UPPSC 2022 Prelims GS Paper-I · Question 25 of 150

Correct — C, Argon. A gas can act as a greenhouse gas only if it absorbs the long-wave infrared radiation that the warmed Earth sends back upward, and that requires a molecule whose vibrations change its electrical dipole. Argon is a noble gas and its molecules are single atoms — one atom has nothing to vibrate against, so it has no infrared-active vibration at all and is transparent to terrestrial radiation. This is the same reason nitrogen and oxygen, which together make up about 99 per cent of dry air, are not greenhouse gases either: they are symmetrical two-atom molecules whose stretching produces no change in dipole. Argon is the third most abundant constituent of dry air at roughly 0.93 per cent by volume, and it still contributes nothing to the greenhouse effect. The other three options — CFCs, carbon dioxide and methane — all have the required asymmetric vibrations and are all recognised greenhouse gases.

• (a)Chlorofluorocarbon — CFCs are greenhouse gases, and unusually powerful ones: each molecule traps far more heat than a molecule of carbon dioxide, which is why even their tiny atmospheric concentrations matter. They are better known for a different offence — destroying stratospheric ozone, which is why they are being phased out under the Montreal Protocol — but that does not stop them being greenhouse gases as well.
• (b)Carbon dioxide — Carbon dioxide is the reference greenhouse gas; global warming potentials of all other gases are quoted relative to it, and it is the largest single driver of human-caused warming because of the sheer quantity emitted by burning fossil fuels.
• (d)Methane — Methane is a greenhouse gas and, molecule for molecule over the short term, a considerably stronger one than carbon dioxide, though it is far less abundant and breaks down in the atmosphere within a couple of decades. Its major sources are wetlands, flooded paddy fields, livestock, landfills and fossil-fuel extraction.

The greenhouse effect works on a division of labour in the atmosphere. Short-wave sunlight passes through and warms the surface; the surface re-radiates that energy as long-wave infrared; and a handful of trace gases absorb the infrared and re-emit part of it downward, keeping the surface far warmer than it would otherwise be. Whether a gas can do this is decided by molecular structure, not by abundance. Monatomic gases such as argon, helium and neon have no vibrational modes; symmetrical diatomic gases such as nitrogen and oxygen have vibrations that do not change the dipole moment. Only molecules with three or more atoms, or two different atoms, absorb in the infrared — hence water vapour, carbon dioxide, methane, nitrous oxide, ozone and the halocarbons.

The reliable shortcut is to look for the noble gas. Group 18 elements — helium, neon, argon, krypton, xenon, radon — are monatomic and chemically inert, and none of them is a greenhouse gas. Once you see argon in a list of otherwise familiar climate gases, the answer is settled without knowing anything about global warming potentials. The trap the question sets is abundance: argon is the third commonest gas in dry air, well ahead of carbon dioxide, and candidates who reason 'more of it in the air, so it must matter for warming' get it wrong. Abundance is irrelevant if the molecule cannot absorb infrared.

• Argon is a monatomic noble gas making up about 0.93 per cent of dry air — the third most abundant atmospheric gas after nitrogen and oxygen
• A greenhouse gas must absorb long-wave infrared, which requires a vibration that changes the molecule's dipole moment; monatomic and symmetric diatomic gases cannot do this
• Principal greenhouse gases: water vapour, carbon dioxide, methane, nitrous oxide, ozone and the halocarbons (CFCs, HFCs)
• The Kyoto Protocol basket covers carbon dioxide, methane, nitrous oxide, HFCs, PFCs and sulphur hexafluoride; CFCs are handled separately under the Montreal Protocol
• Without the natural greenhouse effect the Earth's average surface temperature would be around –18°C instead of about 15°C

• Assuming an abundant gas must matter for the climate — argon, nitrogen and oxygen dominate the atmosphere and warm it not at all
• Mixing up the two atmospheric problems: CFCs deplete ozone AND warm the planet, but carbon dioxide does only the second
• Confusing nitrous oxide (N₂O, a genuine greenhouse gas) with the nitrogen oxides NO and NO₂, which are air pollutants

UPPSC asks this as one-line elimination — 'which is NOT a greenhouse gas', 'which is not a noble gas', 'what would Earth's temperature be without the greenhouse effect'. UPSC prefers the mechanism and the sources: which radiation greenhouse gases absorb, which crop emits the most methane, what HFCs are used for.

• practice — not a real PYQ

  Which one of the following gases in the atmosphere does NOT absorb long-wave terrestrial radiation?

  • (a)Water vapour
  • (b)Nitrous oxide
  • (c)Nitrogen
  • (d)Ozone

  Answer(c) Nitrogen — a symmetric diatomic molecule whose vibration produces no dipole change, so it is infrared-inactive; water vapour, nitrous oxide and ozone are all greenhouse gases.

• practice — not a real PYQ

  Chlorofluorocarbons are being phased out primarily under which international agreement?

  • (a)Kyoto Protocol
  • (b)Montreal Protocol
  • (c)Basel Convention
  • (d)Stockholm Convention

  Answer(b) Montreal Protocol — it addresses ozone-depleting substances, with the Kigali Amendment later extending it to HFCs; the Kyoto Protocol covers a different basket of greenhouse gases.