Why aren’t nitrogen and oxygen greenhouse gases?

We’ve said that nitrogen and oxygen are not greenhouse gases, which means that these gases that make up most of Earth’s atmosphere have essentially no effect on our planet’s temperature. But you might wonder why they aren’t greenhouse gases.

Remember that greenhouse gases are gases that are particularly good at absorbing infrared light. So on a simple level, the reason that nitrogen and oxygen are not greenhouse gases is because they do not absorb infrared light. But going deeper, you might wonder why these molecules don’t absorb infrared light and others do (like carbon dioxide, methane, and water vapor).

You can get a hint of the answer by looking at the molecular formulas. In our atmosphere, nitrogen and oxygen both take the form of molecules in which two atoms are bound together; that is, nitrogen is in the form N₂ and oxygen in the form O₂. In order to absorb photons of infrared light, molecules must be able to vibrate and rotate. This turns out to be fairly difficult for symmetric molecules with only two atoms, particularly when both atoms are the same, as in N₂ and O₂; that is why these molecules are not greenhouse gases and don’t warm a planet.

In contrast, vibration and rotation are relatively easy for many molecules with more than two atoms, which is why water vapor (H₂O), carbon dioxide (CO₂), and methane (CH₄) all absorb infrared light effectively, making them greenhouse gases. In fact, the same is true of many other molecules with more than two atoms, specifically asymmetric molecules and other greenhouse molecules that human activity has been releasing into Earth’s atmosphere include nitrous oxide (N₂O) and industrial chemicals known as halocarbons (which include the chemicals known as CFCs).