FIGURE 62.6 Anthropogenic sources of greenhouse gases in the United States. (a) The largest contributions of methane in the atmosphere arise from gut bacteria that help many livestock species digest plant matter, landfills that experience decomposition in low-oxygen environments, and the production, storage, and transport of natural gas and petroleum products from which methane escapes. (b) The largest contributions of nitrous oxide in the atmosphere arise from the agricultural soils that obtain nitrogen from applied fertilizers, combustion, and industrial production of fertilizers and other products. (c) Nearly all anthropogenic CO₂ emissions come from the burning of fossil fuels.

(Data from http://www.epa.gov/climatechange/ghgemissions/gases/ch4.html, http://www.epa.gov/climatechange/ghgemissions/gases/n2o.html, http://www.epa.gov/climatechange/ghgemissions/gases/co2.html)