Of all the greenhouse gases, carbon dioxide is present at the highest concentration by far. Based on 1990 concentrations, carbon dioxide is said to be responsible for almost 60% of the total greenhouse effect when efficiency and concentration are considered. Its concentration is increasing in the atmosphere due in large part to the extensive burning of coal and other fossil fuels for energy production. Another cause of CO2 increase is the destruction of large areas of trees that leads to a reduction in use of carbon dioxide for photosynthesis.
Methane is present in the atmosphere at less than 1% the levels of carbon dioxide, however it is 25 times more efficient as a greenhouse gas. It contributes to a little over 10% of the total greenhouse effect based on current concentrations. The primary anthropogenic sources are combustion of fossil fuel, and the decomposition of organic materials associated with wetlands, rice paddies, and livestock manure.
Nitrous oxide also occurs in low concentrations relative to carbon dioxide, but it is 230 times more efficient as a greenhouse gas. These factors combine to make it a 6% contributor to total the greenhouse effect. The primary anthropogenic sources are fossil fuel combustion, fertilizers, and deforestation.
The only major greenhouse gases that are not naturally occurring are the chlorofluorocarbons. They come solely from anthropogenic sources such as the production and/or use of foams, aerosols, refrigerants, and solvents. They are present at an extremely low concentration in the atmosphere, however they are 15,000 times more efficient as a greenhouse gas relative to carbon dioxide. As a result, they contribute to approximately 25% of the total greenhouse effect based on 1990 concentrations.
Table 4 lists the major greenhouse gases, their emission levels and sources, and the approximate amount of time they remain in the atmosphere once they are emitted. It also gives their approximate concentrations 100 years ago, today, and projected concentration for the year 2030. Note that concentrations are given in parts per billion (ppb), referring to the proportion of molecules of the gas per billion molecules in the atmosphere. For example, the concentration of CO2 today is 350,000 ppb, meaning that for every billion molecules in the atmosphere, there are 350,000 molecules of CO2.