Methane

Methane (CH₄) is the second most important greenhouse gas produced by human activity, after carbon dioxide. There is far less of it in the atmosphere (about 1.9 ppm compared with over 420 ppm of CO₂), but each molecule absorbs infrared much more strongly. Over 100 years, methane has roughly 28 times the warming effect of the same mass of CO₂. It lasts only about 12 years in the atmosphere before being broken down to CO₂ and water.Methane is produced by methanogenic archaea breaking down organic matter in anaerobic conditions. Its sources include:
• natural: wetlands and bogs, termites, and methane released from sea-floor sediments;
• human: ruminant livestock such as cattle and sheep (microorganisms in the rumen), flooded rice paddies, landfill sites where waste decays, and leaks from the extraction of natural gas, coal and oil.Atmospheric methane has risen from about 0.7 ppm before the Industrial Revolution to about 1.9 ppm, mainly because of more livestock, rice farming and fossil fuel use as the population has grown.There is concern about a positive feedback: warming thaws permafrost, the frozen soil of the Arctic. Organic matter that had been frozen is then decomposed by microorganisms, often anaerobically in waterlogged ground, releasing methane and CO₂ that cause further warming.Worked example: the percentage increase from 0.7 ppm to 1.9 ppm is:(1.9 − 0.7) ÷ 0.7 × 100 = 171%