About half of the sunlight incident on Earth is transmitted through the atmosphere and absorbed at Earth’s surface. The sunlight-warmed surface emits radiation but, because Earth is colder than the sun, this radiation is primarily at infrared wavelengths. Most of this emitted infrared radiation is absorbed by trace gases and clouds in the overlying atmosphere. The atmosphere also emits radiation, primarily at infrared wavelengths, in all directions. Radiation emitted downward from the atmosphere adds to the warming of Earth’s surface by sunlight. This enhanced warming is termed the greenhouse effect.
As a result of the greenhouse effect, Earth’s annual mean surface temperature of 15°C is 33°C higher than an equally reflective planet in Earth’s orbit with no atmosphere.
The term “greenhouse effect“ is something of a misnomer in this context. It is used as an analogy to the trapping of heat by the glass panes of a greenhouse, which let sunlight in. In the atmosphere, however, heat is trapped radiatively, while in an actual greenhouse, heat is mechanically prevented from escaping (via convection) by the glass enclosure.