Methane (CH4) is the second most important GHG for the enhanced greenhouse effect after carbon dioxide (CO2). Methane is emitted by natural and human sources. The main natural sources include wetlands, tundra, oceans and their bottom sediments, and termites. Natural sources make about 36% of methane emissions. Important human sources include landfills, livestock farming (especially enteric fermentation in farm animals), rice farming, biomass burning, as well as the production, transportation and use of fossil fuels. Human-related sources create the majority of methane emissions, accounting for about 64% of the total emissions.
CH4 occurs in the atmosphere in lower concentrations than CO2 and is on average removed from the atmosphere more quickly (lifetime about 12 years), but it has a GWP100yr of about 30.
Natural processes in soil and chemical reactions in the atmosphere represent effective CH4 sinks, however atmospheric CH4 concentration has more than doubled over the last 150 years due to human activities.