Which one among the following is not a greenhouse gas?

Correct — A, Sulphur dioxide. Sulphur dioxide is an air pollutant, not a greenhouse gas. It is the main cause of acid rain, it damages lungs and vegetation, and in the atmosphere it converts to sulphate aerosols which scatter incoming sunlight and therefore have a net cooling effect — the opposite of what a greenhouse gas does. The other three all appear in the basket of greenhouse gases counted under the international climate agreements: methane, nitrous oxide, and the fluorinated gases, which cover hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride.

• (b)Methane — A greenhouse gas roughly eighty times more powerful than carbon dioxide over a twenty-year horizon, though it lasts only about twelve years in the atmosphere. Its main sources are wetlands, flooded rice fields, livestock, landfills and fossil-fuel extraction.
• (c)Nitrous oxide — A greenhouse gas with an atmospheric lifetime of around a hundred and nine years and a global warming potential of roughly two hundred and seventy times that of carbon dioxide. Nitrogenous fertiliser use is its principal anthropogenic source.
• (d)Fluorinated gas — The fluorinated gases — hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride and nitrogen trifluoride — are entirely man-made and are the most potent greenhouse gases by mass, which is why the Kigali Amendment targets hydrofluorocarbons for phase-down.

A greenhouse gas is one that absorbs and re-emits infrared radiation from the Earth's surface, trapping heat in the lower atmosphere. Absorbing infrared requires a molecule whose vibrations change its dipole moment, which is why water vapour, carbon dioxide, methane, nitrous oxide, ozone and the fluorinated gases qualify. The basket recognised under the Kyoto Protocol and carried into the Paris framework is carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride and nitrogen trifluoride. Sulphur dioxide is absent from that list; it belongs with the criteria air pollutants alongside oxides of nitrogen, carbon monoxide and particulate matter.

The temptation is to treat every harmful gas as a greenhouse gas, and sulphur dioxide is harmful enough — it is the classic acid-rain precursor and one of the pollutants monitored in the air quality index. But warming and pollution are different accounting systems. Sulphate aerosols formed from sulphur dioxide actually reflect sunlight and cool the surface, which is why large volcanic eruptions are followed by cooler years, and why cleaning up sulphur emissions has unmasked a little additional warming. This card marks a different letter from the answer we hold on file for this question.

• Sulphur dioxide is an air pollutant and the main precursor of acid rain, not a greenhouse gas.
• Sulphate aerosols derived from it scatter sunlight and exert a net cooling influence.
• Methane, nitrous oxide and the fluorinated gases are all in the Kyoto basket of greenhouse gases.
• Nitrous oxide has an atmospheric lifetime of about 109 years, against about 12 years for methane.
• The Kigali Amendment to the Montreal Protocol phases down hydrofluorocarbons because of their high warming potential.

• Assuming every polluting gas warms the planet; sulphur dioxide does the reverse.
• Forgetting that water vapour is the most abundant greenhouse gas even though it is not in the regulated basket.
• Confusing ozone depletion with global warming; they are different problems with different treaties, although fluorinated gases appear in both.

An odd-one-out item that tests the boundary between air pollutants and greenhouse gases, a distinction asked in almost every recent environment paper.

• practice — not a real PYQ

  Sulphur dioxide released from burning coal is chiefly responsible for

  • (a)ozone depletion
  • (b)acid rain
  • (c)global warming
  • (d)eutrophication

  Answer(b) acid rain — sulphur dioxide oxidises to sulphuric acid in the atmosphere and falls with precipitation.

• practice — not a real PYQ

  Which one among the following greenhouse gases has the longest average residence time in the atmosphere?

  • (a)Methane
  • (b)Nitrous oxide
  • (c)Water vapour
  • (d)Carbon monoxide

  Answer(b) Nitrous oxide — about 109 years, against roughly 12 years for methane and only days for water vapour.