The greenhouse effect is the process by which radiation from a planet’s atmosphere warms the planet’s surface to a temperature above what it would be in the absence of its atmosphere If a planet’s atmosphere contains radiatively active gases the atmosphere radiates energy in all directions. Part of this radiation is directed towards the surface, warming it.
On Earth, solar radiation at the frequencies of visible light largely passes through the atmosphere to warm the planetary surface. The surface itself emits energy at the lower frequencies of infrared thermal radiation. Infrared radiation is absorbed by greenhouse gases in the atmosphere. These gases also radiate energy, some of which is directed to the surface and lower atmosphere. The mechanism is named after the effect of solar radiation passing through glass and warming a greenhouse, but the way it retains heat is fundamentally different as a greenhouse works by reducing airflow, isolating the warm air inside the structure so that heat is not lost by convection]
If an ideal thermally conductive blackbody were the same distance from the Sun as the Earth is, it would have a temperature of about 5.3 °C. However, since the Earth reflects about 30%of the incoming sunlight, this idealized planet’s effective temperature (the temperature of a blackbody that would emit the same amount of radiation) would be about −18 °C.The surface temperature of this hypothetical planet is 33 °C below Earth’s actual surface temperature of approximately 14 °C. The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect