# Does atmospheric $CO_2$ heat up when or while or after it reradiates IR back towards Earth?

I get that $$CO_2$$ absorbs radiation at a frequency that is associated with the energy levels of atoms bumping around and the molecule rotating, and by electrons.

1) Maybe that is not thermal energy, because it is not of the translation of the whole molecule, which would be heat? So it does not get "hotter" before it reradiates?

2) Maybe a little of the IR absorbed into linear motion, or heat, but not much, so the CO2 remains approximately at the same temperature?

3) In summary, does the CO2 get hotter?

4) (In a discussion about Global Warming, I claimed that the CO2 is analogous to a mirror, and that heating up is a secondary effect, if any.)

5) Thank you for your answers. With them and my other reading I hope to clarify: From K. McClary I see that the CO2 could quickly reradiate the photon and so would have netted no gain in energy. But, assuming that you mean 10-9 milliseconds for an average time for a collision, which could transfer the energy to other rotation, vibration, and translation modes, more generally the energy of the original photon gets spread out in a local temperature increase of the nearby molecules. When that energy is radiation in random directions, half of it goes downward towards the Earth. That "one half" is the dominant effect for AGW. Then when the Earth's surface warms, it in turn warms the lower atmosphere.
If the radiation in/out of the CO2/air is balanced, the temperature of the air would not directly increase. Is it balanced, and if not, is that small imbalance unimportant?

• Heat is equivalent to energy, and energy doesn't have a temperature. Temperature is how concentrated the heat contained in a substance is, and it is defined as the average kinetic energy of a large number of atoms/molecules. In my opinion, it's not possible to answer your question as stated. Mar 28 '17 at 1:55

I don't quite get the point of your question. If it's regarding factors contributing to global warming, as suggested by your final parenthetical remark, then "$CO_2$ getting hotter" isn't relevant. Firstly, hotter or not, it almost immediately comes into thermal equilibrium with the rest of the atmosphere, so it's the temperature profile of the atmosphere that's relevant. Secondly, $CO_2$ is about 350 parts per million (a little more by weight), so it doesn't contribute much overall heat energy to the atmosphere. Thirdly, "mirror" is about half right, i.e., it's more like a half-coated mirror. The energy radiated by the earth that it absorbs was on its way "out", i.e., away from the earth. But after the $CO_2$ gets it, that radiation is re-radiated in pretty much all directions, so roughly half gets returned to earth. This is similar to the reason why Fraunhofer lines on the sun appear dark. (P.S. By the way, you might want to take out that quantum-electrodynamics tag)

• I think most of the absorbed radiation goes into heating the atmosphere, very little is "re-radiated", so the "half-coated mirror" analogy is not good. Mar 28 '17 at 3:48
• @KeithMcClary See, e.g., scied.ucar.edu/…
– user89220
Mar 28 '17 at 4:21
• "What happens is that the air mass containing the absorbing CO2 (including O2, N2, etc.) molecule warms due to the absorbed energy, and (statistially) some other GHG molecules will radiate at some frequency in their spectra in a matching amount, due to the changed temperature of the air mass. Not the same molecule in 999/1000 cases. " Is this the correct analysis? Mar 28 '17 at 5:11
• @KeithMcClary I don't know for sure, i.e., if it's correct or not. Your "skepticalscience" link argument sounds reasonable, but I'm not sure precisely how factual. For one thing, absorbed photons typically excite higher-energy vibrational/rotational states until re-emitted, not really part of the $\frac32kT$-energy that's typically exchanged during collisions. But like I said, I don't know all the details. In any event, I was only addressing the OP's much more elementary-level question, and I think we're now agreed that the remarks relevant to that level question are pretty much okay.
– user89220
Mar 28 '17 at 5:40

To understand something about CO2 and radiation try considering simple lab type experiments first.

Take a container of pure CO2, say 1 cubic metre, and place it in a vacuum. Using a back body source of radiation measure the temperature (with a thermistor) of the centre and boundaries of the cube of gas as you turn the radiator on and off. I claim that we will observe an increase in the temperature of the gas (at all points) when the radiator is on. Does anyone disagree ?

Now filter the spectral output of the black body radiator so that only those frequencies which CO2 absorbs are emitted. Again turn the radiator on and off. I claim that we will still observe an increase in the temperature of the gas (at all points) when the radiator is on. Does anyone disagree ?

I'll wait a bit to see if there are any disagreements first before I move on to the next real world experiment anyone can do, lets say a couple of days.

Over and out (for now !).

Wien's Law gives the central wavelength of radiation based on a body's temperature. Based on the Earth's temperature range (say zero to 50 degrees C) the corresponding peak wavelength is between 10.6 and 7 micrometres, which is within the band that CO2 has no absorption - hence, there is no heating effect bar some from the tail ends around the central wavelength peaks.