Despite their relative scarcity, the so-called greenhouse gases play an important role in the regulation of the Earth's energy balance. Greenhouse gases in the atmosphere trap infrared heat energy trying to escape back to space. In doing so they raise the temperature of the lower atmosphere and the Earth's surface in contact with it. This warming process is called the natural greenhouse effect, but during the last 200 years, mankind's pollution of the atmosphere with extra greenhouse gases has enhanced this natural greenhouse effect that may be contributing to global warming.
Greenhouse gases include any gas in the atmosphere that is capable, as a result of its particular molecular structure, of absorbing infrared radiation or heat. They are called greenhouse gases because they behave like glass in a greenhouse gas, allowing sunlight to pass through but trapping the heat formed and preventing it from escaping, thereby causing a rise in temperature. Natural greenhouse gases include water vapour or moisture, carbon dioxide, methane, nitrous oxide and even ozone, which is more commonly associated with the ozone layer and ultraviolet radiation. The amounts of all these gases in the atmosphere are now being increased as a result of man-made processes, such as fossil fuel burning and deforestation. The atmospheric concentration of carbon dioxide, for example, has increased by 30% since the 18th century, whilst levels of methane have more than doubled. Water vapour, whilst not directly released by man-made processes in substantial quantities, may be increasing as a result of climate feedback effects.
In addition to the man-made increase of naturally occurring greenhouse gases, mankind has released some completely new chemicals into the atmosphere, including the CFCs or chlorofluorocarbons. Although these have now been banned in an attempt to save the ozone layer, they will remain in the atmosphere for at least another 50 years. Although their abundance in the atmosphere is very low, molecule for molecule they can be thousands of times better at absorbing heat than carbon dioxide, and consequently contribute significantly to the enhanced greenhouse effect. Furthermore, their replacements, the HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons), whilst being relatively harmless to the ozone layer, are equally potent greenhouse gases, and at present their phase-out dates are not due for another 20 to 30 years.