The principal greenhouse gases
Most infrared trapping is due to water vapor. If the planet warms considerably because of increases in man-made greenhouse gases, even more water vapor could enter the atmosphere, leading to yet more warming in a 'feedback' effect.

Next in importance is carbon dioxide or CO2. Both water vapor and CO2 absorb at many infrared radiation (IR) wavelengths between 3 and 30 microns (a micron is one-millionth of a meter), but neither absorbs much between 8-12 microns, which accounts for a great deal of the Earth's surface radiation. That potential escape route, however, is plugged by another class of greenhouse gases that is highly absorbing in the 8-to-12 micron range, most notably methane, nitrous oxide and halocarbons, which include chlorofluorocarbons (CFCs) and their relatives.

Methane is emitted by landfills, wetlands, digestive systems, coal mining, natural gas leaks and vegetation burning in the tropics, among other causes. Per molecule, it's 60 times more effective at absorbing Earth's surface radiation than is CO2.

Nitrous oxide (N2O) is produced by soils, fertilizer application, combustion and various industrial processes. Per molecule, it's about 270 times as effective as CO2 at trapping IR.

Only halocarbons have no natural source. CFCs were widely used as refrigerants and in plastic foams before an international treaty called the Montreal Protocol constrained their production in 1987. The treaty does not affect many of their replacement compounds known as 'HFCs'. Some halocarbons are thousands of times more effective per molecule at trapping heat than CO2.

CO2 concentrations have increased c. 30 % since 1800, Methane levels have more than doubled and Nitrous oxides concentrations have increased about 8 %. All of the halocarbons have been added in the last few decades.
See Pie Chart.

Excerpted from 'Climate Change Primer'
in the Washington Post 1997