I’m looking into the answers to some “yes, but …” questions on the greenhouse effect and I’m not finding a satisfying answer to the question in the title. The Wikipedia article on atmospheric gases says the following:
Now to my mind “heating” means “causes the temperature to rise” and increased temperature means increased kinetic energy, but I’m also under the impression that when freely moving gas molecules absorb photons the energy entirely goes to excite electrons and is lost to that molecule again when emitted.
Is Wikipedia using “heating” wrong/in-a-way-I-disapprove-of, or am I wrong about what happens when you pass radiation through a gas?
Yes, but I don’t think that is really the main mechanism by which greenhouse gasses cause global warming. As I understand it this is mainly a matter of the gasses trapping some of the radiation emitted by the Earth, that would otherwise be lost into space. When the Earth (the ground and the oceans) have been warmed by the Sun, they begin to re-emit some of that energy as infra-red radiation. In the absence of greenhouse gasses that would simply go off into space and be lost, but the gasses absorb some of it (in the way your Wiki quote describes), and then re-emit the energy again in random directions (still as infra-red), much of it back towards the Earth again.
“Yes” is not a good answer to a question containing an or.
(ETA: And if you’re referring to the question in the title it appears to me the heating is insignificant.)
I tried to do some math and assuming an average CO2 molecule in a 290 K gas and a photon with 10 micro meter wavelength* I got an increase in momentum for the CO2 molecule after absorption of 0.002 percent, while kinetic energy would have to increase by 3300 percent if that’s where the energy went.
Now these were quick calculations with high risks of misplacing a decimal point or leaving out a very large or very tiny constant, but if they’re right, then my understanding of what happens to gasses exposed to radiation was correct.
I didn’t look up whether or not CO2 would absorb that photon, but we’re talking orders of magnitude here.
Your understanding is kind of incorrect, yeah. A photon of that energy would generally cause the molecule to start vibrating, rather than causing its translational motion to change. (There would be a slight recoil effect, as you note, but as far as linear motion and kinetic energy go, it’s effectively an inelastic collision.) Collisions with other molecules can then cause the vibrational energy to be converted to translational motion, and the average speed of the molecules goes up.
Oh, and you might be interested in the equipartition theorem, which roughly says that the thermal energy of a system in equilibrium is equally shared among all of its possible degrees of freedom. In particular, if you have a bunch of molecules and set them all vibrating via infrared light, then after a while (when the system has returned to equilibrium) some of that fraction of energy will have been transferred into extra motion of the moelcules.
Ah, that makes sense. I hadn’t much thought about vibrational energies overlapping with the energies of infrared photons. Having just a high school physics understanding of this particular topic I’d somehow not noticed the magnitudes of the energies involved.
You seem to be describing fluorescence, and conflating that with ordinary absorption and heating. But even with fluorescence, some of the energy that is absorbed is converted into vibrational energy. AFAIK, atmospheric gases don’t fluoresce in any significant way.
Yes, I was referring to the title question, and the burden of the rest of my post was indeed that that heating was not significant wrt the greenhouse effects of the gasses in question.
Yes, I was unaware that “ordinary absorption and heating” applied also to gases under atmospheric pressure and below. And I’d still like to learn more, for instance
is the interaction between solar radiation and the main gases in the atmosphere N[sub]2[/sub] and O[sub]2[/sub] also “ordinary absorption and heating”? The Wikipedia article on the greenhouse effect states that these gases “are not able to directly absorb or emit infrared radiation”.
As a general rule, that will serve you well in a wide variety of physical situations: Whenever the question is “where did the energy go?”, the answer is almost always “heat”.
Yes, but convection is heat, conduction is heat, IR is heat, visible light is heat. When the issue is getting sufficient understanding to trounce someone who thinks his terrible understanding of physics is valid critisism of climate research, it’s not answer enough.
One of his three questions is:
Now my original flawed understanding was that the energy wasn’t in a form transmittable by collision, but I now understand that was completely wrong. I understand where he’s coming from though, or … I don’t understand why he thinks it’s relevant, but looking at the Wikipedia entry on the greenhouse effect for instance, one illustration (a horrible one in my opinion) has some of the infrared radiation is absorbed and re-emitted by the greenhouse gas molecules. The other has everything going via “Heat and energy in the atmosphere” with “Thermal radiation into space” being what leaves the atmosphere. Now I assume from the responses so far that the latter is more correct.
The radiation is “re-emitted” in the sense that it is absorbed by the gas, which heats up the gas (its temperature goes up), which causes the gas to emit more infrared radiation. In fact, the temperature goes up just enough to make the gas emit as much radiation as it is absorbing.
A couple of comments on CO2 absorbing infrared light and how it is converted into heat.
When CO2 absorbs light in the IR, it excites vibrations in the molecule. The CO2 molecules are colliding with other molecules in the atmosphere. At a pressure of 1 atmosphere and typical room temperature, these collisions happen about once every picosecond (yes, once every 10^(-12) sec). It takes about, on average roughly 10 collisions to transfer the energy in the CO2 vibration to the air molecules it collides with. This energy goes into rotations and translations of the other molecules (rotational and translational energy interconvert very efficiently). This is how it increases the average kinetic energy of the gas sample = it heats it up.
Why doesn’t the vibrationally excited CO2 emit IR light? It can, but IR emission is pretty slow - it takes milliseconds, so this is not very likely to happen to a vibrationally excited molecule unless it is at pressures far below 1 atmosphere.
As for the other questions you asked, CO2 is only 0.03% of dry air, so why is it so much more important as a greenhouse gas than N2 (78% of dry air), O2 (21%) and Argon (1%)? N2, O2 and Ar don’t absorb IR light. Argon is simple - it’s an atom, so it doesn’t have any vibrations. N2 and O2 do have vibrations (one each), but these vibrations don’t absorb light in the IR. In order to absorb light in the IR, the dipole moment of a molecule has to change during the vibration. N2 and O2 don’t have a dipole moment. If you stretch or contract the bond (that’s what vibration does), the dipole moment remains zero.
So, CO2 is an important greenhouse gas because it is the most abundant molecule in dry air which absorbs light in the IR.
