Most people regard the 'greenhouse effect' as one of the most serious environmental issues facing the earth. In fact, the 'greenhouse effect' is a natural atmospheric system to trap in heat, which is provided by gases such as carbon dioxide and methane. These gases allow short-wave radiation to pass through the atmosphere, but absorb some of the long-wave radiation trying to escape. Without this effect, the earth would have an average temperature of minus 17oC.
To understand how the greenhouse effect works, view this animation.