The mysteries of carbon dioxide, nitrous oxide, and methane

cientists, or at least some of the more excitable ones, are now claiming that human breathing causes global warming. The idea is being widely ridiculed, as it should be, but the subtext (as people like to say) is what's important. Note what's missing: any mention of carbon dioxide (CO₂). Just nitrous oxide (N₂O) and methane (CH₄).

This is highly significant. It could be a sign that the CO₂ narrative is starting to fall apart scientifically. A close watch of the two science canaries (Nature and Lancet) shows increasing emphasis on CFCs, nitrous oxide, and methane. It sounds like they're preparing us for a switch from CO₂ to something else. Look for statements like “even worse than CO₂” to show up in the popular press, followed by the blog parrots and online encyclopedias.

Surprisingly, there's much about CO₂ that's not very well understood. For instance, to absorb infrared, a molecule must have a dipole moment. CO₂ is linear and symmetric, so it has no dipole moment until it is bent. According to Thomas and Stamnes ^[1], it acquires an induced dipole moment, but in order to do so, it must absorb a photon, which it can't do because it doesn't have one.

I've been looking for an answer to this question for a week now, and the best answer I can find is that a photon excites vibrational modes in molecules. These vibrations, according to the Raman spectroscopy guys, yield scattered photons which are reduced in energy by the amount of energy used to induce the vibration.

CO₂ absorption in Earth's atmosphere is saturated at 400 ppm. The Green curve is the spectral flux calculated as Δν of 3 cm⁻¹ with no greenhouse gases. The Black curve is with all gases at their standard concentrations and CO₂ at 400 ppm. The Red curve is with CO₂ at 800 ppm. A similar saturation result was obtained with the methane band at 1300 cm⁻¹. Source: [2]

Fine, but for this to make sense you still need absorption. It can't be through collision, which would put the absorption way into the microwave region. I'm still searching for a good explanation as to how it does this magic trick, and why even Arxiv has no articles on it.

Of course there's no doubt that CO₂ absorbs IR. But it would be a good topic to study exactly how it does it as Striebel et al. ^[6] have recently done with dipoles.

Another mystery is why so few people realize that CO₂'s 15 μm absorption band (around 660 cm⁻¹), which is said to be the strongest one, is saturated. Absorption by any molecule follows a logarithmic curve: the effect of a change from 1 to 2 ppm is the same as a change from 200 to 400 ppm or 400 to 800 ppm. Many articles show this ^[3], including the sophisticated radiative transfer calculations by Wijngaarden and Happer ^[2], who performed line by line calculations from over 1/3 of a million transition lines to study spectral overlap. Saturation also significantly affects methane forcing ^[4]. They write:

For current atmospheric concentrations, the per-molecule forcings of the abundant greenhouse gases H₂O and CO₂ are suppressed by four orders of magnitude. The forcings of the less abundant greenhouse gases, O₃, N₂O and CH₄, are also suppressed, but much less so. For current concentrations, the per-molecule forcings are two to three orders of magnitude greater for O₃, N₂O and CH₄, than those of H₂O or CO₂. Doubling the current concentrations of CO₂, N₂O or CH₄ increases the forcings by only a few per cent.

This is also confirmed in the textbooks, namely that CO₂ is maxed out and has absorbed about as much as it can. These authors of one ^[5] also show the legendary skepticism so often missing in science:

[W]henever you see in a textbook a statement of the form “Einstein [or Newton or some other scientific worthy] said . . .” replace “said” with “did not say” and what follows is more likely to be true.

My admittedly amateur playing around with the HITRAN spectra seems to confirm that no matter how high you make the CO₂, the absorption stays about the same once you get above 200 ppm. I thought I was doing something wrong, but apparently not.

One counter-argument is that as the center of the absorption band gets saturated, more absorption occurs in the “edges” of the band. This isn't possible: gases don't absorb in bands. Bands are an artifact of modeling, where the overall absorption in some region is smoothed out to make calculations easier. In gases, there are countless extremely narrow transition lines that remain narrow due to a phenomenon called CFR: if a photon gets absorbed, it ‘forgets’ its original wavelength. When it's emitted, it stays within the narrow absorption line. Those transition lines are often converted to Lorentzian, Doppler, and Voigt peak shapes, which owe most of their width to pressure broadening, collisions, and Doppler shifts. The best way to figure these things out is by line-by-line calculations, that is by studying each absorption line separately. This is understandably a pain in the butt, as there are millions of them.

This is totally different from what we're used to when dealing with solids and liquids. Thermal emission from the ground is a continuum. Absorption by gases in the atmosphere isn't. Yet Internet blogs and, increasingly, scientific books simply repeat each other's simplified narrative about global warming.

I understand that people on both sides want simple answers. We all remember the idiotic arguments people had about whether Covid masks worked. One side said they worked perfectly and the other said they didn't work at all. Each decided what to believe for political reasons, and then claimed the science was behind them, and they were both mistaken.

To be honest, until we start seeing something more substantial than a computer model, GW will rank in importance just above those bastards who put apostrophes in every word that ends with an 's'. I'd even glue myself naked to the Mona Lisa (and what a horrifying sight that would be) if I thought there was any more to this than speculation. Yet somehow it went straight from a hypothesis to a dogmatic assertion that we're going extinct unless we dismantle our economy within the next seven years.

Carbon dioxide is one of the simplest molecules, but its physical chemistry is sometimes taken for granted. It's worth studying on that basis alone. Ten years from now, after the warming zealots have all died from the toxic effects of inhalation of orange dye, CO₂ will be a backwater again. Then maybe they'll give it back to the biologists where it belongs.

dec 17 2023, 11:57 am