http://en.wikipedia.org/wiki/Greenhouse_effect
Greenhouse effect

The greenhouse effect, discovered by Joseph Fourier in 1824 and first investigated quantitatively by Svante Arrhenius in 1896, is the process in which the emission of infrared radiation by an atmosphere warms a planet's surface. The name comes from an incorrect analogy with the warming of air inside a greenhouse compared to the air outside the greenhouse. The Earth's average surface temperature is about 20-30°C warmer than it would be without the greenhouse effect [1]. In addition to the Earth, Mars and especially Venus have greenhouse effects.
In common usage, "greenhouse effect" may refer either to the natural greenhouse effect due to naturally occurring greenhouse gases, or to the enhanced (anthropogenic) greenhouse effect which results from gases emitted as a result of human activities (see also global warming, scientific opinion on climate change and attribution of recent climate change).