Question

Gases that comprise > 99% of your atmosphere (N2, O2, Ar) are not greenhouse gases, but CO2 is considered to be a greenhouse gas. Why?

Answer 1

The definition of a greenhouse gas is one that will absorb significant amounts of infrared radiation. Those gas molecules in the Earth's atmosphere with three or more atoms are called "greenhouse gases" because they can capture outgoing infrared energy from the Earth, thereby warming the planet. The greenhouse gases include water vapor with three atoms (H₂O), ozone (O₃), carbon dioxide (CO₂), and methane (CH₄).

A CO2 molecule, having THREE atoms (with one of them different than the other two), is able to "wiggle" in such a way as to make that happen. In contrast, the other three molecules can't do this in any way that will create positive and negative ends. So CO2 can absorb this energy while many other molecules cannot.