They're in the news almost as much as Brad and Angelina, and you know they have something to do with global warming. But what exactly is a greenhouse gas? In a nutshell, they're gases in the atmosphere-both naturally occurring and caused by humans-that absorb and emit radiation.
Ultraviolet radiation and sunlight hit the Earth and its atmosphere. Most of this is reflected back into outer space, thanks to the ozone layer, clouds, and ice on the Earth's surface. Approximately 25 percent is absorbed by the planet, and is then re-emitted as infrared radiation. Some of this re-emitted radiation is absorbed by greenhouse gases in the atmosphere, exciting the molecules and causing them to emit the radiation back to the planet again, as heat.
The more of these gas molecules there are in the atmosphere, the more infrared radiation can be absorbed and sent back to the Earth's surface. Increasing concentrations of greenhouse gases in the air will result in an increase of infrared radiation being trapped and re-emitted.
The greenhouse gases that we have the widest understanding about are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs, used for aerosols). CO2 currently contributes the most to the greenhouse effect - the amount of radiation being trapped and re-emitted by greenhouse gases - but the other gases combined will soon be as important to the greenhouse effect as CO2.
These gases are increasing in concentration in the atmosphere, and they have higher rates of absorption of infrared radiation. CFCs, for instance, can absorb 20,000 times as much heat as CO2. However, international decisions to curb CFC emissions have helped stabilize CFC levels in the air.
Greenhouse gases are not, inherently, a bad thing. In fact, they help make this planet livable. Without them, the average temperature of the Earth's surface would be slightly less than 0 degrees Fahrenheit! With the greenhouse effect, the average surface temperature is 59 degrees Fahrenheit. But the growing concentration of greenhouse gases in the atmosphere has the power to raise the average temperature.