The relative contribution of different greenhouse gases to global warming is given below. Identify the incorrect pair.
- (a)CH₄ – 20 %
- (b)CFCs – 14 %
- (c)CO₂ – 40 %
- (d)N₂O – 6 %
Correct — C, CO₂ – 40 %.
The pairs come from the pie chart in the NCERT Class XII Biology chapter on Environmental Issues, which gives CO₂ 60 %, CH₄ 20 %, CFCs 14 % and N₂O 6 %. The paper has changed one figure: CO₂ – 40 % is the mismatch, because the textbook share of carbon dioxide is 60 %.
A sum check confirms that a pair is wrong even without recalling each number: the textbook shares add to 100, while the printed figures add to 80. Knowing that carbon dioxide is the dominant gas pins the 20-point gap to option (c).
Carbon dioxide leads on volume, not potency. Its warming potential is the unit of measurement (GWP 1), yet it is emitted in far larger quantities than the other three and a large fraction of it lingers for centuries, so it carries about three-fifths of the total.
- (a)CH₄ – 20 % — This pair matches the textbook figure, so it is not the incorrect one. Methane is the second-largest slice, ahead of CFCs and N₂O, despite far lower emissions than CO₂, because each molecule traps heat about 28 times as strongly over a century.
It would be the right answer to a question asking for methane's share of global warming.
- (b)CFCs – 14 % — Correctly paired. CFCs take third place at 14 % even though their concentration is measured in parts per trillion, because their per-molecule warming potential runs into the thousands.
Their production is controlled under the Montreal Protocol (1987), so this slice has been slowly declining since the figure was drawn. The option correctly states the CFC share in the textbook chart.
- (d)N₂O – 6 % — Correctly paired. Nitrous oxide is the smallest of the four slices at 6 %; it comes mainly from nitrogen fertilisers and soils, and each molecule warms roughly 270 times as strongly as CO₂ over a century.
The hazard is swapping its 6 % with the 14 % of CFCs. As printed, the option gives nitrous oxide's share correctly.
Greenhouse gases differ on two axes: how much of each is emitted, and how strongly each unit warms, measured as global warming potential (GWP) relative to CO₂ = 1 over 100 years. A gas's contribution to warming is the product of the two.
CO₂ has the weakest per-molecule effect but by far the largest emissions and a very long residence time, so it dominates. Methane (GWP about 28) and nitrous oxide (about 270) are emitted in much smaller amounts, taking second and fourth place.
CFCs have GWPs in the thousands, so even trace concentrations produce a substantial slice.
The chart with CO₂ 60 %, CH₄ 20 %, CFCs 14 % and N₂O 6 % appears in the NCERT Class XII Biology chapter on Environmental Issues, and these pairs are taken from it. It covers the human-emitted long-lived gases, so water vapour, the largest natural greenhouse contributor, is not one of its slices.
The figure is indicative rather than a current inventory. In present-day radiative-forcing accounting by WMO and NOAA, CO₂ still contributes roughly two-thirds of the forcing from long-lived greenhouse gases, with methane second.
- The NCERT Class XII Biology chapter on Environmental Issues gives the relative contributions as CO₂ 60 %, CH₄ 20 %, CFCs 14 % and N₂O 6 %.
- The four textbook shares add to 100 %; the figures printed in this question add to 80 %.
- Global warming potential is expressed relative to carbon dioxide, whose 100-year GWP is defined as 1.
- Methane's 100-year GWP is about 28 and nitrous oxide's about 270 in IPCC AR5/AR6 values.
- Methane's atmospheric lifetime is about a decade, against centuries for a sizeable part of emitted CO₂.
- CFCs deplete stratospheric ozone as well as warming the climate; their production is controlled under the Montreal Protocol (1987).
- The Kyoto Protocol's Annex A lists CO₂, CH₄, N₂O, HFCs, PFCs and SF₆ (NF₃ added by the Doha Amendment); CFCs are handled under the Montreal Protocol instead.
- Nitrous oxide is released mainly from nitrogen fertilisers, soils and livestock waste; methane from paddy fields, ruminants, landfills and fossil-fuel leaks.
Textbook shares from the NCERT pie chart; the four add to 100, and the CO₂ row is the altered pair.
- Treating 40 % as plausible because 'CO₂ is under half' sounds reasonable; the textbook share is 60 %, and the printed four add to 80, not 100.
- Confusing per-molecule warming potential with share of warming: CFCs and N₂O have the strongest molecules but the smaller slices.
- Swapping the two small figures, CFCs 14 % and N₂O 6 %, when the paper has printed each of them correctly.
- Mixing this chart with the composition of air (N₂ 78 %, O₂ 21 %), which is a different pie altogether.
- Expecting the incorrect pair to carry an implausible number; 40 % is a believable figure, so the pair fails on recall, not on plausibility.
This idea can be asked as gas–percentage pairs with one mismatch, as here; as an ordering of the four gases by contribution; or simply as which gas contributes most.
A related shape swaps share for potency and asks which gas has the highest warming potential per molecule, where the ranking is reversed: CFCs and N₂O lead, and CO₂ is the unit.
RPSC_2016_PRE_Prelims2016_Q942016Same NCERT pie chart, asked as an ordering of the four gases (CO₂ > CH₄ > CFCs > N₂O) rather than as gas–percentage pairs. The UKPSC item needs the actual figures; the RPSC item needs their rank.
MPPSC_2019_PRE_PaperI_Q32019Coarser version of the same fact: it asks which single gas contributes most to global warming (carbon dioxide). The UKPSC stem goes further by testing the size of each share, not just the leader.
- practice — not a real PYQ
Consider the following statements about the NCERT figure on relative contribution of greenhouse gases to global warming: 1. Carbon dioxide contributes the largest share because it has the highest global warming potential per molecule. 2. CFCs contribute a larger share than nitrous oxide. 3. Methane's share equals the combined share of CFCs and nitrous oxide. Which of the statements given above is/are correct?
- (a)1 and 2 only
- (b)2 only
- (c)2 and 3 only
- (d)1, 2 and 3
Answerc — Statement 2 is correct (CFCs 14 % against N₂O 6 %) and statement 3 is correct (CH₄ 20 % = 14 % + 6 %).Statement 1 is wrong: CO₂ leads on the volume emitted, not on potency; its GWP is 1, the lowest of the four. So (a) and (d) fail by including statement 1, and (b) fails by leaving out statement 3.
- practice — not a real PYQ
Which one of the following greenhouse gases has the highest global warming potential per molecule over a 100-year horizon?
- (a)Carbon dioxide
- (b)Methane
- (c)Nitrous oxide
- (d)CFC-12
Answerd — CFC-12 has a 100-year GWP in the thousands, far above nitrous oxide (about 270) and methane (about 28). Carbon dioxide is the reference gas with GWP 1, so (a) is the lowest, not the highest.The question is about potency per molecule, which is the reverse of the share-of-warming ranking, where CO₂ leads.
- practice — not a real PYQ
The production and consumption of chlorofluorocarbons (CFCs), which contribute to ozone depletion as well as global warming, are controlled under which one of the following?
- (a)Kyoto Protocol
- (b)Montreal Protocol
- (c)Paris Agreement
- (d)Basel Convention
Answerb — The Montreal Protocol (1987) sets the phase-out schedule for CFCs and other ozone-depleting substances.The Kyoto Protocol covers greenhouse gases not already controlled by Montreal, so CFCs sit outside its Annex A; the Paris Agreement works through nationally determined contributions rather than gas-specific production controls; the Basel Convention deals with transboundary movement of hazardous wastes.