BharatNotes What is Greenhouse Gases (Chemistry)?
Greenhouse gases (GHGs) are atmospheric gases that are transparent to incoming short-wave solar radiation but absorb and re-emit the long-wave infrared (IR) radiation that the warmed Earth radiates back outward. This re-emission warms the lower atmosphere — the greenhouse effect. Without it the planet's average surface temperature would be roughly -18 degC instead of about +15 degC; the problem is the enhanced greenhouse effect from human emissions.
The chemistry: why only some gases trap heat
A gas absorbs IR radiation only when a molecular vibration produces a change in dipole moment (a quantum-mechanical selection rule). This single rule explains the whole phenomenon:
- N2 and O2 (about 99% of dry air) are symmetric diatomic molecules. Their stretching vibration causes no dipole change, so they are IR-inactive — they are not greenhouse gases.
- CO2 is linear and symmetric, yet its asymmetric stretch (around 4.3 micrometre) and bending modes (around 15 micrometre) do change the dipole moment, making it strongly IR-active.
- H2O, CH4, N2O and O3 are bent or polar/asymmetric molecules with several IR-active modes, so they absorb across multiple bands.
Global Warming Potential (GWP) and lifetimes
GWP combines a molecule's IR-absorbing efficiency with how long it survives in the atmosphere, relative to CO2 over (usually) 100 years.
| Gas | Pre-eminent source | Atmospheric lifetime | GWP-100 (IPCC AR6, 2021) |
|---|---|---|---|
| CO2 | Fossil fuels, deforestation | ~Centuries (no single value) | 1 (reference) |
| CH4 (methane) | Agriculture, fossil fuels, wetlands | ~12 years | ~27-30 |
| N2O (nitrous oxide) | Fertilisers, combustion | ~109 years | 273 |
| SF6 (sulphur hexafluoride) | Electrical switchgear | 800-3,200 years | ~23,500-25,200 (highest of common GHGs) |
Methane is short-lived but punches far above CO2 per molecule; SF6 is the most potent common GHG by GWP.
Current status (verified)
NOAA global annual averages: CO2 = 422.8 ppm (2024), methane = 1,921.8 ppb (2024), nitrous oxide = 337.7 ppb (2024) — all record highs. The May 2025 monthly CO2 average (Scripps) was about 430 ppm.
Treaty mapping (a UPSC trap)
- CO2, CH4, N2O, HFCs, PFCs, SF6 (and NF3) — controlled under the UNFCCC / Kyoto Protocol and now the Paris Agreement.
- CFCs, HCFCs, halons (ozone-depleting) — controlled under the Montreal Protocol, 1987; HFCs are being phased down under its Kigali Amendment (2016).
UPSC angle
Master three points: (1) the dipole-moment reason why N2/O2 are not GHGs; (2) the GWP ranking (SF6 highest, CO2 reference, methane far stronger than CO2 per molecule but short-lived); (3) the Montreal-vs-Kyoto/Paris division of gases. These recur in Prelims factual questions and as the scientific backbone of GS3 climate-mitigation answers.
