All molecules store heat by wiggling. Wiggling includes vibrating, twisting, flying around, and generally acting in a disordered way. Hot things wiggle faster, and for a given temperature, heavy things wiggle slower.

Molecules can absorb or emit light. In many cases when a molecule absorbs light it starts wiggling and jiggling faster (having heated up), and when it releases light, it tends to wiggle slower (having cooled down). So that’s heat in a nutshell: when it’s absorbed into matter, heat is molecular jiggling; when it’s flying through space, heat is light.

In a previous post, I mentioned that oxygen molecules absorb ultraviolet light, which eventually leads to the formation of the ozone layer. In that case, the oxygen molecules (which have two atoms) collide with the light, which makes them wiggle strongly enough to fall apart. Then the atoms recombine to form a different molecule called ozone (which has three oxygen atoms).

Carbon dioxide also absorbs light, but differently than oxygen. Carbon dioxide does not readily convert to ozone, for example, although it contains two oxygen atoms. This kind of situation creeps up frequently in chemistry and physics; the study of how molecules absorb light is called spectroscopy. The ability of molecules to selectively absorb and emit light explains why different chemicals can have different colors. I think the first important result of spectroscopy (historically) was that sunlight contains many different colors, not all of them visible to the human eye. (Isaac Newton figured that one out.)

Imagine two glass containers, one filled with pure carbon dioxide, the other filled with dry air. There’s a thermometer in each container. Put them beside each other in the sun. For our thought experiment, let’s stop the rotation of the Earth so the sun is heating the two containers perfectly evenly over time.

Do the containers eventually reach the same temperature? (Yes.) But now, let’s take them out of the sun and put them in two buckets with the same amount of ice water. Can the two sun-warmed glass containers melt an equal quantity of ice? (No-that’s because carbon dioxide has a different heat capacity than normal air.)

How does the gas inside each container get warmed? The dry air gets warmed indirectly: the container wall absorbs some light and gets warmer, and then the air collides into the warmed glass and begins to jiggle faster. In contrast, the pure carbon dioxide absorbs the sunlight more directly (this is a key result in climate science). Both containers look transparent-visible light is not significantly absorbed by air or by carbon dioxide-but the pure carbon dioxide absorbs infrared-colored light much better than dry air.

Microwaves (as in, the things produced by microwave ovens) are another color of light, like green or ultraviolet. This color of light happens to match the jiggling rate of water, so when water is placed in the microwave oven, it absorbs the microwave light and starts jiggling faster. The whole idea(l) of a microwave oven is that the color of the light is appropriate to make water specifically jiggle. Sunlight, by contrast, makes pretty much every substance jiggle, because it has a wide range of colors of light, from gamma rays to radio waves.

So it turns out that carbon dioxide wiggles and jiggles in a different way than oxygen or nitrogen. A loose way of explaining this is that a three-partner dance (three atoms in carbon dioxide) has a different dynamic than a two-partner dance (two atoms in molecular oxygen or nitrogen). This difference makes carbon dioxide much better at absorbing and storing heat. And this makes the climate science of carbon dioxide a key factor in the climate change debate.


Ryan MB Hoffman has a B.Sc. in Biochemistry from Queen’s University in Kingston, Ontario, and a Ph.D. in Biochemistry from the University of Alberta in Edmonton, Alberta. He is mostly interested in how protein molecules fluctuate throughout their functional processes. During his doctoral work he studied troponin, which is a switch that regulates striated muscle contraction. He works as a post-doctoral scholar at the University of California, San Diego, at the Center for Theoretical Biological Physics. He is active with the Intrinsically Disordered Proteins subgroup of the Biophysical Society. Ryan likes to remind people that his contributions to TRN are performed entirely using his personal resources.