Greenhouse gases are gases in an atmosphere that absorb electromagnetic radiation and emit electromagnetic radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The main greenhouse gases in the Earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. In our solar system, the atmospheres of Venus, Mars and Titan also contain gases that cause greenhouse effects. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth’s surface would be on average about 33°C (59°F) colder than at present. Source: Wikipedia article on ‘greenhouse gas’.