Greenhouse gases are gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds. This property causes the greenhouse effect. Greenhouse gases are essential to maintaining the temperature of the Earth; without them the planet would be so cold as to be uninhabitable. Anthropogenic sources of greenhouse gases include modern industrial, transportation, residential, commercial and agricultural processes. Venus and Mars also have atmospheric gases that cause greenhouse effects.