Topic 17: Global Warming & Ozone Depletion

Greenhouse gases and global warming potential, climate forcing and feedback mechanisms, cryosphere loss, stratospheric ozone depletion, ozone depleting substances (CFCs/HFCs), and the Vienna Convention–Montreal Protocol–Kigali Amendment framework.

GS Paper III — Environment & Ecology Reading time: 16 min Prelims: Very High Yield Mains: GS-III

1Greenhouse Effect — Basics
2Conceptual Clarity — Greenhouse Effect vs Global Warming vs Climate Change
3Major Greenhouse Gases & GWP
4Climate Forcing & Feedback Mechanisms
5Cryosphere Loss & Impacts
6Stratospheric Ozone & the Ozone Hole
7Ozone Depleting Substances (ODS)
8Vienna Convention & Montreal Protocol
9Kigali Amendment (2016)
10Current Affairs
11Prelims PYQs
12Mains PYQs
13Revision Box

Conceptual Clarity — Why this Topic Matters

UPSC tests global warming & ozone in three distinct ways:

Definitional / static: direct recall — GWP hierarchy (SF6 > CFC > HFC > N2O > CH4 > CO2), which gases are ODS vs GHG, treaty years (Vienna 1985, Montreal 1987, Kigali 2016), the greenhouse-effect mechanism.
Statement-elimination: the classic trap — is HFC an ODS? (No, zero ODP but high GWP). Is the ozone hole caused by CO2? (No). Does GWP compare to CO2? (Yes). One false statement per set.
Applied / current: ozone-recovery assessments, India's Kigali/Cooling Action Plan schedule, 2024 breaching 1.5°C — feeding prelims news-recall + Mains GS-III international-governance answers (see Current Affairs section).

Focus especially on ODS vs GHG (HFC distinction), the Vienna–Montreal–Kigali chain, and positive vs negative climate feedbacks — the three highest-frequency themes.

1. Greenhouse Effect — Basics

Greenhouse effect: natural process — GHGs absorb + re-radiate outgoing longwave (infrared) radiation from Earth's surface, trapping heat.
Keeps planet's average surface temp habitable at ~15°C (without it: ~-18°C).
Mechanism: shortwave solar radiation passes in, warms surface; surface re-emits longwave IR; GHGs absorb + re-radiate part of it back down, warming lower atmosphere further.
Global warming: human-amplified enhancement of this effect via GHG emissions (fossil fuels, deforestation, industry, agriculture) since Industrial Revolution.
Raised global mean surface temp ~1.1-1.2°C above pre-industrial levels (IPCC AR6, 2021-2023).

Fig 17.1 — The greenhouse effect: shortwave in, longwave trapped and re-radiated by GHGs

2. Conceptual Clarity — Greenhouse Effect vs Global Warming vs Climate Change

Don't confuse these three

Greenhouse effect: natural physical mechanism itself (always present, life-sustaining).
Global warming: specific trend of rising average global surface temp, caused by enhanced (human-amplified) greenhouse effect.
Climate change: broader umbrella term — all long-term shifts in temperature, precipitation, wind, extreme weather, sea level, cryosphere.
Global warming = one driver/symptom of climate change; climate change also covers regional cooling anomalies, altered monsoons, shifting storm tracks.

3. Major Greenhouse Gases & Global Warming Potential (GWP)

Global Warming Potential (GWP) measures how much heat a GHG traps in the atmosphere relative to the same mass of CO2 over a specified time horizon (typically 100 years), where CO2 is assigned a reference value of 1.

Gas	Key sources	GWP (100-yr, approx.)
Carbon dioxide (CO2)	Fossil fuel combustion, deforestation, cement	1
Methane (CH4)	Livestock, paddy fields, landfills, gas leaks	~28-36
Nitrous oxide (N2O)	Nitrogenous fertilisers, industrial processes	~265-298
Tropospheric ozone (O3)	Secondary pollutant, vehicle/industrial emissions	Short-lived, high localised forcing
CFCs (e.g., CFC-12)	Older refrigerants, aerosols (largely phased out)	~10,000+
HFCs (e.g., HFC-134a)	Modern refrigerants, ACs (CFC/HCFC replacements)	~1,400-4,000
Sulphur hexafluoride (SF6)	Electrical switchgear insulation	~23,500 (highest common GHG)

HFCs were adopted as ozone-safe CFC replacements under the Montreal Protocol but turned out to be extremely potent GHGs — this contradiction is precisely what the Kigali Amendment (Section 9) was designed to fix.

4. Climate Forcing & Feedback Mechanisms

Radiative forcing is the change in energy balance (W/m²) at the top of the atmosphere caused by a factor (e.g., added GHGs, aerosols, land-use change) — positive forcing warms the climate system, negative forcing cools it.

Feedback loops

Positive (amplifying) feedback: Ice-albedo feedback — melting ice/snow exposes darker ocean/land (lower albedo), absorbing more solar radiation, causing further warming and melting. Water vapour feedback — warming increases atmospheric water vapour (itself a GHG), amplifying further warming. Permafrost-carbon feedback — thawing permafrost releases stored CH4/CO2, adding to warming.
Negative (dampening) feedback: Increased cloud cover (in some conditions) can reflect more sunlight; enhanced plant growth under higher CO2 (CO2 fertilisation) can increase carbon uptake, partially offsetting emissions (though this effect saturates and is outweighed by other factors).

5. Cryosphere Loss & Impacts

The cryosphere (all frozen water on Earth — glaciers, ice sheets, sea ice, permafrost, snow cover) is highly sensitive to warming and is retreating globally.

Glacier retreat

Himalayan glaciers (including Gangotri, source of the Ganga) are retreating; threatens long-term freshwater security for glacier-fed river basins.

Arctic sea ice decline

September Arctic sea ice extent has declined sharply since satellite records began (1979), amplifying Arctic warming via ice-albedo feedback ("Arctic amplification").

Permafrost thaw

Siberian/Arctic permafrost thaw releases trapped CH4/CO2 and destabilises infrastructure built on frozen ground.

Cryosphere loss directly contributes to sea-level rise (thermal expansion + land-ice melt) and disrupts freshwater supply for billions dependent on glacier/snowmelt-fed rivers (Ganga, Indus, Brahmaputra, Mekong).

6. Stratospheric Ozone & the Ozone Hole

Ozone layer: stratosphere (~15-35 km altitude), absorbs 97-99% of sun's harmful UV-B radiation.
Protects life from DNA damage, skin cancer, cataracts, crop/marine ecosystem harm.
Naturally formed + destroyed continuously (Chapman cycle); anthropogenic ODS disrupt this balance.
Antarctic ozone hole: severe seasonal thinning (discovered 1980s), forms each austral spring (Sept-Nov).
Extreme cold triggers Polar Stratospheric Clouds (PSCs) that activate Cl/Br compounds — catalytically destroy ozone once sunlight returns.

Fig 17.2 — Catalytic chlorine cycle: a single Cl radical destroys thousands of ozone molecules before removal from the stratosphere

7. Ozone Depleting Substances (ODS)

Substance	Former/current use	Ozone-depleting?
CFCs (chlorofluorocarbons)	Refrigerants, aerosol propellants, foam blowing	Yes — high ODP, being phased out globally
Halons	Fire extinguishers	Yes — very high ODP
HCFCs (hydrochlorofluorocarbons)	Transitional CFC replacements	Yes — lower ODP than CFCs, being phased out
Methyl bromide	Agricultural fumigant	Yes — moderate ODP
HFCs (hydrofluorocarbons)	Current AC/refrigerant of choice	No ODP, but very high GWP (addressed by Kigali)

8. Vienna Convention & Montreal Protocol

The Vienna Convention for the Protection of the Ozone Layer (1985) was a framework treaty establishing international cooperation to research and monitor ozone depletion — it set no binding reduction targets but created the platform for future action.

Montreal Protocol (1987), under Vienna Convention, widely regarded as most successful international environmental agreement.
Set binding, phased timelines for eliminating ODS production/consumption.
Applies CBDR — developing countries get longer compliance timelines + financial/technical aid via Multilateral Fund.
Result: ozone layer on track to recover to 1980 levels by mid-century (UNEP/WMO assessments).

Universal ratification: the Montreal Protocol is the first and only UN treaty ratified by all 198 UN member states — a rare full-consensus environmental success story frequently cited in Mains answers on international environmental governance.

Treaty / Instrument	Year	Binding targets?	What it covers
Vienna Convention	1985	No (framework only)	Cooperation, research, monitoring of ozone layer — the umbrella treaty
Montreal Protocol	1987	Yes	Binding phase-out of ODS (CFCs, halons, HCFCs, methyl bromide); CBDR + Multilateral Fund
London / Copenhagen / Beijing Amendments	1990s–2000s	Yes	Progressively tightened ODS phase-out schedules
Kigali Amendment	2016 (in force 2019)	Yes	Phase-down of HFCs (GHGs, not ODS); India freeze 2028, ~85% cut by 2047

Prelims hook: Vienna = framework (no targets); Montreal = ODS phase-out; Kigali = HFC phase-down. Ozone treaties, NOT under UNFCCC/Kyoto/Paris — a common wrong-basket distractor. Note "phase-out" (ODS, eliminate) vs "phase-down" (HFC, reduce).

9. Kigali Amendment (2016)

Kigali Amendment to Montreal Protocol (adopted 2016, in force 2019) extends mandate to phase down HFCs.
HFCs: not ozone-depleting, but potent GHGs used as CFC/HCFC replacements.
Sets binding HFC-reduction schedules, again on CBDR lines — developed countries cut earliest.
India/China/other developing countries follow later schedule: India's freeze year 2028, ~85% cut below baseline by 2047.

The Kigali Amendment is projected to avoid up to 0.4°C of global warming by 2100 — making it one of the single most impactful climate mitigation measures embedded within ozone-layer governance architecture.

10. Current Affairs (2024–2026)

The fastest-moving, most exam-relevant edge — verify each against the latest WMO/UNEP release before your attempt.

2024 breached 1.5°C for a full year check for latest update or data

WMO confirmed 2024 as the warmest year on instrumental record and the first full calendar year to breach 1.5°C above pre-industrial levels — 2023 and 2024 rank as the two hottest years, intensifying urgency around the Paris Agreement's temperature goal.

Ozone layer on recovery path check for latest update or data

UNEP/WMO Scientific Assessments continue to confirm the Antarctic ozone hole is recovering — on track for 1980 levels by ~2066 (Antarctic), ~2045 (Arctic), ~2040 (rest of world) — though annual hole size still fluctuates with meteorology.

India's HFC phase-down & Cooling Action Plan check for latest update or data

India continued implementing Kigali commitments via the India Cooling Action Plan (ICAP) and HFC phase-down schedule (freeze 2028), with industry shifting toward low-GWP refrigerants and energy-efficient cooling.

11. Prelims PYQs

UPSC CSE 2023

Q. Consider the following statements regarding Global Warming Potential (GWP):

1. GWP measures the heat-trapping ability of a gas relative to carbon dioxide over a specified time period.
2. Methane has a higher GWP than carbon dioxide.
3. Sulphur hexafluoride has a lower GWP than carbon dioxide.

Which of the statements given above is/are correct?

(a)1 and 2 only
(b)2 and 3 only
(c)1 and 3 only
(d)1, 2 and 3

Ans: (a). SF6 has the highest GWP (~23,500), not lower than CO2 — so statement 3 is false.

UPSC CSE 2022

Q. The Kigali Amendment to the Montreal Protocol is related to which of the following?

(a)Phasing out of chlorofluorocarbons
(b)Phasing down of hydrofluorocarbons
(c)Reduction of carbon dioxide emissions
(d)Ban on production of methyl bromide

Ans: (b). Kigali (2016) phases down HFCs — potent GHGs, not ODS.

UPSC CSE 2021

Q. With reference to the Antarctic ozone hole, consider the following statements:

1. The ozone hole forms primarily during the austral spring (September-November).
2. Polar Stratospheric Clouds play a role in activating chlorine compounds.
3. The ozone hole is caused primarily by rising carbon dioxide concentrations.

Which of the statements given above is/are correct?

(a)1 and 2 only
(b)2 and 3 only
(c)1 and 3 only
(d)1, 2 and 3

Ans: (a). Ozone hole is from Cl/Br radicals, NOT CO2 — statement 3 is the classic trap.

UPSC CSE 2020

Q. Which of the following is/are ozone-depleting substance(s)?

1. Chlorofluorocarbons
2. Halons
3. Hydrofluorocarbons

Select the correct answer using the code given below:

(a)1 only
(b)1 and 2 only
(c)2 and 3 only
(d)1, 2 and 3

Ans: (b). CFCs + halons deplete ozone; HFCs have zero ODP (they are GHGs).

UPSC CSE 2019

Q. Which of the following gases is/are considered greenhouse gas(es)?

1. Carbon dioxide
2. Methane
3. Nitrous oxide

Select the correct answer using the code given below:

(a)1 only
(b)2 and 3 only
(c)1 and 3 only
(d)1, 2 and 3

Ans: (d). All three (CO2, CH4, N2O) are greenhouse gases.

UPSC CSE 2017

Q. "Climate Action Tracker" which monitors the emission reduction pledges of various countries is a:

(a)Database created by coalition of research organisations
(b)Wing of "International Panel of Climate Change"
(c)Committee under United Nations Framework Convention on Climate Change
(d)Agency promoted and financed by European Union and headquartered in Brussels

Ans: (a). An independent research-coalition database, not a UN/UNFCCC body.

UPSC CSE 2016

Q. The Montreal Protocol, an international treaty, is mainly associated with:

(a)Conservation of coral reefs
(b)Regulation and phasing out of ozone-depleting substances
(c)Prevention of trade in endangered species
(d)Reduction of greenhouse gas emissions under the Kyoto framework

Ans: (b). Montreal Protocol = phasing out ODS (ozone treaty, not Kyoto/GHG).

UPSC CSE 2014

Q. In the context of WHO Air Quality Guidelines, regarding "particulate matter":

1. Particulate matter is generally categorised into PM10 and PM2.5 based on aerodynamic diameter.
2. PM2.5 poses a greater health risk due to deeper penetration into lung tissue.

Select the correct answer using the code given below:

(a)1 only
(b)2 only
(c)Both 1 and 2
(d)Neither 1 nor 2

Ans: (c). Both correct — PM10/PM2.5 by aerodynamic diameter; PM2.5 penetrates deeper into lungs.

12. Mains PYQs

GS-III, 2023

Q. "Rapid economic progress in developing countries seems to have an adverse effect on the global climate." Explain with justification.

Model Answer Framework

Introduction: Frame the growth-emissions link — developing economies (China, India, Southeast Asia) industrialising rapidly; note China now world's largest CO2 emitter, India 3rd, though per-capita emissions remain low.
Body — how growth harms climate:
- Coal-heavy energy mix — India ~70%+ electricity from coal; cheapest baseload for development
- Rising vehicle ownership, urbanisation, construction (cement/steel = hard-to-abate sectors)
- Land-use change — deforestation for agriculture/infrastructure releases stored carbon
- Rising cooling demand — AC/refrigeration drives HFC + electricity growth (India Cooling Action Plan flags this)
Body — the counter-argument (balance):
- CBDR principle — historical responsibility lies with the developed West; developing nations have a right to development
- Low per-capita emissions — India ~1.9 t CO2/capita vs USA ~14 t
- Leapfrogging — solar (ISA), LED (UJALA), EV push (FAME) decouple growth from emissions
- India's Panchamrit / Net-Zero-2070 pledge at COP26
Conclusion: Growth need not be climate-hostile — sustainable-development pathway (renewables + efficiency + climate finance from developed nations) reconciles both.

250 words · 15 marks

GS-III, 2022

Q. Describe the various causes and the effects of ozone layer depletion. What steps are being taken at national and international levels to protect the ozone layer?

Model Answer Framework

Introduction: Define stratospheric ozone (O3) as Earth's UV shield in the 15–35 km layer; depletion = thinning caused by ozone-depleting substances (ODS).
Body — causes:
- CFCs, halons, carbon tetrachloride, methyl bromide release Cl/Br radicals in the stratosphere
- Catalytic cycle — one Cl atom destroys ~100,000 O3 molecules
- Antarctic ozone hole — Polar Stratospheric Clouds activate radicals during Sept–Nov spring
Body — effects:
- Human health — skin cancer, cataracts, immune suppression
- Ecology — damage to phytoplankton (ocean food-chain base), crop yield loss
- Materials — degradation of plastics, paints
Body — steps taken:
- International — Vienna Convention (1985) framework; Montreal Protocol (1987) binding ODS phase-out, universally ratified; Kigali Amendment (2016) phases down HFCs
- National — India's Ozone Cell (MoEFCC), HCFC Phase-out Management Plan, ratified Kigali (freeze 2028)
Conclusion: Montreal Protocol = most successful environmental treaty; ozone layer projected to recover by ~2040 (mid-latitudes), ~2066 (Antarctic).

250 words · 15 marks

GS-III, 2021

Q. Discuss global warming and mention its effects on the global climate. Explain the control measures to bring down the level of greenhouse gases which cause global warming, in the light of the Kyoto Protocol, 1997.

Model Answer Framework

Introduction: Define global warming as human-amplified rise in average surface temperature (~1.1–1.5°C above pre-industrial) driven by GHG accumulation.
Body — effects on global climate:
- Sea-level rise, glacier/ice-sheet melt, ocean acidification
- More frequent extreme events — heatwaves, cyclones, erratic monsoon
- Shifting agro-climatic zones, biodiversity loss, climate migration
Body — Kyoto Protocol (1997) mechanisms:
- First binding emission-reduction targets for Annex-I (developed) nations, based on CBDR
- Three flexible market mechanisms — Clean Development Mechanism (CDM), Joint Implementation (JI), Emissions Trading
- Limitations — USA never ratified, no targets for developing giants; superseded by Paris Agreement (2015) with NDCs
Body — control measures: renewable transition (ISA, solar/wind), energy efficiency (BEE, UJALA), afforestation carbon sinks, carbon pricing, EV adoption.
Conclusion: Kyoto pioneered market-based climate governance; Paris + Net-Zero pledges now carry the mandate forward.

250 words · 15 marks

GS-III, 2017

Q. "In spite of adverse environmental impact, coal mining is still inevitable for development." Discuss.

Model Answer Framework

Introduction: Coal supplies ~55% of India's commercial energy and ~70%+ of electricity — central to the development-vs-climate tension.
Body — adverse impacts:
- Largest CO2 source; air pollution (PM, SOx, NOx, mercury)
- Land degradation, deforestation, acid mine drainage, groundwater contamination
- Displacement of tribal/forest communities (Fifth Schedule areas)
Body — why still inevitable:
- Cheapest, most abundant domestic baseload (energy security, low import dependence)
- Employment — millions dependent on mining; Coal India as PSU backbone
- Renewables intermittent — grid-scale storage still maturing
Body — way forward: clean-coal tech (supercritical/ultra-supercritical, CCS), just transition for coal regions, phased renewable substitution per Net-Zero-2070.
Conclusion: Coal is a transitional necessity, not a permanent choice — managed decline aligned with climate commitments.

250 words · 15 marks

GS-III, 2015

Q. How far do you agree that the causes and consequences of ozone layer depletion are similar to that of the global warming? Discuss the international mechanism to combat both these hazards.

Model Answer Framework

Introduction: Both are atmospheric problems from human-made gases, but distinct in cause, location and chemistry — a common exam trap.
Body — similarities:
- Both driven by anthropogenic emissions of specific gases
- Overlap — CFCs/HFCs are both ODS and potent GHGs; both threaten health and ecology
- Both need global cooperation (no single nation can fix)
Body — differences:
- Cause — ODS (Cl/Br radicals) deplete ozone; GHGs (CO2, CH4, N2O) trap heat
- Layer — depletion in stratosphere; warming from troposphere
- Effect — ozone loss → UV exposure; warming → temperature/climate change
Body — international mechanisms:
- Ozone — Vienna Convention (1985), Montreal Protocol (1987), Kigali Amendment (2016)
- Warming — UNFCCC (1992), Kyoto Protocol (1997), Paris Agreement (2015)
Conclusion: Partly similar in spirit and overlap (HFCs), but scientifically distinct — Montreal succeeded where climate governance still struggles.

250 words · 15 marks

15-Minute Revision Box

Must-Remember Facts — Global Warming & Ozone Depletion

Warming & GHGs:

Greenhouse effect = natural, life-sustaining; global warming = human-amplified rise; climate change = umbrella term
GWP hierarchy: SF₆ (~23,500) > CFCs (~10,000+) > HFCs (~1,400-4,000) > N₂O (~265-298) > CH₄ (~28-36) > CO₂ (1, reference)
Positive feedback: ice-albedo, water vapour, permafrost-carbon (amplify) | CO₂ fertilisation = weak negative feedback

Ozone & Treaties:

Antarctic ozone hole forms Sept-Nov via Polar Stratospheric Clouds activating Cl/Br radicals; one Cl destroys ~100,000 O₃
Vienna Convention (1985) = framework, no targets; Montreal Protocol (1987) = binding ODS phase-out, universally ratified
Kigali Amendment (2016, in force 2019): HFC phase-down; India freeze 2028, ~85% cut by 2047; avoids up to 0.4°C by 2100
HFCs: zero ODP but very high GWP — replaced ODS but created a new GHG problem, fixed by Kigali

Topic 17: Global Warming & Ozone Depletion

Table of Contents

Conceptual Clarity — Why this Topic Matters

1. Greenhouse Effect — Basics

2. Conceptual Clarity — Greenhouse Effect vs Global Warming vs Climate Change

Don't confuse these three

3. Major Greenhouse Gases & Global Warming Potential (GWP)

4. Climate Forcing & Feedback Mechanisms

Feedback loops

5. Cryosphere Loss & Impacts

Glacier retreat

Arctic sea ice decline

Permafrost thaw

6. Stratospheric Ozone & the Ozone Hole

7. Ozone Depleting Substances (ODS)

8. Vienna Convention & Montreal Protocol

9. Kigali Amendment (2016)

10. Current Affairs (2024–2026)

11. Prelims PYQs

12. Mains PYQs

Model Answer Framework

Model Answer Framework

Model Answer Framework

Model Answer Framework

Model Answer Framework

15-Minute Revision Box

Must-Remember Facts — Global Warming & Ozone Depletion