Any physical body absorbs and emits electromagnetic radiation when its temperature is above absolute zero. Planck's law determines the radiant flux of a body at a specific wavelength. In atmospheric chemistry, the calculation involving the earth's blackbody radiation shows that the earth's surface temperature would be below the freezing point of water if it did not have an atmosphere which absorbed some of the outgoing radiation.
[Physical Review A;v45; 8471-8487;1992] [Science;v232; 1517-1522;1986]
Source: Atmospheric Chemistry Glossary