What is a Greenhouse Gas?

These lambs, bathed in the reddish glow of an infrared lamp, are taking advantage of a property of infrared light: it causes electrons to vibrate. When electrons vibrate the molecules they hold together move as well. And we are quite adept at noticing an increase in molecular movement: we call it an increase in temperature.

Any moving object, whether it is a hammer or a molecule, has energy associated with it. Energy associated with the movement of large objects is called kinetic energy. When you a drop a hammer on your foot, kinetic energy in the hammer is transferred to your foot. That’s why it hurts.

Energy associated with the movement of atoms and molecules is called thermal energy. Thermal energy that is transferred to an object (for example, a lamb sitting under an infrared lamp) is called heat.

In the atmosphere some gases absorb infrared and produce heat. But not all gases are greenhouse gases. To make a good greenhouse gas, a molecule must possess an uneven charge distribution, called a dipole moment.

When a molecule with a dipole moment vibrates, the opposite charges alternately move closer together and farther apart. It takes energy to move the charges farther apart, which can be supplied by infrared light. Energy is released as heat when the charges move closer together, causing an increase in temperature. The water molecule has partial negative (δ-) and partial positive (δ+) charges and therefore possesses a dipole and is a potent greenhouse gas (Figure 1).

Figure 1: Water has a dipole moment and acts as a greenhouse gas (left). Oxygen does not possess a dipole and does not act as a greenhouse gas (right).

Carbon dioxide possessed two dipole moments but the molecule is linear, so the dipole moments cancel each other out. For this reason you might not expect carbon dioxide to act as a greenhouse gas. However, certain vibrations can torque the molecule, producing a temporary induced dipole (Figure 2). When the induced dipole disappears, energy is re-released as heat. In other words, carbon dioxide acts as a greenhouse gas.

Figure 2. Carbon dioxide is a greenhouse gas because molecular vibration can torque the molecule, producing an induced dipole.

You don’t need fancy lab equipment to observe carbon dioxide behaving as a greenhouse gas. Just take two, 2-liter soda bottles and fill each half way with water. Drop an Alka-Seltzer tablet in one (the tablet produces carbon dioxide when it dissolves). Cap the bottles and place in the warm sun, measuring the temperature at the beginning and again an hour later. The Alka-Seltzer bottle, which contains more carbon dioxide, will be several degrees warmer than the bottle with water only.