MPSC 2026 Prelims Paper-I · Question 34 of 100

Correct — option (1), 'Carbon dioxide, Methane, Nitrous oxide, Fluorinated Gases'. The stem asks for the ranking by percentage of contribution, which is a question about shares of total greenhouse gas emissions, and on the standard global inventory used by the IPCC the order is not in doubt. Carbon dioxide is by far the largest contributor, at roughly three-quarters of the total once emissions from fossil fuel combustion and industrial processes are added to those from deforestation and other land use. Methane comes second at around a sixth of the total. Nitrous oxide is third, at roughly six per cent. Fluorinated gases — the hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride and nitrogen trifluoride — are last, at about two per cent. Carbon dioxide, methane, nitrous oxide, fluorinated gases: option (1). The reason this question is worth more than the four names it contains is the distinction it forces. There are two entirely different ways to rank greenhouse gases, and they give almost opposite answers. Ranked by potency per unit mass — the global warming potential, conventionally measured over a hundred years with carbon dioxide set at 1 — methane is roughly thirty times as strong as carbon dioxide, nitrous oxide roughly three hundred times, and sulphur hexafluoride runs into the tens of thousands. On that measure carbon dioxide is the weakest of the four. Ranked by actual contribution, which is potency multiplied by the quantity emitted and by how long the gas survives in the atmosphere, carbon dioxide dominates overwhelmingly, because it is emitted in volumes larger by orders of magnitude and a substantial fraction of it remains in the atmosphere for centuries. The stem says 'percentage of their contribution', so it is the second ranking that is being asked for, and a candidate who answers with the potency ranking will produce a confident wrong answer.

• (2)Carbon dioxide, Methane, Fluorinated Gases, Nitrous oxide — Right at the top and wrong at the bottom: it correctly places carbon dioxide first and methane second, then swaps the last two, promoting fluorinated gases above nitrous oxide. Nitrous oxide contributes roughly six per cent of global greenhouse gas emissions, against about two per cent for the fluorinated gases, so the order is the reverse of what this option states. The swap is tempting because fluorinated gases have spectacular global warming potentials per molecule, but they are emitted in very small quantities, and the ranking asked for is by contribution, not by strength.
• (3)Methane, Carbon dioxide, Nitrous oxide, Fluorinated Gases — Places methane above carbon dioxide. Methane is indeed the more powerful gas per unit mass, roughly thirty times as effective on a hundred-year basis, and it is the second largest contributor overall — but it accounts for around a sixth of emissions against carbon dioxide's three-quarters, so it cannot lead the list. This option is what a candidate produces by remembering that 'methane is far more potent than carbon dioxide' and applying that fact to a question that asks about percentage contribution instead.
• (4)Methane, Nitrous oxide, Fluorinated Gases, Carbon dioxide — Ranks the gases by potency rather than by contribution and, in doing so, puts carbon dioxide last — the gas responsible for the largest share of warming placed at the bottom of the list. It is the fullest expression of the trap the question is built around. Even on a pure global warming potential ranking the order would not be quite this, since fluorinated gases outrank nitrous oxide per unit mass; but the deciding point is simpler, namely that the stem asks for percentage of contribution, on which carbon dioxide is first by a wide margin.

The greenhouse effect is the trapping of outgoing long-wave terrestrial radiation by gases whose molecules absorb in the infrared. Incoming solar radiation is short-wave and passes largely unimpeded; the warmed surface re-radiates at longer wavelengths, and greenhouse gases absorb part of that and re-emit it in all directions, including downwards. Without it the earth's mean surface temperature would be far below freezing; the problem is not the effect but its enhancement by human emissions. How much any one gas contributes depends on three things multiplied together: how strongly a molecule absorbs infrared, how much of the gas is emitted, and how long it persists before being removed. Global warming potential captures the first and the third in a single index, expressed relative to carbon dioxide over a chosen time horizon, usually a hundred years. It does not capture the second, which is why a gas with a modest global warming potential can dominate the total while gases with enormous potentials remain marginal. Carbon dioxide is the extreme case: weakest of the four per molecule, and yet responsible for the largest share of warming, because it is emitted in vastly greater quantity and a large part of each pulse remains in the atmosphere for centuries.

The four categories in this question are the ones used in every national and global inventory, and each has a characteristic source profile. Carbon dioxide comes from the combustion of coal, oil and gas, from cement manufacture and from deforestation and land use change. Methane comes from enteric fermentation in ruminant livestock, from flooded paddy fields, from landfills and from leakage in coal mining and the oil and gas industry; its atmospheric lifetime is only about a decade, which makes it the most attractive target for rapid climate action. Nitrous oxide comes overwhelmingly from agriculture — nitrogenous fertiliser applied to soils and from manure management — with a long atmospheric lifetime of a century or more. Fluorinated gases are wholly industrial: hydrofluorocarbons in refrigeration and air conditioning, perfluorocarbons from aluminium smelting and electronics, and sulphur hexafluoride as an insulator in high-voltage electrical switchgear. The first six of these gases form the basket regulated by the Kyoto Protocol, with nitrogen trifluoride added later, and the hydrofluorocarbons are separately subject to phase-down under the Kigali Amendment to the Montreal Protocol.

• Shares of global greenhouse gas emissions on the standard IPCC-based inventory: carbon dioxide roughly three-quarters of the total, counting fossil fuel and industrial sources together with forestry and land use; methane around a sixth; nitrous oxide about six per cent; fluorinated gases about two per cent.
• Global warming potential over a hundred years, with carbon dioxide set at 1: methane roughly 30, nitrous oxide roughly 300, and sulphur hexafluoride in the tens of thousands. Potency per unit mass and share of total contribution therefore rank the gases in almost opposite orders.
• Sources by gas — carbon dioxide from fossil fuel combustion, cement and land use change; methane from enteric fermentation in livestock, paddy cultivation, landfills and fossil fuel leakage; nitrous oxide chiefly from nitrogenous fertilisers and manure; fluorinated gases entirely from industry, refrigeration and electrical equipment.
• Atmospheric lifetime differs sharply: methane persists roughly a decade, nitrous oxide for over a century, many fluorinated gases for thousands of years, and carbon dioxide has no single lifetime because a substantial fraction of each emission stays in circulation for centuries. Short-lived methane is therefore the fastest lever on near-term warming.
• Water vapour is the most abundant greenhouse gas in the atmosphere, but it is treated as a feedback rather than a forcing: its concentration is governed by temperature rather than directly by emissions, so it amplifies warming caused by the other gases rather than initiating it.

• Ranking by potency when the question asks for contribution. Methane, nitrous oxide and the fluorinated gases all beat carbon dioxide per unit mass, but carbon dioxide dominates the total; the stem's wording decides which ranking is wanted, and here it says 'percentage of their contribution'.
• Assuming a spectacular global warming potential implies a large share. Sulphur hexafluoride is thousands of times more potent than carbon dioxide and still contributes a fraction of a per cent, because the quantity emitted is minute — potency without volume produces little warming.
• Forgetting that water vapour, though the most abundant greenhouse gas, is not listed among the gases to be controlled. It is a feedback governed by temperature rather than a directly emitted forcing agent, which is why inventories and treaties omit it.

Greenhouse gas questions come in four shapes. First, the ranking question used here, which is decided entirely by whether the stem asks about share of emissions or about potency. Second, the source-to-gas match — livestock and paddy to methane, fertiliser to nitrous oxide, switchgear to sulphur hexafluoride — which is straight recall. Third, the global warming potential comparison, where the expected knowledge is the order of magnitude rather than the exact figure. Fourth, the treaty question, asking which gases fall within the Kyoto basket or the Kigali phase-down. The safest preparation is a four-row table: gas, share of global emissions, approximate hundred-year global warming potential, principal sources and atmospheric lifetime — because every one of the four shapes can be read off it, and because holding share and potency side by side is what prevents the ranking error.

No directly related past PYQ was found.

• practice — not a real PYQ

  Among the following greenhouse gases, which has the highest global warming potential per unit mass over a hundred-year horizon ?

  • (a)Carbon dioxide
  • (b)Methane
  • (c)Nitrous oxide
  • (d)Sulphur hexafluoride

  Answer(d) Sulphur hexafluoride — its hundred-year global warming potential runs into the tens of thousands with carbon dioxide set at 1, against roughly 30 for methane and roughly 300 for nitrous oxide. It is used as an insulating gas in high-voltage electrical switchgear, and despite its extraordinary potency it contributes only a tiny share of total warming because so little of it is emitted.

• practice — not a real PYQ

  Global emissions of nitrous oxide, a major greenhouse gas, arise principally from which activity ?

  • (a)Combustion of coal in thermal power stations
  • (b)Application of nitrogenous fertilisers to agricultural soils and manure management
  • (c)Leakage of refrigerants from air conditioning equipment
  • (d)Manufacture of cement and lime

  Answer(b) Application of nitrogenous fertilisers to agricultural soils and manure management — agriculture dominates the nitrous oxide account, which is why the gas is central to any discussion of emissions from Indian farming. Refrigerant leakage releases hydrofluorocarbons, and cement manufacture releases carbon dioxide, so each of the other options names a real source of a different gas.