Arrange the following major greenhouse gases in descending order according to the percentage of their contribution to global warming.

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2026 Q34 — Environment & Ecology PYQ with Answer

State PCS 2026 Q34 — Environment & Ecology PYQ with Answer