The Green House Effect occurs when infrared heat energy emitted from surfaces in a space is reflected back to the surfaces by an insulator. The constant reflection of infrared heat increases the temperature of the space over time. In the case of an actual greenhouse, the suns visible light enters the greenhouse through glass panes. Plants and dark surfaces in the greenhouse absorb the visible short wave radiation and emit infrared long wave radiation. The glass panes reflect the long wave radiation, preventing the heat energy from leaving the greenhouse, increasing temperatures inside.
The same concept applies to the earths relationship with the sun. The earths atmosphere acts as the green house glass, allowing visible light in and reflecting a portion of the infrared radiation emitted from its surface. Substantial increases of carbon dioxide and other insulating gases in the atmosphere over the last decade has resulted in more infrared radiation being reflected back to earths surface causing average temperatures to rise. This trend has been termed global warming.