(Aka. GHE) The heating of a planet that is caused by atmospheric gases that trap the sun's heat.
Without the greenhouse effect (and an atmosphere), the Earth's average surface temperature (which is presently 15 °C) could be as low as -18 °C.
1. The phenomenon was discovered by Joseph Fourier in 1824.
2. The term's first Oxford English Dictionary citation is from 1937: (from G. T. Trewartha, Introduction to Weather and Climate, i. 25): "The so-called greenhouse effect of the atmosphere."
3. Click here to read an excellent, concise summary of basic GHE principles by Real Climate's Rasmus Benestad.