Nitty Gritty Details about the Greenhouse Effect

[jeskill]

Hi People,

 

So, I feel like I don't really have a complete understanding of the greenhouse effect and I would like to understand it in a bit more depth. My questions for all you people well-versed in Chemistry and/or Physics are as follows:

 

How much of the greenhouse effect is caused by GHGs trapping and re-radiating ...

• light energy radiating from the sun?
• light energy reflected from the surface of the earth?
• or light energy that's been absorbed by the surface of the earth and re-radiated outward?
• am I missing anything?

When energy is absorbed by a CO2 molecule and re-radiated, how much energy is lost? Can energy that's been re-radiated by a carbon dioxide molecule be re-absorbed by another GHG molecule? If energy is being lost through every transaction, is there a point at which the energy is no longer able to be absorbed by the carbon dioxide molecule?

 

The reason I'm asking is that I have this very basic conceptual model in my head whereby light energy is radiated by the sun, passes through the atmosphere, is reflected (or absorbed/re-radiated) by the earth, then is absorbed/re-radiated by a GHG molecule, then is absorbed/re-radiated by the earth ..... kind of like a ping pong ball that loses energy after each bounce. I just want to know, is this at all accurate?

 

Cheers

Jes

[swansont]

For the overall numbers, the graph iNow posted is from 2007, but I don't know how much newer data might have changed it

http://www.scienceforums.net/topic/36089-global-warming/page__p__478791#entry478791

 

The forcings are of order 1 W/m^2 out of roughly 1350 W/m^2 that hits the top of the atmosphere.

 

Almost none of the effect is from trapping of light that is reflected, because the GHGs are pretty transparent at visible wavelengths. The effect comes from absorption and emission of blackbody radiation, which shifts the spectrum from visible to the IR, where the GHGs can absorb it. Changes in reflection have an effect because light that is not reflected is absorbed and then the energy can get trapped, so a lowering of the albedo (e.g. from a loss of ice, or increase in black carbon deposits) results in warming.

[jeskill]

For the overall numbers, the graph iNow posted is from 2007, but I don't know how much newer data might have changed it

http://www.sciencefo...791#entry478791

 

The forcings are of order 1 W/m^2 out of roughly 1350 W/m^2 that hits the top of the atmosphere.

 

Almost none of the effect is from trapping of light that is reflected, because the GHGs are pretty transparent at visible wavelengths. The effect comes from absorption and emission of blackbody radiation, which shifts the spectrum from visible to the IR, where the GHGs can absorb it. Changes in reflection have an effect because light that is not reflected is absorbed and then the energy can get trapped, so a lowering of the albedo (e.g. from a loss of ice, or increase in black carbon deposits) results in warming.

 

Thanks for the response. I understand your explanation. One more questions: can GHGs absorb a wide spectrum of IR, or just specific narrow bands?

[mississippichem]

Thanks for the response. I understand your explanation. One more questions: can GHGs absorb a wide spectrum of IR, or just specific narrow bands?

 

Generally speaking they absorb many frequencies of narrow bands. IR radiation is around the energy of molecular vibrations so molecules with more vibrational degrees of freedom (this depends on the molecular geometry) will have more modes of infrared absorption. Carbon dioxide is a linear molecule so will have fewer unique vibrational degrees of freedom than for example methane. To be observable, that vibrational degree of freedom must result in a net dipole moment change for the molecule.

[swansont]

CO2 absorbs at about 14 microns and 4 microns, with some width to those lines. Those are the important ones for the global warming discussion.

 

http://en.wikipedia.org/wiki/File:Atmospheric_Transmission.png

[Essay]

Hi People,

 

So, I feel like I don't really have a complete understanding of the greenhouse effect and I would like to understand it in a bit more depth. My questions for all you people well-versed in Chemistry and/or Physics are as follows:

 

How much of the greenhouse effect is caused by GHGs trapping and re-radiating ...

• light energy radiating from the sun?
• light energy reflected from the surface of the earth?
• or light energy that's been absorbed by the surface of the earth and re-radiated outward?
• am I missing anything?

When energy is absorbed by a CO2 molecule and re-radiated, how much energy is lost? Can energy that's been re-radiated by a carbon dioxide molecule be re-absorbed by another GHG molecule? If energy is being lost through every transaction, is there a point at which the energy is no longer able to be absorbed by the carbon dioxide molecule?

 

The reason I'm asking is that I have this very basic conceptual model in my head whereby light energy is radiated by the sun, passes through the atmosphere, is reflected (or absorbed/re-radiated) by the earth, then is absorbed/re-radiated by a GHG molecule, then is absorbed/re-radiated by the earth ..... kind of like a ping pong ball that loses energy after each bounce. I just want to know, is this at all accurate?

 

Cheers

Jes

 

 

You've got a good picture, I'd say; especially regarding the path of a "given photon" ping ponging.

(but I'm speaking from a biochemist's perspective)

 

It's something I've wondered about too. I know some basics and learned some additional stuff recently (and try to "figure out" the stuff inbetween). I don't think energy is actually "lost," but after a photon is absorbed it can "downshift" to lower frequencies when it is "regenerated" (if I can be imprecise with terminology), but only if some kinetic energy is co-generated to balance the loss in frequency. Plus this only works for certain permitted "balances" of kinetic and EM energy. ...or words to that effect?

I think the right combination of collision and absorption can even generate the emission of a photon of a higher wavelength, but that would be rare... if even possible.

Hey, I'd welcome any corrections on this (above & below).

 

...but, so from another perspective....

Absorbed frequencies activate different (permitted) vibrational modes; and occasionally thru collisions, some of the vibrational energy is transferred away. In that case, a lower energy photon may be (re)emitted. But if no vibrational transfer (energy loss) occurs, the absorbed photon would be re-emitted unchanged (not the "same" photon, but one of the same frequency)... and as you say it may be then re-absorbed by another GHG with the proper (resonant) structure... or travel on out to space, or travel back to the surface.

 

...and by "resonant structure" I am trying to describe the way molecules absorb EM radiation in quantized, discrete frequency bands (as opposed to being "invisible" to large ranges of non-absorbed frequencies).

 

...and again, I'll welcome any help with my lack of proper terminology (or QM/other concepts).

===

 

The vibrational modes of molecules are more easily understood, and GHG's are notably different from the main components of our atmosphere, nitrogen and oxygen.

 

O2 & N2 are diatomic (duh), but this means that they don't have many vibrational modes (and especially few modes which resonate with IR frequencies). Nor do they resonate with visible light either, or else our atmosphere would be rather opaque.

 

But GHG's are often triatomic (H2O, CO2, O3, N2O) or larger molecules such as CH4 and the CFC's.

 

These triatomic (and larger) molecules have many more modes of vibration, which happen to resonate with (absorb) certain IR frequencies. My professor gyrated and danced around to demonstrate the many extra vibrational modes of triatomic molecules, in his lecture on GHG's. But there is a .pdf that covers this stuff too, and gives some relative percentages for the incoming, re-radiated, and outgoing light/heat that you mentioned.

 

see:

 

http://ats150.atmos....nergyBudget.pdf

(page 1 & 2 especially, for percentages)

 

&

 

http://ats150.atmos....houseEffect.pdf

(page 3 for vibrations of triatomic GHG's, but all... overall)

 

===

 

~

[jeskill]

That's great Essay!

 

My 2nd year chemistry class material was popping up in my brain as I read what you wrote. It may not all be lost afterall . I think my problem is that I've been thinking of energy transfer within a molecule in the same way that I think of energy transfer through trophic levels. Which is silly, really, because they're completely different scales with completely different processes.

[JohnB]

Great pdfs Essay.

 

Jeskill, I would also suggest Kevin Trenberths paper on the Earths Energy Budget as a good read as to the current state of thinking. He puts numbers to what is going where which can help in the visualisation.

 

http://content.imamu.edu.sa/Scholars/it/net/trenbert.pdf

[AbeMichelson]

What you are describing is the Raman effect. 1 out of a million photons lose or gain energy due to a transfer of kinetic energy. The effect is used to get IR-style information (vibration of bonds) using visible light (laser). And guess what? Raman used the sun as his source in the original Nobel winning experiment in the 20s!