Which of the following greenhouse gases, trapped in large quantity in…

Concept — A greenhouse gas can be held out of the atmosphere whenever physical conditions keep it in a trapped, condensed phase rather than as free gas. The most important such store is the gas clathrate hydrate: an ice-like lattice of water molecules in which each cage encloses one small gas molecule, and which is stable only at low temperature and high pressure. Frozen ground and cold, pressurised sediment therefore act as long-term gas reservoirs, and any warming that raises temperature or relieves pressure breaks the lattice down and returns the stored gas to the air.

Application — Applying that principle to the permafrost and hydrate deposits named in the question:

1. Permafrost is ground that has remained below 0 degrees Celsius for at least two consecutive years, and it locks in enormous amounts of dead organic matter that never fully decayed.

2. In that waterlogged, oxygen-free material, methanogenic archaea break the organic carbon down, and the gas this anaerobic decomposition produces is methane.

3. At the low temperature and high pressure of frozen ground and cold sea-floor sediment, the methane is held inside clathrate cages; these deposits are for that reason called methane hydrates, and they hold a carbon stock comparable to all other fossil-fuel reserves combined.

4. Global warming thaws the permafrost and warms the sediment, so the hydrate lattice loses stability and the stored gas is vented into the atmosphere.

5. Since methane is itself a potent greenhouse gas, that release drives still more warming, which thaws still more ground; this is the classic positive climate feedback. The gas released is therefore methane.

Cross-check — Cross-checking the other gases listed against the same store:

• Nitrous oxide is emitted as a free gas by microbial activity in soils and is not held in water-cage deposits, so no large frozen reservoir of it exists to be destabilised.

• Chlorofluorocarbons are manufactured halocarbons only about a century old, so no natural process has buried them in Arctic ground or sea-floor sediment.

• Hydrofluorocarbons are synthetic refrigerant gases introduced even later as replacements for chlorofluorocarbons, so they too have no natural frozen store.

Answer — Methane (CH4).