Last night’s Cosmos episode, the twelfth of thirteen, gave a straightforward and understandable explanation of how carbon dioxide warms the Earth. I thought it was very well done.
Naturally, the howling began soon after, as deniers pulled out their favorite misunderstandings and/or misrepresentations of the facts. Everyone is entitled to their own opinion regarding what we should do about carbon dioxide and climate change. Everyone is not entitled to their own facts. The next time a denier goes on about the unsettled science, say this:
Fact: the Earth emits much more infrared that it absorbs from the Sun (why? I’ll explain below)
Fact: carbon dioxide absorbs infrared light. (why? I’ll explain that below, too)
Fact: when carbon dioxide absorbs infrared light, it re-emits a large percentage of that light right back to Earth (this is simple geometry – the atmosphere is not very far above the Earth)
Fact: this re-emitted infrared light adds heat energy to the Earth (this is just physics – all light carries energy, E = hf)
Now, the denier must explain in what weird universe adding heat energy to the Earth doesn’t affect temperature. It really is that simple.
OK, details, just because they’re great science. First, the Earth emits much more infrared than it absorbs from the Sun. The Sun is quite a bit hotter than the Earth, so hot that it glows white-hot. The Sun sends a great deal of visible light toward the Earth.
The Earth glows, too, but it glows in invisible infrared light. Consider that the Earth needs to radiate away the same amount of energy as it absorbs – otherwise the Earth would get hotter and hotter, until it, like the Sun, glowed white hot. Clearly the Earth is not glowing white-hot. Most of the high-energy light from the Sun is absorbed by the Earth and re-emitted as much lower-energy infrared light.
With no atmosphere, that infrared light would just travel out into space. This is what happens, for instance, on the Moon. On Earth, though, the carbon dioxide (and other gases like water vapor and methane) in the atmosphere redirect some of that infrared light back. Why? It’s because of the shape of carbon dioxide.
Consider an oxygen (O2) or nitrogen (N2) molecule:
These molecules are made of just two atoms. Suppose someone asked you to bend this molecule. You can’t bend it in the middle, because there’s no atom there, just empty space. The only way to affect its shape is to move the atoms closer together or further apart. It’s like trying to break a baseball bat by pulling it longways. Pretty hard to do. This is why the air is see-through. Not just infrared light, but even visible light is of too low an energy to vibrate oxygen or nitrogen molecules (actually, blue and purple light do have enough energy to affect the molecules somewhat, which is why the sky is blue).
now consider a carbon dioxide molecule (CO2)
This molecule has three atoms. It can vibrate in a new way. In addition to the really hard direction of pushing and pulling length ways, this molecule can bend in the middle. Just like breaking that baseball bat is a lot easier if you push up in the middle and down on both ends, so the CO2 molecule can vibrate back-and-forth along this shearing direction with a much lower energy input. It turns out that energy level is smack in the middle of the infrared spectrum.
These facts can’t be denied. CO2 does absorb infrared light and send it back to Earth, causing the Earth to heat up. You can have your own opinion regarding what we should do about this extra heat (I have some strange ideas myself), but you’re not entitled to your own facts.