Greenhouse gases are the atmospheric compounds that trap heat radiated from Earth's surface — driving global warming and climate change.
The six major GHGs regulated under the Kyoto Protocol are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF₆). Understanding their sources, Global Warming Potentials (GWP), lifetimes, and shares of total warming is essential for competitive exams. Questions on GHG names, GWP values, major sources, the greenhouse effect mechanism, and India's emissions profile appear in UPSC Prelims, SSC CGL, Banking, and State PSC exams under Environment and Science.
⚡ Quick Facts
- CO₂ is the most abundant anthropogenic GHG — accounting for ~76% of total global GHG emissions in CO₂ equivalent terms.
- Methane (CH₄) is about 80 times more potent than CO₂ over 20 years — but breaks down in only ~12 years, making methane cuts the fastest way to reduce near-term warming.
- Water vapour (H₂O) is the most abundant GHG by volume — but it is a feedback, not a forcing; it is NOT counted in emission targets.
- Global Warming Potential (GWP) measures how much heat a gas traps relative to CO₂ over 100 years — CO₂ has GWP = 1 (the reference point).
- SF₆ (sulphur hexafluoride) has the highest GWP of any regulated GHG at 23,900 — and persists in the atmosphere for 3,200 years.
Water vapour = most abundant GHG — but NOT included in emission inventories (it is a feedback, not a forcing). CO₂ = most abundant anthropogenic GHG. SF₆ = highest GWP (23,900) — NOT methane. Methane = most potent short-term (80× over 20 years) — but CO₂ contributes more to total warming. India did NOT sign the Global Methane Pledge (COP26, 2021). Kigali Amendment (2016) targets HFCs (not CFCs — those are covered by the original Montreal Protocol).
✅ My Progress Tracker
🌡️ Major Greenhouse Gases — Complete Reference
| # ↕ | Gas ↕ | Formula | GWP (100-yr) ↕ | Atm. Lifetime | Share of GHG | Category ↕ | Key Exam Fact |
|---|---|---|---|---|---|---|---|
| 1 | Carbon Dioxide | CO₂ | 1 (reference) | 50–200 years | ~76% | Kyoto 6 | Most abundant anthropogenic GHG; reference gas for GWP; fossil fuel combustion + deforestation + cement |
| 2 | Methane | CH₄ | 28–36 (80–85 over 20 yr) | ~12 years | ~16% | Kyoto 6 | 80× more potent than CO₂ over 20 years; only 12-yr lifetime; livestock + rice paddies + landfills; India = major source |
| 3 | Nitrous Oxide | N₂O | 265–298 | ~114 years | ~6% | Kyoto 6 | Ozone-depleting + GHG; "laughing gas"; agricultural soils + nitrogen fertilisers; very long-lived |
| 4 | Hydrofluorocarbons (HFCs) | Various (HFC-134a etc.) | 12–14,800 (varies by type) | 1–270 years | ~2% (rising fast) | Kyoto 6 / Synthetic | Replaced ozone-depleting CFCs; Kigali Amendment (2016) phases down HFCs; AC + refrigeration; India ratified 2021 |
| 5 | Perfluorocarbons (PFCs) | CF₄, C₂F₆ etc. | 7,390–12,200 | 2,600–50,000 years | <1% | Kyoto 6 / Synthetic | Extremely long-lived; CF₄ persists 50,000 years; aluminium smelting + semiconductor manufacturing |
| 6 | Sulphur Hexafluoride | SF₆ | 23,900 🔥 (highest) | ~3,200 years | <1% | Kyoto 6 / Synthetic | Highest GWP of any regulated GHG; used in high-voltage electrical switchgear; lasts 3,200 years |
| 7 | Nitrogen Trifluoride | NF₃ | 17,200 | ~500 years | Trace | Synthetic | Semiconductor + solar panel + LCD manufacturing; added to Kyoto basket under Paris Agreement |
| 8 | Ozone (tropospheric) | O₃ | ~30 (indirect) | Days to weeks | ~3–5% radiative forcing | Natural / Indirect | Secondary pollutant; formed from NOx + VOCs + sunlight; significant warming agent AND air pollutant |
| 9 | Water Vapour | H₂O | Very high (not applicable) | Days | Largest natural GHG | Natural / Feedback | Most abundant GHG by volume — but NOT counted in emission targets; it is a feedback (amplifies warming) not a forcing |
| 10 | Black Carbon (Soot) | C (aerosol) | ~3,200 (short-lived) | Days to weeks | Significant | Short-lived Forcer | NOT a gas — it's an aerosol; major health + climate impact in India; cooking stoves + diesel engines; very short-lived but locally intense |
| Gas | Formula | GWP (20-year) | GWP (100-year) | Atmospheric Lifetime | Key Note |
|---|---|---|---|---|---|
| Carbon Dioxide | CO₂ | 1 | 1 | 50–200 years | Reference gas; all others compared to this |
| Methane | CH₄ | 80–85 | 28–36 | ~12 years | Most potent short-term; fastest route to near-term cooling |
| Nitrous Oxide | N₂O | 268 | 265–298 | ~114 years | Long-lived; ozone-depleting; from agriculture |
| HFC-134a | CH₂FCF₃ | 3,710 | 1,430 | ~14 years | Common refrigerant; Kigali Amendment targets this |
| HFC-23 | CHF₃ | 12,400 | 14,800 | ~222 years | Most potent common HFC; by-product of HCFC-22 production |
| Carbon Tetrafluoride (PFC) | CF₄ | 4,880 | 7,390 | 50,000 years | Longest-lived GHG; from aluminium smelting |
| Sulphur Hexafluoride | SF₆ | 18,300 | 23,900 | ~3,200 years | Highest GWP of any regulated GHG; electrical switchgear |
| Nitrogen Trifluoride | NF₃ | 17,200 | 17,200 | ~500 years | Solar panels + semiconductors + LCD screens |
| Water Vapour | H₂O | Very high | N/A (feedback) | Days | Most abundant GHG; feedback not forcing; not counted |
| Ozone (tropospheric) | O₃ | Short-term warming | ~30 indirect | Days to weeks | Secondary pollutant; air quality + climate impact |
| Stage | Process | Key Points for Exams |
|---|---|---|
| 1. Solar Radiation | Sun emits short-wave radiation (UV + visible light) | ~30% reflected by atmosphere, clouds, and surface; 70% absorbed |
| 2. Earth Absorbs | Earth's surface absorbs solar radiation and warms | Surface averages ~+15°C with greenhouse effect (vs –18°C without it) |
| 3. Earth Radiates | Warm Earth emits long-wave infrared (heat) radiation back towards space | Longer wavelength — can be absorbed by GHG molecules |
| 4. GHG Absorption | GHG molecules absorb outgoing infrared radiation and re-emit in all directions | Some heat returns to Earth's surface — the greenhouse effect |
| 5. Enhanced Effect | Human GHG emissions increase concentrations, trapping more heat | CO₂ rose from 280 ppm (pre-industrial) to ~420 ppm (2023); +1.2°C warming locked in |
| Natural GHE | Without any GHGs, Earth would be ~33°C colder | Natural greenhouse effect is essential for life — it's the enhanced effect that is the problem |
| Sector | Share of Global GHG | Key Gases | Key Exam Fact |
|---|---|---|---|
| Energy (electricity + heat) | ~35% | CO₂ | Largest source globally; coal-fired power plants are the main driver |
| Industry | ~24% | CO₂, CH₄, N₂O, HFCs | Cement, steel, chemicals; PFCs + SF₆ from industrial processes |
| Agriculture (AFOLU) | ~24% | CH₄, N₂O | Livestock (enteric fermentation), rice paddies, manure, nitrogen fertilisers; India's agriculture = major CH₄ source |
| Land Use Change / Forests | ~11% | CO₂ | Tropical deforestation (Amazon, Congo, SE Asia); carbon released from cleared forests |
| Transport | ~16% | CO₂ | Road transport dominates; aviation growing fast; shipping significant |
| Buildings | ~6% | CO₂ | Heating, cooling, cooking; improving efficiency is key |
| Waste | ~3% | CH₄, N₂O | Landfills (anaerobic decomposition); wastewater treatment |
| Aspect | Detail |
|---|---|
| Global rank by total GHG emissions | 3rd largest total emitter (after China and USA) — as of 2023 |
| Per capita emissions | Among the lowest for major economies — ~2–2.5 tonnes CO₂e per person per year (world avg ~6) |
| Total annual emissions | ~3.5–4 billion tonnes CO₂ equivalent (2023 estimate) |
| Largest sector (India) | Energy sector (electricity generation — coal accounts for ~70% of India's power) |
| Agriculture emissions | Rice paddies = major CH₄ source; livestock = CH₄; fertilisers = N₂O; India is one of the world's largest rice producers |
| India's NDC target | 45% reduction in emission intensity of GDP from 2005 levels by 2030 |
| India's Net Zero target | 2070 |
| India's carbon sink | Forests and tree cover (~26% of geographic area); target = additional 2.5–3 billion tonnes CO₂e sink through forests by 2030 |
| Global Methane Pledge (COP26) | India did NOT sign the Global Methane Pledge (because of rice paddy agriculture and livestock concerns) |
| Kigali Amendment | India ratified in 2021; major HFC user (large AC market) |
| CO₂ current level | ~420 ppm (2023); pre-industrial was ~280 ppm; a 50% increase |
⚖️ Compare Two Greenhouse Gases
📝 Key Notes & Memory Tips
The six GHGs with binding reduction targets under the Kyoto Protocol (1997) are: CO₂ (carbon dioxide), CH₄ (methane), N₂O (nitrous oxide), HFCs (hydrofluorocarbons), PFCs (perfluorocarbons), and SF₆ (sulphur hexafluoride). Under the Paris Agreement, NF₃ (nitrogen trifluoride) was also added. CO₂ = the reference gas (GWP = 1). All other gases are converted to CO₂-equivalent (CO₂e) using their GWP values for comparison. Mnemonic: "Carbon Methane Nitrogen HFCs PFCs Sulphur".
Methane has a GWP of 80–85 times CO₂ over 20 years — making it an extremely potent short-term climate forcer. But its atmospheric lifetime is only ~12 years. This means cutting methane now will show results within a decade or two — the fastest way to slow near-term warming. The 2021 Global Methane Pledge (COP26) aimed for 30% reduction by 2030 — signed by 150+ countries. India did NOT sign this pledge — due to concerns about rice paddy CH₄ emissions and livestock. This is a direct exam fact.
SF₆ (Sulphur Hexafluoride) has the highest GWP of any regulated GHG at 23,900 over 100 years. It is used in high-voltage electrical switchgear and lasts ~3,200 years. PFCs (Perfluorocarbons) like CF₄ are even longer-lived — persisting up to 50,000 years in the atmosphere. Despite being emitted in tiny volumes (compared to CO₂ or CH₄), their extreme potency and near-permanence make them serious concerns. They are entirely synthetic — no natural sources.
HFCs (hydrofluorocarbons) were introduced in the 1990s as replacements for ozone-depleting CFCs (which were banned by the Montreal Protocol, 1987). But HFCs turned out to be powerful GHGs — with GWPs up to 14,800. The Kigali Amendment to the Montreal Protocol (adopted in Kigali, Rwanda, 2016) set binding schedules to phase down HFC production and use. If fully implemented, it is expected to avoid 0.5°C of warming by 2100 — one of the most effective single climate actions. India ratified the Kigali Amendment in 2021.
Water vapour (H₂O) is the most abundant greenhouse gas in the atmosphere by volume and is an extremely potent heat-trapper. However, it is classified as a feedback, not a forcing: as CO₂ and other GHGs warm the planet, more water evaporates, increasing atmospheric water vapour, which causes further warming (a positive feedback loop). Humans cannot directly control atmospheric water vapour. Therefore it is NOT included in emission inventories or climate targets. Climate targets focus on the forcing gases (CO₂, CH₄, N₂O etc.) that humans directly emit.
Six Kyoto Protocol GHGs:
"Carbon Methane Nitrogen HFCs PFCs Sulphur"
→ CO₂ | CH₄ | N₂O | HFCs | PFCs | SF₆
GWP ranking (increasing power over 100 years):
"CO₂ (1) → CH₄ (30) → N₂O (300) → HFCs (up to 14,800) → SF₆ (23,900)"
Methane vs CO₂ key facts:
"Methane = More Potent, Much Shorter (80× over 20yr; 12yr lifetime)"
→ Most potent short-term | Must act fast (12-year decay)
Water vapour rule:
"Most Abundant But Not Counted — Feedback Not Forcing"
🃏 Flashcards
Click a card to flip · Use arrows to navigate
🧩 Practice Quiz
5 questions · Answer all · Check your score
Carbon dioxide (CO₂) is the most abundant anthropogenic greenhouse gas, accounting for approximately 76% of total global GHG emissions in CO₂ equivalent terms. Its primary sources are fossil fuel combustion (coal, oil, natural gas), deforestation, and cement production. CO₂ is also the reference gas for Global Warming Potential (GWP = 1), against which all other GHGs are compared.
Methane has a Global Warming Potential of approximately 80–85 times that of CO₂ over a 20-year period — making it an extremely potent short-term climate forcer. However, methane breaks down relatively quickly — its atmospheric lifetime is about 12 years, compared to CO₂ which can persist for 50–200 years. This makes reducing methane emissions one of the fastest ways to slow near-term warming.
Sulphur Hexafluoride (SF₆) has the highest Global Warming Potential of the Kyoto Protocol's six regulated GHGs — approximately 23,900 times that of CO₂ over 100 years. It also has an atmospheric lifetime of approximately 3,200 years. SF₆ is used mainly in high-voltage electrical switchgear and semiconductor manufacturing. Despite being emitted in tiny volumes, its extreme potency and longevity make it a significant concern.
The Kigali Amendment to the Montreal Protocol (adopted in Kigali, Rwanda in 2016) set a schedule to phase down Hydrofluorocarbons (HFCs) — synthetic refrigerants used in air conditioning, refrigeration, and industrial processes. HFCs were introduced to replace ozone-depleting CFCs but turned out to be powerful greenhouse gases. Full implementation is expected to avoid up to 0.5°C of global warming by 2100. India ratified the amendment in 2021.
Water vapour is indeed the most abundant greenhouse gas in the atmosphere by volume. However, it is classified as a feedback rather than a forcing — as CO₂ and other GHGs warm the planet, more water evaporates, increasing atmospheric water vapour, which causes further warming (a positive feedback loop). Humans do not directly control atmospheric water vapour concentrations. Climate targets focus on the forcing gases (CO₂, CH₄, N₂O etc.) that are directly caused by human activities.
✅ Key Takeaways
❓ Frequently Asked Questions
Greenhouse gases (GHGs) are atmospheric compounds that absorb and re-emit infrared (heat) radiation, trapping heat that would otherwise escape to space. When sunlight hits Earth, the surface warms and radiates heat back as infrared radiation. GHG molecules (CO₂, CH₄, N₂O, etc.) absorb this outgoing radiation and redirect some of it back to Earth — the greenhouse effect. Without any greenhouse gases, Earth would be approximately 33°C colder (average –18°C rather than +15°C). Human activities — burning fossil fuels, agriculture, deforestation, industrial processes — have increased GHG concentrations far above natural levels, enhancing the greenhouse effect and causing global warming.
The answer depends on the timeframe. CO₂ is the most important by total contribution — it accounts for ~76% of anthropogenic GHG emissions (CO₂e) and has a very long atmospheric lifetime (50–200 years). Methane (CH₄) is the most important for near-term action — it has a GWP of 80–85 times CO₂ over 20 years and reduces in about 12 years, meaning cutting methane now will show results in decades. Water vapour is the most abundant GHG overall, but it's a feedback (responds to warming), not a direct human emission. SF₆ has the highest GWP (23,900) but is emitted in tiny volumes.
Global Warming Potential (GWP) is a measure of how much heat a GHG traps in the atmosphere relative to CO₂ over a given time period (usually 100 years). CO₂ has a GWP of 1 and serves as the reference gas. Key values over 100 years: CO₂ = 1; CH₄ = 28–36; N₂O = 265–298; HFC-134a = 1,430; SF₆ = 23,900. GWP is used to convert all GHGs to CO₂ equivalent (CO₂e) for reporting and comparison. The formula: CO₂e = mass of gas × GWP. India's NDC targets are expressed in CO₂e intensity per unit of GDP.
GHGs appear in UPSC Prelims (Environment and Ecology + Science), SSC CGL, Banking PO, RBI Grade B, and all State PSC exams. Key exam patterns include: six Kyoto Protocol gases (CO₂, CH₄, N₂O, HFCs, PFCs, SF₆), GWP ranking (SF₆ highest at 23,900; methane most potent short-term), greenhouse effect mechanism (how it works), water vapour distinction (feedback not forcing), Kigali Amendment (HFC phase-down 2016, India ratified 2021), India NOT signing Global Methane Pledge (COP26), India's rank (3rd total, lowest per capita), and India's NDC and Net Zero targets. This page covers all major GHG patterns for 2026 exams.
