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 17^oC.

To understand how the greenhouse effect works,  view this animation.

The problem is that, over the last few hundred years, human activity has dramatically increased the quantities of these gases in the atmosphere, making the 'greenhouse' effect far more efficient at retaining heat.