4.2.3 Carbon Cycle: Methane

• Methane (CH₄) is a simple hydrocarbon
• It is present as a gas in the atmosphere, and underground, and is the main component of natural gas fossil fuel
• Methane can be produced by the naturally occurring process known as methanogenesis by organisms known as methanogens
  • Before methanogenesis can occur, a series of bacteria convert organic matter into a compound called acetate, as well as carbon dioxide and hydrogen
  • A group of single-celled organisms called archaeans then carry out methanogenesis via two different mechanisms:

CO₂ + 4H₂ → CH₄ + 2H₂O

CH₃COOH → CH₄ + CO₂

• • CH₃COOH is acetic acid, a compound that can be formed from acetate

• Archaeans carry out methanogenesis in a range of environments
  • Waterlogged mud
    • E.g. in naturally occurring wetlands, or in man-made rice fields

  • The guts of ruminant mammals such as cattle
  • Landfill sites containing organic matter such as food waste
  • Anaerobic digesters used for the break down of organic waste

• Depending on the location of methanogenesis, the methane is either released directly into the atmosphere or into the surrounding soil
  • Methane can accumulate in the ground, but it can gradually make its way to the surface where it is released into the atmosphere

• Human activities such as livestock farming and landfill disposal of waste food are leading to an increase in the release of methane into the atmosphere
  • Note that anaerobic digesters are sealed units for the production of biogas, and the methane produced is collected and used for fuel

• The lifetime of methane gas once it reaches the atmosphere is around 10-12 years
• This is because methane is oxidised in the atmosphere
  • This oxidation involves a reaction with molecules called hydroxyl radicals (·OH)
    • Hydroxyl radicals are highly reactive molecules that form in the presence of sunlight from other gases in the atmosphere e.g. nitrous oxides

  • Hydroxyl radicals react with methane in a series of reactions that produce carbon dioxide and water

• Methane oxidation keeps levels of methane in the atmosphere relatively constant
  • Recent increases in atmospheric methane have led to concern that levels of hydoxyl radicals in the atmosphere may be declining