A '''greenhouse gas''' is any gas which contributes to the [[greenhouse effect]].
The most important greenhouse gas is [[water vapor]]. <ref>Water vapor has the largest greenhouse effect. [http://earthobservatory.nasa.gov/Library/glossary.php3?mode=all (NASA)]</ref>
Also important are [[carbon dioxide]], [[methane]], [[nitrous oxide]], and [[ozone]].<ref>http://yosemite.epa.gov/oar/globalwarming.nsf/content/emissions.html</ref>
The nature of a greenhouse gas is to pass [[visible light]], i.e., incoming energy from the sun, but to be opaque to the [[infrared]], i.e., heat energy being re-radiated outward from the earth's surface. This functions much like the [[glass]] on a [[man-made]] [[greenhouse]], and serves to trap warmth at the surface and in the lower [[atmosphere]].
==References==