Infrared absorption by N2 vs CO2

In this weeks issue of Norway’s leading engineering journal a reader’s letter poses the following challenge/question:

To me that looks like something I’d grade a high school student down for writing, since I found what looks to me like coherent explanations and empirical data just by opening the Wikipedia articles on Nitrogen, CO[sub]2[/sub] and greenhouse gases. But just so I don’t get caught by some nitpick gotcha after I berate the journal for giving an obvious moron a platform I thought I’d check with the teeming millions.

  1. Is the differing properties of greenhouse gases and non-greenhouse gases a mystery to researchers in any relevant way?

  2. Is there some obvious AGW-denier source for this letter writer’s confusion that I could refer to in my response?

CO2 doesn’t reflect radiation more efficiently than oxygen or nitrogen, it absorbs it. If it reflected radiation it would be an anti-greenhouse gas, equivalent to most particulates, that would cool the Earth by reflecting solar radiation back into space.

The person who asked this question has made the mistake of accepting that " The Greenhouse Effect" actually operates like a greenhouse. The glass in greenhouse makes it hot by allowing in visible light, but when that visible light is converted to infrared, not allowing the IR to escape. Because the IR radiation is trapped in the structure, it eventually warms all the matter within the structure, including the air, the ground and the plants.

The “Greenhouse Effect” works in almost exactly the opposite way. CO2 won’t allow IR light back into space. Instead it absorbs the light, converting it into heat.

In short: Actual greenhouse uses a material that absorbs very little IR radiation, instead reflecting the radiation emitted from the warmed ground back into the ground and air, heating them even further. “Greenhouse Effect” uses a material that absorbs a lot of IR radiation, which warms that material directly.

Huge difference and something that makes “Greenhouse Effect” utterly misleading and leads to silly questions like the one in the OP. Of course no scientist can epxlain why CO2 is more efficient at reflecting heat radiation, because every interested scientist knows that CO2 is more efficient at absorbing heat radiation.

To answer the thrust of the question, rather than the erroneous and leading question asked, yes, it’s explained comprehensively by quantum mechanics. There is no mystery surrounding the whys and wherefores of EMR absorption. There are literally thousands of book and theses written on every tedious detail of the subject. this forms the basis of every spectroscopic analysis technique as well as large amounts of modern optics.

In simple terms it comes down to an interaction between the energy level of the electrons in the atoms and the binds between them, the length of the bonds between atoms and whether those factors allow incoming photons can provide enough energy to to kick them to a higher energy level. You need to understand some quantum physics to understand it in any more depth, but anybody with a basic understanding of quantum physics doesn’t find it at all mysterious.

To point out how utterly un-mysterious all this is, spectroscopic analysis relies on how utterly predictable this all is to map the structure of molecules. It is because we know that a C=O double bind always absorbs light at this point on the spectrum and re-emits it at this point, and that an adjoining metal group will always pull the absorption down X nM and push the emmission up Y nM, that we can use the absorption and emission spectra to say whether an utterly novel, synthesised molecule contain a double or single bond on the 17th carbon atom on the second side chain.

There isn’t even a hint of a mystery here. We know how absorption works and we know why it works.

Well, I don’t. 20+ years after doing analytical and quantum chemistry, I’ve forgotten most of it. But any undergrad analytical chemist or physicist can explain it in detail, as can endless websites.

Your description of what happens in a “greenhouse” is wrong. Professor Robert W. Wood proved by experiment that a “greenhouse” is confined space heating not back radiation of IR. Professor Woods experiment was done in 1909. In 2009 Dr. Nasif Nahle repeated Professor Woods experiment and verified the conclusion that the “greenhouse” is confined space heating.
Many other of your comments are also wrong.
One kick in the ass is enough for this post.

Uhh. Yeah. Sure. :rolleyes:

Trying to understand this… so the atmosphere and greenhouse glass both let in visible light, right?

Then, with greenhouses, Wood thought the primary heating mechanism is the prevention of convection (“confined space heating”, as cleanwater2 calls it), not IR reflection. Blake disagrees and believes it’s the IR reflectivity of glass.

The atmosphere, on the other hand, doesn’t reflect IR, it absorbs and re-emits it? The anti-global-warming implication then is that the earth isn’t sealed like a greenhouse is, so convection will dissipate the heat to space…? Or what? But doesn’t the atmosphere only go so high, after which radiative heat transfer must occur for additional heat to escape? (And that’s the part that greenhouse glass and greenhouse gases both prevent, the former by reflection and the latter by reemission?)

Yes.

Without a doubt, the prevention of convection is a major factor. Obviously a greenhouse where all the warm air is allowed to escape would never warm up. I don’t think anybody has ever doubted that.

However a greenhouse heats up far faster than a simple lack of convection should allow. We can measure how warm the surfaces in a greenhouse are, how warm the air is and how warm the glass is. The temperature of the glass at a given temperature is always less than it would be if the glass were IR absorbant *or *IR transparent, and the temperature of the air is much higher than expected. IOW the heat isn’t being transmitted to the glass and taken away by convection on the outside as you would expect of, say, brick, and the IR radiation isn’t simply being transmitted straight through, and hence lost to the outside air. The ability of the glass to reflect the IR back to be absorbed by the floor and the plants is a major factor in the warming.

FWIW, Woods conducted his experiment over a century ago, and was explicitely referring to only the temperature of the glass, not to the temperature within the room. There has been plenty of work since that has shown that IR reflectance of glass is a major contributor to greenhouse temperature.

Nasif Nahle is a self-proclaimed “scientist on Biology”, and one of the few nutjobs out there who actually believe that Carbon Dioxide cannot be a source of Global Warming, that the increase in CO2 in the atmosphere isn’t caused by humans. I probably don’t have to say anything else about his credibility.

Largely, yes. SOme reflectance occurs.

I think that’s Nahle’s point, but he’s a bit incoherent. He also overlook the fact that space is a vacuum, and you can’t have a transference to a vacuum via convection because there are no atoms in a vacuum. You can transfer heat to the edge of space via convection and then hope it radiates away, but you can transfer heat from a greenhouse bench to the glass via convection and hope it radiates away as well, plus the glass will actually remove heat via convection.

So even if greenhouses did work solely via preventing convection, it still doesn’t apply to global warming.

Exactly.

Not really.
If we accept that glass IR reflectivity is a significant part of the warming of a greenhouse, it’s true there.

But if we only believe that the glass stops convection and the reflection is irrelevant, then the glass isn’t stopping radiative cooling at all. Heat can still radiate through glass at levels about equal to if the glass weren’t there.

In the case of greenhouse gases, radiation into space from the edge of space isn’t what’s being prevented There’s not enough gas up there to make much difference. It’s the radiation of IR light into space from the lower levels of the atmosphere that’s being prevented.

N2 doesn’t absorb at all in the infrared. For an IR photon to be absorbed, the molecule must change its dipole moment when it vibrates. Since N2 has no dipole moment, stretched or not, you cannot excite vibrations with IR photons.

Any gas will absorb IR to some degree, nitrogen is no exception. Nitrogen, like all other gases, has an instantaneous dipole moment and, in the atmosphere, an induced dipole. The IR absorption of Nitrogen is pretty weak, but it’s measurable.

Dude, the first involves a collision-induced dipole that requires a gas mixture, and the second is a quadrupole-mediated absorption. Both completely different.