Why does carbon dioxide trap heat in the atmosphere, while gases like nitrogen and oxygen do not? The answer lies in a physics idea called resonance.

In physics, many systems can vibrate. A simple example is two balls attached together by a spring. You can imagine moving one ball a little, and the spring transmitting that force to the other ball. If you now start pushing and pulling the first ball gently, applying a force side-to-side, the whole system will gently oscillate.

Now the interesting thing is that every system has a particular set of frequencies that it naturally wants to move with. And if you move the first ball backwards and forwards with exactly one of those ‘resonant frequencies’, you see a large response for a tiny input force. This is the phenomenon of resonance.

This model of balls and springs is a really excellent model for a molecule. A carbon dioxide (CO₂) molecule consists of atoms joined together in a line by chemical bonds. These bonds act a bit like tiny springs, allowing the atoms to vibrate in particular ways. Each type of vibration has its own resonant frequency.

We can interact with our system of a single molecule by shining light on it. Light is an electromagnetic wave. Different types of light have different frequencies: visible light has higher frequencies, while infrared light has lower ones.

If you shine light on a molecule which has the resonant frequency of that molecule, it interacts with it very strongly. The molecule starts vibrating more and more, and ends up absorbing the energy in that light. Whereas if you shine light on a molecule of a different frequency, it will not interact.

The resonant frequencies of a CO₂ molecule happen to match frequencies in the infrared part of the spectrum. So visible light from the sun largely passes by Carbon dioxide on its journey down to the surface of the Earth. The sunlight reaches the Earth’s surface, and warms the Earth’s surface. This warm surface then gives off some light in the infrared part of the spectrum – thermal light.

When infrared light from the Earth’s warm surface passes through the atmosphere, CO₂ molecules can resonate with it and absorb its energy. The light is absorbed by the CO2 molecule and heats the surrounding air instead of escaping to space. The more Carbon Dioxide, the less light escapes, the hotter our planet becomes.

This selective absorption of infrared radiation is a key part of the greenhouse effect — and it arises from the physics of resonance.
 

Hannah Christensen
Associate Professor of Physical Climate
University of Oxford