UPPSC 2019 Prelims GS Paper-I · Question 27 of 150

Correct — D, Methane. A transplanted paddy field is kept under standing water, and waterlogged soil quickly runs out of oxygen. In those anaerobic conditions methane-producing archaea (methanogens) decompose the organic matter in the soil and release methane (CH4). The gas escapes to the atmosphere in three ways — bubbling up through the water (ebullition), diffusing across the water surface, and, most of all, travelling up the aerenchyma, the air channels inside the rice plant itself. Methane is a potent greenhouse gas: over a hundred years one tonne of it traps roughly 28-30 times as much heat as a tonne of carbon dioxide (IPCC AR5 global warming potential). That is why flooded rice cultivation is counted among the largest agricultural sources of global warming, alongside enteric fermentation in cattle.

• (a)Nitrogen — Nitrogen gas (N2) already makes up about 78% of the atmosphere and is not a greenhouse gas at all — it is a symmetric diatomic molecule whose vibrations do not change its dipole, so it cannot absorb infrared radiation. Do not confuse it with nitrous oxide (N2O), a genuinely potent greenhouse gas released from nitrogen-fertilised soils; N2O is a different molecule and is not what this option says.
• (b)Carbon dioxide — Carbon dioxide is a greenhouse gas, but it is not what distinguishes a paddy field. The signature of a flooded, oxygen-free paddy soil is methanogenesis; the growing rice crop is in fact absorbing CO2 by photosynthesis. The question asks for the gas characteristically emitted from paddy fields, and the key marks methane.
• (c)Carbon monoxide — Carbon monoxide is a toxic pollutant from incomplete combustion (vehicles, stubble burning, cooking fires). It is not a significant direct greenhouse gas — it affects climate only indirectly, by using up hydroxyl radicals and so prolonging the life of methane — and it is not a characteristic paddy-field emission.

Methane is produced by anaerobic decomposition — the breakdown of organic matter by microbes where there is no free oxygen. Any waterlogged, organic-rich environment therefore emits it: natural wetlands and marshes, flooded rice paddies, the rumen of cattle and buffaloes, landfill sites, and the flooded reservoirs behind dams. Coal mines and oil-and-gas systems add methane of geological origin. Methane's atmospheric lifetime is only about a decade — far shorter than CO2's — but its warming punch per tonne is far greater, which makes it a 'short-lived climate pollutant' and an attractive target for quick climate action.

Read the question as two conditions joined: the gas must be (i) characteristic of paddy fields and (ii) a warming agent. Nitrogen fails the second test outright, carbon monoxide fails both, and carbon dioxide passes the second but is not what a submerged paddy soil specifically produces. Only methane satisfies both, and the mechanism to remember is the single word 'anaerobic' — standing water shuts out oxygen, and methanogens take over.

• Flooded paddy soil turns anaerobic; methanogenic archaea then produce methane, most of which escapes through the aerenchyma (air channels) of the rice plant.
• Methane's 100-year global warming potential is about 28-30 times that of CO2 (IPCC AR5), but its atmospheric lifetime is only around a decade.
• Other major methane sources: natural wetlands, enteric fermentation in ruminants, landfills, coal mining and oil-and-gas leakage — UPSC's 2008 prelims asked exactly this list.
• Alternate Wetting and Drying (AWD), the System of Rice Intensification and direct-seeded rice cut paddy methane by periodically restoring aerobic conditions in the soil, and save irrigation water too.
• Nitrogen (N2) is not a greenhouse gas; nitrous oxide (N2O), which comes mainly from nitrogen-fertiliser use, is — the two names are the exam's favourite confusion.

• Confusing nitrogen (N2 — inert, not a greenhouse gas) with nitrous oxide (N2O — a strong greenhouse gas).
• Picking carbon dioxide reflexively because it is the best-known greenhouse gas, when the question points at a specific anaerobic source.
• Thinking methane comes only from cattle — wetlands and rice paddies are comparably large sources.
• Treating carbon monoxide as a greenhouse gas; it is a pollutant with only an indirect climate effect.

UPSC asks it as a list ('which of these are sources of methane') or as reasoning ('why does extensive rice cultivation contribute to global warming'); UPPSC asks the one-line version — which gas comes from paddy fields, or which gas is not a naturally occurring greenhouse gas.

• practice — not a real PYQ

  Which of the following practices in rice cultivation reduces methane emission from the field?

  • (a)Keeping the field continuously flooded through the season
  • (b)Alternate wetting and drying of the field
  • (c)Increasing the dose of urea
  • (d)Transplanting seedlings into deeper standing water

  Answer(b) Alternate wetting and drying — periodically draining the field lets oxygen back into the soil, which suppresses the methanogens that need anaerobic conditions (it also saves irrigation water and pumping energy).

• practice — not a real PYQ

  Why is methane described as a 'short-lived climate pollutant'?

  • (a)It stays in the atmosphere for centuries but traps little heat
  • (b)It has a much higher warming potential than CO2 but an atmospheric lifetime of only about a decade
  • (c)It directly destroys the stratospheric ozone layer
  • (d)It is produced only by natural, not human, activity

  Answer(b) It has a much higher warming potential than CO2 but an atmospheric lifetime of only about a decade — which is why cutting methane delivers climate benefits far faster than cutting CO2.