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Greenhouse Effect
A greenhouse admits sunlight through large windows, heating the inside of the room; because the glass traps the heat inside, the temperature rises several degrees above what a room would be without the over-sized windows. If the room becomes excessively hot, you can block off some of the sunlight with shades or open the windows to let heat escape. The Greenhouse Effect also happens to the Earth; the Sun warms the ground and the atmosphere prevents some of the heat from escaping into space at night. The Moon, by contrast, has no atmosphere; its temperature soars to 123 degrees Celsius (253 degrees Fahrenheit) when the Sun shines and drops to -233 degrees Celsius (-387 degrees Fahrenheit) almost immediately when darkness falls.
Greenhouse Gases
Some atmospheric gases are better than others at trapping heat; the difference has to do with how their molecules vibrate in response to infrared rays. Gases such as carbon dioxide, methane and some chlorinated fluorocarbon refrigerants let visible sunlight through but are opaque to infrared, trapping heat in the lower atmosphere. Methane has 25 times the heat-trapping power of carbon dioxide, and CFC-12 has 436 times more heat-trapping ability than methane. As the concentration of greenhouse gases increases, they trap more and more infrared energy, gradually raising the average global temperature.
Runaway Greenhouse Effect
Moderate amounts of greenhouse gases are good for life on Earth as they keep conditions from becoming too hot or cold. However, at higher concentrations, the Greenhouse Effect traps so much heat the atmosphere becomes caught in a runaway effect, becoming hotter and hotter until it cannot sustain life.
Planet Venus
Venus is a textbook example of a runaway Greenhouse Effect; its dense atmosphere, having 90 times the surface pressure of Earth, consists of 96.5 percent carbon dioxide. Its atmosphere traps so much heat its temperature has risen to 657 degrees Celsius (1,214 degrees Fahrenheit). Although the global warming phenomenon is slowly raising the Earth̵7;s temperature, it is unlikely to proceed as far as conditions on Venus anytime soon. However, Venus is a striking illustration of the Greenhouse Effect̵7;s power taken to an extreme.