The principal anthropogenic sources are leakages from the production of fossil fuels , human- promoted anaerobic decay in landfills, and the digestive tracts of domestic animals. The sources and absorption of nitrous oxide are much more speculative than those for other greenhouse gases. The principal sources are thought to be bacterial breakdown of nitrogen in soils, particularly forest soils. The primary human-made sources are enhancement of natural processes through application of nitrogen fertilizers, combustion of fuels, and certain industrial processes. The annual increase in concentration in the atmosphere is thought to be on the order of 4 million metric tons.
Some greenhouse gases are more potent at affecting climate change than are others.
As a result, comparable increases in the concentrations of different greenhouse gases can have vastly different heat-trapping effects. Among those identified, carbon dioxide is least effective as a greenhouse gas. It would be useful to determine the precise relative effectiveness of various greenhouse gases in affecting the Earth's climate. This information would help policymakers know whether it would be more effective to concentrate effort on reducing the very small emissions of powerful greenhouse gases, such as carbon tetrafluoride, or whether they should bend their efforts to controlling the very large emissions of relatively ineffective gases, such as carbon dioxide. There has been extensive study of the relative effectiveness of various greenhouse gases in trapping the Earth's heat.