How does carbon dioxide cause global warming?

carbon dioxide

Every sixth grader today knows that carbon dioxide causes global warming. Or at least every sixth grader outside the United States knows this. But why is carbon dioxide a green house gas? I tried to answer this question with the help of an infographic (see above).

From a chemical point of view, heat is motion of molecules and atoms. The more movement is happening on a molecular level, the more heat we feel on a human level (also called macroscopic level). Carbon dioxide is a molecule that is very good at this kind of movement, you could say it is a very athletic molecule. All molecules and atoms in any gas are able to move in all directions in space, but carbon dioxide can even carry out certain movements inside the molecule. These movements are called vibrations. Carbon dioxide can carry out three different vibrations, the symmetric stretch, the asymmetric stretch and the symmetric bend, which can be seen in the infographic above. You can even watch them here. One could say that while other gas molecules or atoms can only run around to create motion, carbon dioxide can additionally jump, wave with its arms and do sommersaults.

The three vibrations are the reason that carbon dioxide can store heat much more efficiently than other gases in the atmosphere where it takes up heat that would normally be lost into space. This means the more carbon dioxide we have in the atmosphere, the more heat is stored on earth. Other green house gases like methane (which eventually converts into carbon dioxide in the atmosphere) have a number of different vibrations as well and work on the same principle. Even water can carry out different vibrations and is a good heat store too. However, it normally falls back to earth as rain or snow within a few days and does not stick around in the atmosphere long enough to do much damage. Carbon dioxide, on the other hand, cannot easily leave the atmosphere and it takes about 100 years for it to do so.

So what about other gases in the atmosphere? Air consistes for the most part of nitrogen (78 %), oxygen (21 %) and argon (1 %). Argon consists of only one single atom and does therefore not have any vibrations. Nitrogen and oxygen both are made up of of diatomic molecules which means two atoms are held together by one bond. This enables them to carry out only symmetric stretches. This is, however, not enough to store heat as efficiently as carbon dioxide does.

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