2.3D Methane (HL) Notes

1. Methane (CH₄) is a potent greenhouse gas with a global warming potential over 80 times that of carbon dioxide ($CO₂$) over a 20-year period.
2. It plays a critical role in the carbon cycle and climate change.

How Methane Forms

1. When dead organic material decomposes under anaerobic (oxygen-poor) conditions, carbon is converted into methane by specific microorganisms known as methanogenic bacteria.
2. This process, called methanogenesis, is part of the carbon cycle, temporarily storing carbon as methane before it is oxidized back into CO₂.
3. This process locks carbon temporarily into CH₄ molecules, which can later escape into the atmosphere or accumulate underground in natural gas deposits.

Organic matter → simple organic acids → CO₂ + H₂ → CH₄ (methane)

Conditions for Methane Production

1. Methane is produced where oxygen availability is extremely low and organic matter accumulates.
2. These conditions prevent complete aerobic decomposition, allowing methanogens to thrive.
3. Key environments include:
   1. Swamps and wetlands: large accumulations of plant material underwater create ideal anaerobic conditions.
   2. Rice paddies: flooded fields restrict oxygen diffusion, leading to methane production from decomposing organic residues.
   3. Landfills: buried organic waste decomposes anaerobically.
   4. Digestive tracts of ruminants: methanogenic microbes inhabit the stomachs of ruminants such as cows, sheep, and goats
   5. Permafrost and peat bogs: frozen organic matter traps methane that can be released upon thawing.

Anthropogenic Methane Sources

1. While methane is produced naturally, human activities have significantly increased emissions:
   1. Agriculture: rice cultivation and livestock farming are major sources.
   2. Landfills and sewage treatment: decomposition of waste materials.
   3. Fossil fuel extraction: methane leaks from coal mining, oil drilling, and natural gas pipelines.

Methane in the Atmosphere

Atmospheric Presence and Fate of Methane

1. Once released into the atmosphere, methane becomes a potent greenhouse gas, absorbing long-wave infrared radiation emitted from Earth’s surface and trapping heat within the troposphere.
2. However, methane’s residence time is relatively short, about 10 years on average.

Oxidation of Methane

1. Methane is eventually oxidized to carbon dioxide and water vapour through reactions involving hydroxyl radicals (OH•) in the atmosphere.
2. This oxidation process regulates atmospheric methane concentrations, converting it into longer-lived greenhouse gases (CO₂), thus extending its influence on climate systems.

CH₄ + OH• → CH₃• + H₂O → CO₂ + H₂O

Methane’s Role as a Greenhouse Gas

1. Methane is the second most significant greenhouse gas after carbon dioxide in terms of its contribution to global warming.
2. It has a global warming potential (GWP) approximately 25 times greater than CO₂ over a 100-year timescale.
3. Over 20 years, its GWP exceeds 80 times that of CO₂, making it particularly dangerous for short-term warming.

Climate Implications and Mitigation

1. Methane accounts for roughly 20 % of global warming since the Industrial Revolution.
2. Because of its short residence time, reductions in CH₄ emissions show measurable climate benefits within decades.

Mitigation strategies

1. Agriculture: Alternate wetting/drying in rice fields; feed additives for livestock to inhibit methane-producing microbes.
2. Waste management: Capture landfill gas for biogas energy.
3. Energy sector: Repair leaking pipelines, improve flaring efficiency, and capture fugitive emissions.