Greenhouse Gases and Global Warming Potential: CO2 vs CH4 vs N2O Comparison
Quick Answer: Global Warming Potential (GWP) measures how much heat a greenhouse gas traps in the atmosphere relative to carbon dioxide over a fixed time horizon (usually 100 years). CO2 is the reference gas with GWP = 1. Among the big three, N2O has the highest GWP (~265–273), followed by CH4 (~28–30) and CO2 (1). The Kyoto Protocol basket covers six original gases, with NF3 added later under the Doha Amendment.
- Quick Answer: What is Global Warming Potential (GWP)?
- What is GWP and How is It Measured?
- Comparison Table: CO2 vs CH4 vs N2O (GWP, Lifetime, Sources)
- Carbon Dioxide (CO2): GWP, Sources and Sinks
- Methane (CH4): GWP and Why It Is a Short-Lived Climate Pollutant
- Nitrous Oxide (N2O): GWP and Agricultural Sources
- The Kyoto Protocol Basket of Greenhouse Gases
- IPCC Reports and GWP Value Updates: AR5 vs AR6
- Global Warming vs Greenhouse Effect: Concept Clarification
- Previous Year UPSC Questions on Greenhouse Gases
- Memory Tricks and One-Line Revision Notes
- Frequently Asked Questions
- Q: What is the GWP of methane over 100 years?
- Q: Which gas has the highest GWP in the Kyoto basket?
- Q: Why does N2O have a higher GWP than CH4 despite being less discussed?
- Q: Is water vapour part of the Kyoto Protocol basket?
- Q: Which gas is both a greenhouse gas and an ozone-depleting substance?
- Related reading
Quick Answer: What is Global Warming Potential (GWP)?
Global Warming Potential (GWP) is an index developed by the IPCC to compare the warming influence of different greenhouse gases. It answers a simple question: how much warmer would the planet get from one tonne of gas X compared to one tonne of CO2? Because CO2 is the most abundant and longest-lasting anthropogenic greenhouse gas, IPCC scientists fixed it as the baseline, so GWP of CO2 = 1 and every other gas is expressed as a multiple of it. For UPSC Environment & Geography, GWP is a high-yield concept that appears in both prelims MCQs and mains answers on climate governance.
What is GWP and How is It Measured?
GWP is calculated using three components, as defined in IPCC assessment reports:
- Radiative forcing: the additional energy (watts per square metre) a gas adds to the Earth-atmosphere system by absorbing outgoing infrared radiation.
- Atmospheric lifetime: how long a gas survives before being removed by chemical reactions or sinks.
- Time horizon: the integration period — the IPCC and UNFCCC reporting use the 100-year GWP (GWP-100) as the standard, though 20-year (GWP-20) and 500-year values also exist.
The time horizon matters greatly: methane’s 20-year GWP is roughly 80, because it is potent but short-lived, while its 100-year GWP falls to about 28–30. UPSC has tested this exact nuance, so always mention the time horizon whenever you cite a GWP value. Full methodology is documented in the IPCC assessment reports.
Comparison Table: CO2 vs CH4 vs N2O (GWP, Lifetime, Sources)
| Gas | GWP-100 (AR5 / AR6) | Atmospheric Lifetime | Major Anthropogenic Sources |
|---|---|---|---|
| Carbon Dioxide (CO2) | 1 (baseline) | Centuries (a fraction persists for 1,000+ years) | Fossil fuel combustion, cement production, deforestation |
| Methane (CH4) | 28 (AR5) / ~27–30 (AR6) | ~12 years | Rice paddies, livestock enteric fermentation, landfills, natural gas leaks |
| Nitrous Oxide (N2O) | 265 (AR5) / ~273 (AR6) | ~110–120 years | Nitrogen fertilisers, manure management, industrial processes |
Carbon Dioxide (CO2): GWP, Sources and Sinks
CO2 is the benchmark gas, so its GWP is exactly 1 for all time horizons. It accounts for the largest share of anthropogenic greenhouse gas emissions. Principal sources include fossil fuel burning in power plants and transport, cement manufacture (which releases CO2 from limestone calcination), and land-use change such as deforestation. Its sinks are equally exam-relevant: the ocean sink absorbs roughly a quarter of emissions (causing ocean acidification) and the land sink — forests, soils and vegetation — absorbs a similar share. Unlike CH4, CO2 has no single atmospheric lifetime; a portion of any CO2 release remains in the air for over a thousand years, which is why cumulative CO2 emissions drive long-term warming.
Methane (CH4): GWP and Why It Is a Short-Lived Climate Pollutant
Methane’s 100-year GWP is about 28 (AR5) to ~30 (AR6), but over a 20-year horizon it is roughly 80 times more potent than CO2. With an atmospheric lifetime of only about 12 years, methane is classified as a Short-Lived Climate Pollutant (SLCP) — cutting methane emissions delivers rapid cooling benefits, which is why the Global Methane Pledge (2021) targets a 30% reduction from 2020 levels by 2030. Key anthropogenic sources:
- Rice cultivation (flooded paddies create anaerobic conditions)
- Livestock enteric fermentation (cattle digestion)
- Landfills and waste decomposition
- Leakage from oil, gas and coal operations
Nitrous Oxide (N2O): GWP and Agricultural Sources
N2O has a GWP-100 of about 265 (AR5) to 273 (AR6) — the highest of the three gases compared here — and a long lifetime of roughly 110–120 years. Its dominant source is agriculture: application of nitrogenous fertilisers and manure, where soil microbes convert excess nitrogen through nitrification and denitrification. Industrial processes (nylon and nitric acid production) and fossil fuel combustion also contribute. Crucially for prelims, N2O is also an ozone-depleting substance — research published in Science identified it as the dominant ozone-depleting emission of the 21st century, since the Montreal Protocol has phased out most CFCs.
The Kyoto Protocol Basket of Greenhouse Gases
The 1997 Kyoto Protocol created a “basket” of greenhouse gases that Annex I countries committed to reduce. The original basket contains six gases:
- Carbon dioxide (CO2)
- Methane (CH4)
- Nitrous oxide (N2O)
- Hydrofluorocarbons (HFCs)
- Perfluorocarbons (PFCs)
- Sulphur hexafluoride (SF6)
The Doha Amendment (2012) added a seventh gas — nitrogen trifluoride (NF3) — for the second commitment period (2013–2020). Note carefully: water vapour, though the most abundant greenhouse gas, is not in the Kyoto basket because human activity does not control it directly. The Paris Agreement and Kigali Amendment (2016) later brought HFC phase-down under the Montreal Protocol framework.
IPCC Reports and GWP Value Updates: AR5 vs AR6
GWP values are revised with every IPCC assessment report as radiative forcing science improves. Comparing the two most-cited reports:
| Gas | AR5 (2013) | AR6 (2021) |
|---|---|---|
| CO2 | 1 | 1 |
| CH4 | 28 | ~27–30 (fossil vs non-fossil) |
| N2O | 265 | ~273 |
In answers, cite AR6 values where possible but note the AR5 figure, since older UPSC question papers and NCERT texts still carry AR5 numbers. Reference material is available at IPCC AR6 and India’s climate data at moes.gov.in.
Global Warming vs Greenhouse Effect: Concept Clarification
A recurring prelims trap conflates two distinct ideas:
- Greenhouse effect (natural): the natural process by which greenhouse gases trap outgoing infrared radiation and keep Earth about 33°C warmer than it would otherwise be — essential for life.
- Global warming (anthropogenic): the enhanced greenhouse effect — additional warming caused by human-emitted gases since industrialisation, raising global mean temperature by about 1.1–1.2°C above pre-industrial levels.
Statements like “the greenhouse effect is harmful” are false — it is the enhancement of it that causes global warming. This distinction has appeared in UPSC prelims in various forms.
Previous Year UPSC Questions on Greenhouse Gases
UPSC has tested this topic in patterns such as:
- Source-matching questions: matching gases (CH4, N2O, CO2) with their sources — know rice paddies = methane, fertilisers = N2O, cement = CO2.
- Kyoto basket questions: identifying which gas is not a Kyoto greenhouse gas — water vapour and ozone are classic distractors.
- GWP ordering: arranging gases by warming potential — remember N2O > CH4 > CO2 among the big three.
- Dual-property gases: which gas is both a greenhouse gas and ozone-depleting? Answer: N2O.
Tip: When a question mixes Kyoto, Montreal and Paris frameworks, check which gas belongs to which treaty — HFCs now sit under Montreal (Kigali Amendment).
Memory Tricks and One-Line Revision Notes
- Kyoto basket mnemonic: “Clever Monkeys Never Hop; Pandas Sit” → CO2, CH4, N2O, HFCs, PFCs, SF6 (Doha added NF3 — “New Friend”).
- GWP order: “N-C-C” — N2O (~273) > CH4 (~30) > CO2 (1).
- Lifetime order: CH4 shortest (~12 yrs), N2O long (~110+ yrs), CO2 longest (centuries to millennia).
- Methane = 80 over 20 years, ~30 over 100 years — always state the horizon.
- N2O = greenhouse gas + ozone depleter (only one with both labels in exams).
- SF6 = highest GWP in the Kyoto basket (~23,500).
- Water vapour = most abundant GHG but not in the Kyoto basket.
Frequently Asked Questions
Q: What is the GWP of methane over 100 years?
Approximately 28 times that of CO2 in IPCC AR5, and around 27–30 in AR6. Over a 20-year horizon, methane’s GWP rises to roughly 80, reflecting its short ~12-year lifetime and strong early radiative forcing.
Q: Which gas has the highest GWP in the Kyoto basket?
Sulphur hexafluoride (SF6), with a GWP of roughly 23,500 over 100 years — far exceeding CO2, CH4 and N2O. It is used mainly in electrical switchgear and is regulated under the Kyoto framework.
Q: Why does N2O have a higher GWP than CH4 despite being less discussed?
Because N2O has a much longer atmospheric lifetime (~110–120 years versus ~12 for methane) and absorbs infrared radiation more effectively per molecule. Its persistence means each emission exerts warming for a century or more, pushing its 100-year GWP (~265–273) well above methane’s.
Q: Is water vapour part of the Kyoto Protocol basket?
No. Water vapour is the most abundant greenhouse gas and contributes the largest share of natural greenhouse warming, but it is not controlled under the Kyoto Protocol because it is not emitted directly by human activity in a controlling way — its concentration depends on temperature.
Q: Which gas is both a greenhouse gas and an ozone-depleting substance?
Nitrous oxide (N2O). With CFCs largely phased out under the Montreal Protocol, N2O is recognised as the dominant ozone-depleting emission of the 21st century — a favourite UPSC cross-concept question.
Related reading
- Carbon Markets in India: CCTS, Offset Mechanism and Article 6 Explained for UPSC Mains
- UPSC Prelims 2025 Admit Card Release Date & Exam Date: Direct Answer, Download Guide & Official Links
Quick revision
- Radiative forcing: the additional energy (watts per square metre) a gas adds to the Earth-atmosphere system by absorbing outgoing infrared radiation.
- Atmospheric lifetime: how long a gas survives before being removed by chemical reactions or sinks.
- Time horizon: the integration period — the IPCC and UNFCCC reporting use the 100-year GWP (GWP-100) as the standard, though 20-year (GWP-20) and 500-year values…
- Rice cultivation (flooded paddies create anaerobic conditions)
- Livestock enteric fermentation (cattle digestion)
- Landfills and waste decomposition
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