Although there is less methane than carbon dioxide in the atmosphere, methane is a more effective heat-trapping greenhouse gas. It comes from the decay of matter without the presence of oxygen. Primary sources include wetlands, rice paddies, animal digestive processes, fossil fuel extraction, and decaying garbage. Soils and oceans are the primary natural sources of nitrous oxide. Humans contribute to nitrous oxide emissions through soil cultivation and use of nitrogen fertilizers, nylon production, and the burning of organic material and fossil fuels.
Ozone exists naturally in the lower atmosphere in minute quantities.
It also can be produced in the lower atmosphere from a reaction involving several human-produced pollutants and sunlight. They are some of the most effective heat-trapping greenhouse gases of all. Over the past 200 years methane concentrations have more than doubled. Nearly 6 billion tonnes of carbon or 22 billion tonnes of carbon dioxide are added to the atmosphere every year through the burning of fossil fuels. The human-made greenhouse gases, such as halocarbons, were invented only in the last 100 years.