Which gas is NOT classified as a greenhouse gas?

Oxygen is not classified as a greenhouse gas because it does not significantly absorb or emit infrared radiation, which is the basis for the greenhouse effect. Greenhouse gases, such as carbon dioxide, methane, and chlorofluorocarbons, have molecular structures that allow them to trap heat in the atmosphere effectively. This process contributes to global warming by preventing heat from escaping into space.

In contrast, oxygen primarily participates in combustion and respiration processes but does not play a role in the Earth's energy balance or contribute to the greenhouse effect in the same way as the other gases mentioned. The presence of oxygen in the atmosphere is essential for life, but its function is different from that of greenhouse gases, which help regulate the Earth's temperature.