Quick Answer: GWP of CO2, CH4 and N2O (IPCC AR6 Values)

Greenhouse Gases Compared: CO2 vs CH4 vs N2O Global Warming Potential

Quick Answer: Among the three major greenhouse gases, nitrous oxide (N2O) has the highest global warming potential with a GWP-100 of approximately 273, followed by methane (CH4) at roughly 27–30, while carbon dioxide (CO2) is the reference gas with GWP = 1. These are IPCC AR6 (2021) values and are the standard figures for UPSC, NEET and SSC preparation.

What is Global Warming Potential (GWP)?

Global Warming Potential (GWP) is a metric that compares how much heat a greenhouse gas traps in the atmosphere over a specific period, relative to an equal mass of carbon dioxide. In simple terms, GWP tells you how many times more “warming” one kilogram of a gas causes compared to one kilogram of CO2.

CO2 is used as the baseline (GWP = 1) because it is the most abundant human-emitted greenhouse gas, its atmospheric concentration is well measured, and it serves as a convenient common currency for comparing emissions. International climate negotiations and national inventories under the UNFCCC report emissions in “CO2-equivalent (CO2e)” using GWP multipliers.

Time horizon matters: GWP is calculated over a chosen time horizon — most commonly 20 years (GWP-20) or 100 years (GWP-100). Short-lived gases such as methane appear far more potent over 20 years than over 100, because much of the gas decays before the century ends. UPSC and NEET questions generally use the 100-year horizon unless stated otherwise, so always check the time horizon stated in the question.

Carbon Dioxide (CO2): Sources, Lifetime and GWP

• GWP: 1 (reference gas, by definition)
• Sources: Fossil fuel combustion (coal, oil, natural gas), deforestation and land-use change, cement production, industrial processes
• Lifetime: CO2 has no single atmospheric lifetime — a portion is absorbed by oceans and vegetation within decades, but a significant fraction persists for centuries to millennia. This is why excess CO2 keeps influencing climate long after emission.

CO2 contributes the largest share of anthropogenic warming not because it is potent per kilogram, but because of the enormous volume emitted — roughly 36–37 billion tonnes from fossil fuels annually.

Methane (CH4): Sources, Lifetime and GWP

• GWP-100: ~27 (non-fossil) to ~30 (fossil methane, including oxidation to CO2) per IPCC AR6
• GWP-20: approximately 80 — methane’s short-term punch is huge
• Atmospheric lifetime: about 12 years
• Sources: Livestock (enteric fermentation), rice paddies, landfills and waste, leakage from oil and gas infrastructure, wetlands, biomass burning

Methane’s short lifetime (~12 years) is highly significant for climate policy: cutting methane emissions produces rapid, near-term cooling, which is why over 150 countries have joined the Global Methane Pledge to cut methane emissions 30% by 2030 (relative to 2020 levels).

Nitrous Oxide (N2O): Sources, Lifetime and GWP

• GWP-100: approximately 273 (IPCC AR6)
• Atmospheric lifetime: about 109 years
• Main human-made sources:
  • Nitrogen-based synthetic fertilizers and manure applied to agricultural soils (largest source)
  • Agricultural residue burning and cultivation of nitrogen-fixing crops
  • Industrial processes — notably adipic acid and nitric acid production
  • Fossil fuel combustion and vehicular emissions (catalytic converters)
  • Wastewater treatment

N2O is also the dominant ozone-depleting emission now that CFCs are phased out under the Montreal Protocol — a favourite multi-link fact for UPSC Prelims.

Comparison Table: GWP, Lifetime and Major Sources

Why Methane Has High GWP but Short Lifetime

GWP and lifetime are inversely linked for gases other than CO2. A gas that decays quickly must trap energy intensely while it exists to register a high GWP — and methane does exactly that, absorbing outgoing infrared radiation in wavelength bands where CO2 absorbs weakly. Over 20 years, one kg of CH4 traps about 80 times more heat than one kg of CO2; over 100 years, the average falls to about 27–30 because most of the methane has already oxidised.

This has a crucial policy implication: methane reductions are the fastest lever for slowing near-term warming, while CO2 reductions are essential for long-term stabilisation. Exams sometimes frame this as “short-lived climate pollutants vs long-lived greenhouse gases” — remember the pairing.

Other Greenhouse Gases: HFCs, PFCs, SF6 and Water Vapour

• Hydrofluorocarbons (HFCs): GWP-100 ranging from ~12 to ~14,800 depending on the variant; being phased down under the Kigali Amendment to the Montreal Protocol (ratified by India in 2021).
• Perfluorocarbons (PFCs): GWP up to ~11,000; lifetimes up to 50,000 years (e.g., CF4).
• Sulphur hexafluoride (SF6): GWP-100 of approximately 25,200 — the highest among commonly used industrial gases; used in electrical switchgear; lifetime ~3,200 years.
• Water vapour: The most abundant greenhouse gas by mass, but it is a climate feedback, not a forcing agent — its concentration depends on temperature rather than direct emissions, so it is excluded from GWP calculations. Warmer air holds more vapour, amplifying warming caused by other gases.

IPCC Reports and Evolving GWP Values

GWP values are refined with each IPCC Assessment Report, which is why older books quote different numbers:

In Prelims, if options contain both AR5 and AR6 values (e.g., 265 vs 273 for N2O), prefer the value consistent with the question’s stated source; when in doubt, the current IPCC AR6 values are the safest choice. For official assessment data, refer to the IPCC website and the UNFCCC reporting guidelines.

Relevance for India: Policies and International Commitments

• Paris Agreement (2015): India is a party; commitments are expressed in CO2-equivalent terms — directly linking GWP to national policy.
• Panchamrit (announced at COP26, Glasgow 2021): Five targets including reaching net-zero by 2070, meeting 50% of energy needs from renewables by 2030, and reducing the emissions intensity of GDP by 45% from 2005 levels by 2030.
• National Action Plan on Climate Change (NAPCC, 2008): Eight national missions including the National Mission for Sustainable Agriculture — relevant to N2O mitigation through efficient fertilizer use (soil health cards, nano-urea).
• Global Methane Pledge: India has not joined this pledge, citing agricultural dependence — a fact frequently tested in UPSC and state PCS exams.
• Green Credit Programme and Mission LiFE promote behavioural and ecosystem-based mitigation aligned with these gas-specific strategies.

Previous Year Questions and Exam-Style MCQs

MCQ 1 (UPSC Prelims style): With reference to greenhouse gases, consider the following statements:

1. Methane has a higher GWP-100 than nitrous oxide.
2. The atmospheric lifetime of methane is around 12 years.
3. SF6 has the highest GWP among commonly used industrial gases.

Which of the statements given above are correct?
(a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3
Answer: (b) — N2O (GWP ~273) is more potent than CH4 (~27–30); statements 2 and 3 are correct.

MCQ 2 (NEET assertion–reason style):

Assertion (A): Reducing methane emissions can slow near-term global warming faster than equivalent CO2 cuts.
Reason (R): Methane has a short atmospheric lifetime but a very high GWP-20 of about 80.

(a) Both A and R are true, and R correctly explains A
(b) Both A and R are true, but R does not explain A
(c) A is true, R is false
(d) A is false, R is true
Answer: (a)

MCQ 3 (SSC style): The reference gas used for calculating Global Warming Potential is:
(a) Methane (b) Nitrous oxide (c) Carbon dioxide (d) Water vapour
Answer: (c)

Frequently Asked Questions

Q: What is the GWP of methane as per IPCC AR6?

The GWP-100 of methane is approximately 27–30 — about 27 for non-fossil methane and about 30 for fossil methane when oxidation to CO2 is included. Over a 20-year horizon, methane’s GWP rises to about 80.

Q: Which greenhouse gas has the longest atmospheric lifetime?

CO2 effectively persists for centuries to millennia because a portion of emissions remains in the atmosphere for a very long time. Among gases with a well-defined lifetime, SF6 lasts about 3,200 years, and some PFCs persist even longer.

Q: Why is water vapour not counted in GWP calculations?

Water vapour is a climate feedback, not a forcing agent. Its atmospheric concentration depends on temperature rather than on direct human emissions — warmer air holds more vapour — so it cannot be assigned a meaningful GWP.

Q: Is N2O more potent than CO2?

Yes. N2O has a GWP-100 of about 273, meaning 1 kg of N2O warms the planet roughly 273 times more than 1 kg of CO2 over a century.

Q: Which exam repeatedly asks about greenhouse gases?

UPSC Prelims (Environment section), NEET (Ecology), SSC CGL/CHSL and state PCS exams regularly test GWP rankings, gas sources, atmospheric lifetimes and India’s climate commitments. Previous year questions frequently ask candidates to rank gases by warming potential.

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