Which gas is a greenhouse gas?

• A. CO2
• B. N2
• C. O2
• D. He

Answer: (A)

The main idea is understanding what makes a gas a greenhouse gas: it must absorb infrared radiation and trap heat in the atmosphere. Carbon dioxide does this well because its molecules have vibrational modes that absorb infrared light at specific wavelengths. When sunlight warms the Earth's surface, the planet emits infrared radiation back toward space, and CO2 absorbs portions of that radiation, slowing its escape and warming the lower atmosphere and surface.

The other gases listed don’t act as greenhouse gases in the same way. Nitrogen and oxygen are diatomic molecules with highly symmetric structures that don’t absorb infrared radiation effectively, so they don’t trap heat in the atmosphere. Helium is a noble gas with no infrared absorption bands relevant to the greenhouse effect. (Water vapor is a potent greenhouse gas, but it’s not among the options.)