Terrestrial radiation is the energy released by the Earth itself as opposed to solar radiation that it receives from the Sun. Apart from the energy generated by the decay of radioactive minerals in rock, the energy that drives terrestrial radiation ultimately comes from the Sun, and it is a major factor in the study of global warming.
Earth's atmosphere generally permits the free passage of sunlight in the visible band of the spectrum. Most of this light reaches the surface, whether oceanic or continental, and is absorbed. There is an upper limit to the energy that can be stored in the Earth's crust this way, however, and once the saturation point is reached, Earth begins to radiate energy back into space.
Some of this terrestrial radiation is intercepted by atmospheric gases. Apart from nitrogen and oxygen, virtually all the common gases in the atmosphere absorb some terrestrial radiation. This trapped energy is kept as heat that drives the global climate system. A greater concentration of greenhouse gases in the atmosphere traps higher amounts of terrestrial radiation and continues to drive the process.
Without a greenhouse effect, the balance between solar and terrestrial radiation would hold Earth's average temperature at close to 255 degrees Kelvin. With gases that absorb energy at terrestrial wavelengths, Earth maintains a temperature that is, on average, 33 degrees warmer.