; Antarctica ~2066
✅ Nobel Prize Chemistry 1995 = Molina, Rowland, Crutzen = CFC-ozone link