The intense warming produced inside a greenhouse when its glass walls retain heat within also occurs on the Earth's surface due to a layer of the atmosphere.

The Earth is continuously struck by the electromagnetic radiation emitted by the sun. Part of this radiation is absorbed by the Earth's atmosphere, but the vast majority strikes the Earth's crust. Of this radiation, part is absorbed by the surface, part is reflected as light radiation of various frequencies (which is why we see objects in their respective colours) and part is reflected as radiation with a longer wavelength (typically infrared).

It is precisely this infrared radiation that generates the greenhouse effect: the atmosphere (like the glass of a greenhouse) is almost completely transparent to visible light but is extremely opaque to infrared radiation. The infrared rays reflected by the surface therefore do not "escape" into space, but remain trapped between the surface and the upper layers of the atmosphere (as in a greenhouse, where they are trapped beneath the glass).

The effect is extremely beneficial for life on Earth, since without it the average temperature would be -19°C.