Showing that CO2 is a greenhouse gas (yes you read that right)

Today an interesting (?) topic came up. Is CO2 actually a greenhouse gas? And if yes, do greenhouse gases actually warm the contained object? I mean, this should be an uncontested topic by now. But I found myself unable to directly provide evidence, an explanation or experimental setups to prove the greenhouse property of CO2 and its friends. So this is why I wrote this little post. Should you ever find yourself in the position of having to defend this basic scientific finding you can use these talking points. Or point your correspontent directly to this page.

TL;DR sources [2] and [3] give you some experiments do perform at home in order to show the effects of greenhouse gases.

Please excuse the use of german sources. This text was originally written as an e-mail in german and is only translated into english for this blog.

First things first: there is no doubt that CO2 is a greenhouse gas. There is also no doubt that greenhouse gases can trap heat. The theory for this is based on the work of Joseph Fourier [0] and was later proven to be correct in the PhD thesis of Svante Arrhenius called "On the influence of carbonic acid in the air upon the temperature of the ground" [1].

In simple terms: gases such as CO2, methane, etc. reflect heat. This happens, because heat is essentially just infrared light. When sunlight hits the earth it is, for the most part, within the visible light spectrum. The wavelength of visible light is between 400 and 700 nm. This makes it fit right through most gases in the atmosphere. Once it hits the ground, part of it is reflected as visible light and part is reflected as heat (i.e. infrared light). Infrared light, however, has a slightly larger wavelength of 780 nm to 1 mm. This is because it contains less energy than visible light (as it has essentially "split" from the light rays on contact with the ground). This wavelength is large enough, so that it cannot pass through the blanket of CO2 and friends in the atmosphere. Instead it is reflected back and "trapped" on the planet. A thicker blanket thus means more heat being trapped on the planet. In fact, without any such blanket it would be way to cold.

Is this provable at all?
Yes, sure. Experiments in [2] and [3] can perfectly display the heat trapping effect of CO2. (For the non-german readers of this: please use a translation service to read these.)

How much of this is man made though?
There is a rapid increase of CO2 in the atmosphere right around the start of the period of industralization [4]. Models based on physical findings have proven to be quite good at predicting the changes of our climate during the last century [5].

The climate has never been stable. A perfect example for this is the medieval warm period [6]. There was no industrialization present and yet the climate was warmer than usual for several centuries. Currently there is no real constent on what brought this effect about and thus it is often compared to the current developments by climate sceptics. But what is known is: The current effect is way more drastic, consistent and faster. Oh, and we also surpassed the warmest point of that period several years ago [6].

[0] http://www.oekosystem-erde.de/html/hintergrund_luft.html#kohlendioxid
[1] https://www.tandfonline.com/doi/abs/10.1080/14786449608620846
[2] https://www.sonnentaler.net/aktivitaeten/meteorologie/klima/klima-planet-ich/ue3/co2.html
[3] https://www.sonnentaler.net/aktivitaeten/meteorologie/klima/klima-planet-ich/ue3/co2-alternative.html
[4] http://wiki.bildungsserver.de/klimawandel/index.php/Datei:Treibhausgase2005.jpg
[5] https://pubs.giss.nasa.gov/abs/ha09210n.html
[6] https://en.wikipedia.org/wiki/Medieval_Warm_Period