A greenhouse gas (GHG) is any gas in the atmosphere that absorbs and emits radiation in the thermal infrared range. These are the fundamental cause of the greenhouse effect, which results in increased temperatures on Earth. The greenhouse effect occurs as the gases reach Earth’s surface. As the short wave energy heats the surface, some of the longer wave energy radiates back into the atmosphere and back into space. Greenhouse gases absorb some of the energy and trap it in the lower atmosphere.