Water vapour, carbon dioxide, methane, ozone and chlorofluorocarbons are called greenhouse gases. These gases are responsible for an effect called greenhouse effect. This effect to similar to what happens in a greenhouse. The sun's rays enter the greenhouse, visible rays are reflected back but the heat is trapped. In a similar way, greenhouse gases trap or absorb the heat reflected by the surface of the earth. They do not let heat escape directly into space. This increases the overall temperature of the surface of the earth. For example, without greenhouse gases, the average temperature of the surface of the earth would have been -18 degrees celsius instead of the current average of 14 degrees celsius. Therefore, greenhouse gases are essential to keep the earth's surface warm.