Whirly-gigs and the role of carbon dioxide in climate change


Solar engine or “whirly-gig”

Pictured here is a device known as a radiometer, although I like to call it the whirly-gig. When you put the whirly-gig in bright light (especially sunlight) the vane inside the glass suddenly starts to turn on its own.

Here is a video of the whirley-gig placed in a sunny spot . We know that objects don’t just move on their own, so where is the energy coming from to make the vane spin?

On the bottom of the device the manufacturer has provided a description of how it works which I’ve shown below.


This description makes a common mistake that I often hear. Notice that it says the vanes spin because the “dark side absorbs heat”. However, that’s not true. This is because heat is a transfer of kinetic energy at the level of atoms and molecules. You get some molecules warm and they move faster, then bump hard into other molecules which transfers some of the kinetic energy, and so on. This cannot be the case for the sun because the space between the Earth and the sun is a vacuum – it’s almost completely empty of atoms and molecules. Heat cannot be transferred through a vacuum.

So how does the sun’s energy make the whirly-gig move? Through a different means of transferring energy – light. In this case, infrared light (that which is beyond the red side of the visible spectrum) from the sun is absorbed by the black side of the vane. When infrared light strikes a molecule, that molecule jiggles. That jiggling is kinetic energy, so in other words, the infrared light makes the molecule hot. That molecule transfers heat to other molecules around it. The air around the vanes gets hot and expands, pushing the vanes and making them spin. Since black absorbs more light of all colors than does white, it is the black side of the vane in the back when the vane is spinning because the black side is the one getting the push from the now warm air behind it.

This same effect is why you get hot on a sunny day when you’re wearing a black shirt. Your shirt if absorbing infrared light, which is converted to heat by the now wiggling molecules, and you feel hotter. It’s also why snow in the shadow of a house or tree melts more slowly. It’s just not getting an infrared light to help get it’s molecules moving to return to the liquid state. It’s also how heat lamps work or those high efficiency space heaters that don’t have a fan – they don’t need it because they aren’t making air hot then blowing it at you. The energy reaches you at the speed of light! I wonder if anyone has used that as their tagline…

Another fun property of infrared light is that while it can make molecules jiggle, the opposite effect can also happen when jiggling gets converted to infrared light. In fact, any object (not at absolute zero) will produce infrared light. We get a glimpse of this when we get a stove burner hot enough to glow red. The burner is glowing before that, just in the infrared which we can’t see. The ability to “see” infrared light is how night vision goggles work – they sense the infrared light produced by bodies of different temperature and then display images on a screen but in visible light. The emission of infrared light by warm bodies is why it gets cooler at night, particularly on a clear night. The warm surface of the Earth produces infrared light that escapes into space. Since there is no sunlight at night to replace the energy lost, the Earth’s surface cools down.

All of this would be just a nice bit of science if it weren’t for one problem. Carbon dioxide is an amazing absorber (and re-emitter) of infrared light. Imagine the case of a clear night when infrared light is leaving the Earth’s surface and cooling it down. Carbon dioxide in the atmosphere absorbs infrared light, both coming in and going out. At night, some of the inferred that would normally be heading out into space gets captured. The carbon dioxide then radiates back some of that and some of that gets back to the Earth’s surface where it does its thing and heats up whatever it strikes.

Now, a certain amount of carbon dioxide in the atmosphere is good. If there were no carbon dioxide present in the atmosphere our planet would be much colder overall. But it’s a delicate balance. The problem is that we humans have been upsetting that balance since we started burning coal, oil and natural gas to power our engines and homes starting about 200 years ago. Every time we burn fuels like coal and oil we get energy out of them. That energy is released because of the formation of strong bonds in the two compounds that form, water and carbon dioxide. The strength of those bonds and the mass of the atoms in them, however, is what makes them very good at capturing infrared light that strikes them.

How much carbon dioxide are we talking about?  In 1958 Charles Keeling began sampling the air on an isolated mountain in Hawaii to find out. Just since then, the concentration of carbon dioxide has gone from just over 300 parts per million to over 400 parts per million. That’s a 28% increase in under 70 years. This is not a natural change. It’s a change due to all six billion of us producing a lot of carbon dioxide by burning fossil fuels.

The same energy that you see manifest in the spinning of the whirly-gig is the same energy that warms up the planet after traveling to us from the sun. Once that energy, in the form of infrared light gets here, and some of that energy leaves the planet, also as infrared light. But some of it gets trapped by carbon dioxide and other molecules in our atmosphere, an atmosphere that we’ve altered to a degree that too much of that infrared light stays here on Earth.

That is the (long) and complicated reason why excess carbon dioxide is the origin of human-caused climate change. If you have questions or comments, especially ways to improve this post, please leave them below. Note that posts are moderated.