The term "greenhouse effect" refers to the increased warmth of the earth's surface and atmosphere which results when gas molecules prevent the escape of infrared radiation from the earth. These "greenhouse gas" molecules (principally water, carbon dioxide, and methane) absorb and re-emit longwave infrared rays emitted from the earth's surface into the atmosphere (see radiational cooling).
It was discovered by Joseph Fourier in 1824 and first investigated quantitatively by Svante Arrhenius in 1896. In the case of the Earth, without these greenhouse gases its surface would be up to 30°C cooler. In addition to the Earth, Mars and especially Venus have greenhouse effects.
This insulating effect has been compared to a blanket or to the glass walls and roof of a greenhouse, but the process is not the same. The name comes from an analogy with the way in which greenhouses on the earth retain heat. The difference is that the atmospheric greenhouse effect blocks infrared radiation, while a terrestial greenhouse just keeps warm air from blowing away.
In common parlance, the term "greenhouse effect" may be used to 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).