A greenhouse gas is any gas which contributes to the greenhouse effect.
The level of carbon dioxide in the atmosphere has increased by about 37 percent since the start of industrialization. It is now at its highest level in at least 650,000 years.
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.
- Water vapor has the largest greenhouse effect, though since the quantity of water vapor in the atmosphere is determined primarily by the temperature it is not regarded as a climate forcing agent. (NASA)