NDA (II) 2018 General Ability Test · Question 114 of 146

Correct — C, Carbon dioxide. Of the four gases listed, carbon dioxide is present in by far the greatest amount. At the time of the September 2018 exam it stood at a little over 400 parts per million, and by 2019 the figure quoted was about 410 ppm. Methane, the nearest rival, was around 1,866 parts per billion — roughly 1·9 ppm, more than two hundred times less. Nitrous oxide was about 332 parts per billion, and the chlorofluorocarbons are measured in parts per trillion, with CFC-11 near 226 ppt and CFC-12 near 503 ppt. Ranked by how much of each is actually in the air, the order is carbon dioxide, then methane, then nitrous oxide, then the CFCs, and that is what the question asks for.

• (a)Chlorofluorocarbon — The CFCs are the least abundant of the four by an enormous margin, present at only a few hundred parts per trillion. They matter because each molecule is a very powerful absorber and because they attack stratospheric ozone, not because there is much of them.
• (b)Nitrous oxide — Nitrous oxide sits at roughly 332 parts per billion, more than a thousand times less abundant than carbon dioxide. Agriculture, chiefly nitrogen fertiliser use, is its main human source.
• (d)Methane — Methane is the second most abundant of the four but is still only about 1·9 parts per million against carbon dioxide's 400-plus. It is the classic trap here because methane is the more potent greenhouse gas molecule for molecule — potency and abundance are different questions.

A greenhouse gas is one whose molecules absorb the long-wave infrared radiation emitted by the warmed surface of the Earth and re-emit it in all directions, keeping heat in the lower atmosphere. Two separate properties decide how much warming a gas actually causes: how much of it is present, and how strongly each molecule absorbs. Carbon dioxide leads on the first, the halocarbons on the second, and methane sits awkwardly in between with a global warming potential roughly twenty-eight times that of carbon dioxide over a century.

One honest caveat belongs on this question. Water vapour, not carbon dioxide, is the most abundant greenhouse gas in the atmosphere and is held responsible for something like 41 to 67 per cent of the natural greenhouse effect. It is not one of the four options, which is why the answer stands, but a well-prepared candidate should know that the full ranking by abundance runs water vapour, carbon dioxide, methane, nitrous oxide, ozone. Water vapour is usually left out of policy discussion because its concentration is set by temperature itself and cannot be controlled directly — it acts as a feedback rather than a driver. As a present-day note, carbon dioxide has continued to climb since this paper was set and passed 420 ppm in the 2020s.

• Carbon dioxide stood at about 410 ppm in 2019, against methane at 1,866 ppb, nitrous oxide at 332 ppb and CFC-12 at 503 ppt.
• Ranked by average global concentration the five most abundant greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and ozone.
• Water vapour is responsible for roughly 41 to 67 per cent of the greenhouse effect, but its concentration is not directly set by human activity.
• Methane is far more potent per molecule than carbon dioxide, which is why abundance and warming impact give different rankings.

• Answering with methane because it is the more powerful greenhouse gas per molecule; the question asks about concentration.
• Forgetting that water vapour outranks carbon dioxide in abundance, and so overstating the claim when writing about it.
• Mixing up the ozone-depletion story, where the CFCs are central, with the global-warming story, where they are a small share.

Asked as a straight ranking item where the trap option is the gas that leads on a different measure.

• practice — not a real PYQ

  Which one of the following greenhouse gases has the highest global warming potential per molecule over a hundred-year horizon?

  • (a)Carbon dioxide
  • (b)Methane
  • (c)Chlorofluorocarbon
  • (d)Water vapour

  Answer(c) Chlorofluorocarbon — the halocarbons are thousands of times more effective per molecule than carbon dioxide, even though they are present only in parts per trillion.

• practice — not a real PYQ

  Which greenhouse gas is the most abundant in the Earth's atmosphere?

  • (a)Carbon dioxide
  • (b)Water vapour
  • (c)Methane
  • (d)Ozone

  Answer(b) Water vapour — it leads the list by average global mole fraction, with carbon dioxide second; its concentration, however, is governed by temperature rather than directly by emissions.