Which of the following statements are correct about the deposits of 'methane hydrate'? 1. Global warming might trigger the release of methane gas from these deposits. 2. Large deposits of 'methane hydrate' are found in Arctic Tundra and under the seafloor. 3. Methane in atmosphere oxidizes to carbon dioxide after a decade or two. Select the correct answer using the code given below.

The correct answer is D, 1, 2 and 3. All three statements are correct.

Methane hydrate, also called gas hydrate or "fire ice", is methane trapped inside a cage of frozen water molecules. It stays stable only under low temperature and high pressure, so it is found in two settings: in the permafrost of the Arctic tundra and in sediments under the seafloor along continental margins, including India's Krishna-Godavari basin. If warming heats the permafrost or the deep sea, the hydrate can break down and release methane, a greenhouse gas many times stronger than carbon dioxide, which would add to the warming in a feedback loop. Once in the air, methane is broken down by hydroxyl radicals and turned into carbon dioxide and water; its lifetime is about a decade.

A, B and C are wrong because each leaves out one correct statement.

Exam tip: methane hydrate = methane in ice cages, under permafrost and seafloor; atmospheric lifetime about 12 years, then oxidised to CO2.