Guest Post by Ira Glickstein
This series began with a mechanical analogy for the Atmospheric “Greenhouse Effect” and progressed a bit more deeply into Atmospheric Windows and Emission Spectra. In this posting, we consider the interaction between air molecules, including Nitrogen (N2), Oxygen (O2), Water Vapor (H2O) and Carbon Dioxide (CO2), with Photons of various wavelengths. This may help us visualize how energy, in the form of Photons radiated by the Sun and the Surface of the Earth, is absorbed and re-emited by Atmospheric molecules. 
DESCRIPTION OF THE GRAPHIC
The animated graphic has eight frames, as indicated by the counter in the lower right corner. Molecules are symbolized by letter pairs or triplets and Photons by ovals and arrows. The view is of a small portion of the cloud-free Atmosphere. (Thanks to WUWT commenter davidmhoffer for some of the ideas incorporated in this graphic.)
- During the daytime, Solar energy enters the Atmosphere in the form of Photons at wavelengths from about 0.1μ (micron – millionth of a meter) to 4μ, which is called “shortwave” radiation and is represented as ~1/2μ and symbolized as orange ovals. Most of this energy gets a free pass through the cloud-free Atmosphere. It continues down to the Surface of the Earth where some is reflected back by light areas (not shown in the animation) and where most is absorbed and warms the Surface.
- Since Earth’s temperature is well above absolute zero, both day and night, the Surface radiates Photons in all directions with the energy distributed approximately according to a “blackbody” at a given temperature. This energy is in the form of Photons at wavelengths from about 4μ to 50μ, which is called “longwave” radiation and is represented as ~7μ, ~10μ, and ~15μ and symbolized as violet, light blue, and purple ovals, respectively. The primary “greenhouse” gases (GHG) are Water Vapor (H2O) and Carbon Dioxide (CO2). The ~7μ Photon is absorbed by an H2O molecule because Water Vapor has an absorption peak in that region, the ~10μ Photon gets a free pass because neither H2O nor CO2 absorb strongly in that region, and one of the 15μ Photons gets absorbed by an H2O molecule while the other gets absorbed by a CO2 molecule because these gases have absorption peaks in that region.
- The absorbed Photons raise the energy level of their respective molecules (symbolized by red outlines).
- The energized molecules re-emit the Photons in random directions, some upwards, some downwards, and some sideways. Some of the re-emitted Photons make their way out to Space and their energy is lost there, others back down to the Surface where their energy is absorbed, further heating the Earth, and others travel through the Atmosphere for a random distance until they encounter another GHG molecule.
- This frame and the next two illustrate another way Photons are emitted, namely due to collisions between energized GHG molecules and other air molecules. As in frame (2) the Surface radiates Photons in all directions and various wavelengths.
- The Photons cause the GHG molecules to become energized and they speed up and collide with other gas molecules, energizing them. NOTE: In a gas, the molecules are in constant motion, moving in random directions at different speeds, colliding and bouncing off one another, etc. Indeed the “temperature” of a gas is something like the average speed of the molecules. In this animation, the gas molecules are fixed in position because it would be too confusing if they were all shown moving and because the speed of the Photons is so much greater than the speed of the molecules that they hardly move in the time indicated.
- The energized air molecules emit radiation at various wavelengths and in random directions, some upwards, some downwards, and some sideways. Some of the re-emitted Photons make their way out to Space and their energy is lost there, others back down to the Surface where their energy is absorbed, further heating the Earth, and others travel through the Atmosphere for a random distance until they encounter another GHG molecule.
- Having emitted the energy, the molecules cool down.
DISCUSSION
As in the other postings in this series, only radiation effects are considered because they are the key to understanding the Atmospheric “Greenhouse Effect”. I recognize that other effects are as important, and perhaps more so, in the overall heat balance of the Earth. These include clouds which reflect much of the Sun’s radiation back out to Space, and which, due to negative feedback, counteract Global Warming. Other effects include convection (wind, thunderstorms, …), precipitation (rain, snow) and conduction that are responsible for transferring energy from the Surface to the Atmosphere. It is also important to note that the Atmospheric “Greenhouse Effect” and a physical greenhouse are similar in that they both limit the rate of thermal energy flowing out of the system, but the mechanisms by which heat is retained are different. A greenhouse works primarily by preventing absorbed heat from leaving the structure through convection, i.e. sensible heat transport. The greenhouse effect heats the earth because greenhouse gases absorb outgoing radiative energy and re-emit some of it back towards earth.
That said, how does this visualization help us understand the issue of “CO2 sensitivity” which is the additional warming of the Earth Surface due to an increase in atmospheric CO2? Well, given a greater density of CO2 (and H2O) molecules in the air, there is a greater chance that a given photon will get absorbed. Stated differently, a given photon will travel a shorter distance, on average, before being absorbed by a GHG molecule and be re-emitted in a random direction, including downwards towards the Surface. That will result in more energy being recycled back to the Surface, increasing average temperatures a bit.
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I like that animation. I have never been very good at that. 🙂
It is worth noting that gases can gain energy from collisions as well as by absorbing a photon. A molecule doesn’t care where it got the energy from. Once a molecule has more energy than the molecules around it, it will dissipate through releasing a photon or it could also lose it by another collision. Each molecule is having billions of such collisions per second under normal atmospheric conditions.
You are correct in keeping it simple as the more complex the animation is, the harder it is to follow.
Convection currents are the “great equalizer”…. I have a question I’m hoping someone can answer – I thought Al Gore got his graph backwards and the increase of CO2 followed warming – why are we still working with the assumption it creates it???
The Ideal Gas Law PV=nRT quite accurately predicts the atmospheric temperature of Earth, Mars, Venus and Titan (at all altitudes) based on a simple pressure effect. Why add the unnecessary complication of the highly dubious Greenhouse Effect to an elegant and simple solution?
Can we have one with the number of different GHG molecules represented proportionally for reference? We might not be able to see the CO2 one among all the others, though….
Don’t know how I missed this, but here’s a catchy Harrington Bro’s tune for part of the soundtrack: http://www.youtube.com/watch?v=9Ts4WxMHw4E&feature=related
Much of Newfoundland (“The Rock”) is at higher Lat than Minnesota. Though not as biting as “Hide the Decline,” tune-wise the M4GW guys have some professional competition…
Let us use the back radiation according to the global warming science theory to warm our houses without burning fossil fuels.
It is simple: paint a black square on your ceiling.
Say the black paint covers 10% of the ceiling and let us compare the backradiation of the black square to the back radiation of the Co2 molecules in the air .
Let us assume that the Co2 molecules in the airabove the ceiling are all present next to each other on the ceiling and radiating all the energy back to the floor . The backradiation of the Co2 molecules on the ceiling would be much stronger than the the real situation in the air.
According to the global warming theory doubling the Co2 concentration would increase the temperature with one degree
Instead of doubling the Co2 concentration we have a painted a black square.
The surface of the black square is 250 times bigger than the Co2 surface
The black paint will absorb and emit the whole infrared band from the floor
whilst the Co2 surface will only backradiate from a narrow band in the spectrum.
Because of its size the black square we can compare this to a (at least) 250 times increase in Co2 concentration.
In other words we can expect an enormous increase in temperature in the room without using energy. Be carefull don’t paint the whole ceiling black !!!.
Ira,
I think a review of classical thermodynamics is in order here.
You can not win
You must lose
You can not get out of the game.
Very interesting discussion thread. I appreciate the simplification and the limitation of the argument, since this basic physical concept seems to confuse the waters in more cases than not.
In the meantime, I’ve been pondering the question of atmospheric absorption of energy lately, and maybe you could pull out some interesting figures for the audience. Do you happen to have in your files the energy dependent photon absorption curves for various important gases in the air (i.e. N2, O2, H2O vapor, CO2, and why not Ar, H2, He, CH4, CO, and N2O)? I imagine the non-asymmetric molecules will not be very opaque, but it’s still a nagging question in my mind.
Also, for the sake of education it my be instructive to show what the blackbody radiation spectra of a surface at, say 240K (Arctic), 285K (a normal summer night), and 310K (a hot day) look like.
Thanks for the good work.
Although there clearly is a greenhouse effect it is infinitesimal compared to the energy retaining effect of the oceans.
Furthermore the non radiative processes seem to mostly or entirely negate its effect with the real reason for the Earth’s temperature being down to oceanic and not atmospheric energy retention.
Really, we should regard the oceans as part of the atmosphere for energy budget purposes.
The recent suggestion that removing CO2 would cause a snowball Earth cannot be right because there would still be solar shortwave into the oceans keeping them liquid and so a continuing water cycle.
As far as I can see a removal of CO2 would just mean a little less energy in the atmosphere, lower the height of the tropopause a little for a shallower troposphere and redistribute the pressure patterns for a slightly slower water cycle for no significant temperature effect.
The cooling effect of less CO2 would be all or mostly offset by the slowing of the water cycle just as the warming effect of more CO2 seems to be all or mostly offset by a speeding up of the water cycle.
Sounds all very well BUT.
This explanation completely ignores convection which is probably the most important heat distribution system. When this LW radiation adds energy to a GHG, ie warms up a CO2 molecule, it will immediately transfer this energy to molecules of lower energy levels, ie warm up the surrounding gasses. This is as per the 2nd law which states, through inference, that heat cannot be stored.
When a parcel of gas becomes warmer than that surrounding it will convect due to density difference and rising air cools adiabatically. This cooling air will be warmer than that surrounding to continue convecting but cooler than the surface so energy will not be radiated to the surface because the 2nd law of thermodynamics forbids this to happen, energy only able to flow from a high to lower level.
If it were possible to get energy to flow from cold to hot then we would have all our energy requirements solved. We would have a perpetual motion machine. This is impossible.
So the above explanation may work in the laboratory but the atmosphere is a different place. If convection were not a major player in heat transport then we would see far less cloud than we do, most cloud being due to convection.
When we look at geological history there were periods when atmospheric CO2 levels were many times today’s. There were ice ages and periods of warmth both independent of the atmospheric CO2 levels. Present day research shows that CO2 levels rise after rises in temperature which goes a long way to prove that CO2 does not drive temperature.
bananabender. Go ahead, try to explain the temperature of the Earth using only the ideal gas law! It can’t be done, you see, if you actually start to think about what you do.
“The Photons cause the GHG molecules to become energized and they speed up and collide with other gas molecules”
I’m not really convinced about that, or better it could be not always true. In the particular case of the CO2 absorption band at 15um, as far I know, the energy of the photon is converted into molecular bending which just polarize the molecule. From my point of view, this shouldn’t increase the speed at all, but it should increase the probability of share energy with the surrounding molecules because the polarized molecule needs only two simultaneous collisions to share energy, while the unpolarized one needs three simultaneous collisions, one in one direction applied to the C atom and the other two applied in the opposite direction, or at least applied having angles on the O atoms which can bend the aligned O=C=O chain. If I’m right here, the CO2 molecules should be better as “photons absorbers” than “photon emitters”. So, it should have very little efficiency as “optical photon spreader”, but it should be a very efficient “thermodynamic photon sharer”.
Ira
Sorry to poop the party but over at climaterealists http://climaterealists.com/index.php?id=7457
there is an excellent proof using the physics of black body spectrum which disproves the greenhouse theory conclusively.
So a single CO2 molecule can both absorb photon energy and then re emit this energy in random directions? Given the fact that atmospheric CO2 only amounts to 0.039% of the atmosphere the net amount of re re radiated photons hitting the earth must be truly tiny, so small in fact as to be barely measurable.
“The greenhouse effect heats the earth because greenhouse gases absorb outgoing radiative energy and re-emit some of it back towards earth.”
The problem I have with this explanation is that the amount of re radiated energy directed downwards by such a trace gas as CO2 would be infinitesimally small and it is not constant and because it is not constant and the amount is so small this heating would be lost almost immediately within the far greater ‘noise’ signal of the other atmospheric gases which exist in far far greater amounts.
The obvious question is one of proportions and percentages and given that atmospheric CO2 is present in such trace amounts just how much is CO2 heating the earth by? And given that global temperatures have not increased in the last decade and most of the 20th centurys small net warming occurred BEFORE the steep rise in atmospheric CO2 then I have strong reservations about CO2 being responsible for any measurable warming whatsoever.
Here is the problem as I see it, the rise in atmospheric CO2 and the rise in global temperatures should be linked together and yet they are not, we have seen a fall in global temperatures even as CO2 increased and if CO2 was the driver of increasing temperatures we would have seen a greater increase in global temperatures at the same time as increasing CO2 levels yet the opposite has happened. CO2 rose quickly post war while global temperatures rose much more slowly post war and even as CO2 has been rising since 1998 global temperatures have not measurably increased at all.
Careful – you’ll get Louis, Richard and the “Violation of the 2nd Law of Thermodynamics” crew on to you with outrageous statements like:
“The greenhouse effect heats the earth because greenhouse gases absorb outgoing radiative energy and re-emit some of it back towards earth.”
I am happy with this description overall but I am not sure about this part:
“The Photons cause the GHG molecules to become energized and they speed up and collide with other gas molecules, energizing them”
It is a long time since I studied absorption and scattering but this is how I see it.
Photons interact with electrons in molecules. Photons with energies which excite any of the vibrational or rotational modes of the molecule will be absorbed, other wavelengths will generally not be absorbed. There are some inelastic scattering modes which result in photon absorbtion and re-emission at slightly different wavelengths but I do not think that this band broadening is what you are describing.
A CO2 molecule colliding with, say, an N2 molecule, will pass on energy as well as receive energy during that collision. However the CO2 molecule has vibrational and rotational modes in the long wave energy band but N2 does not so any energy passed to the N2 molecule will not be radiated away whilst energy absorbed by the CO2 molecule might well be. On balance however I do not see this as being significant overall since the nett effect of the collisions should be small.
Was it not you who showed the radiation plot looking up from the earth at night? This showed that there is virtually no atmospheric radiation outside of the absorption bands of the greenhouse gases. I do not understand why you feel a need to include these small scattering effects in your description while you exlude major effects like convection which change the altitude and therefore amount of radiation to space far more significantly.
If all that is going on is absorption and re-emitting radiation there would be no heating of the atmosphere. Heating happens because of the re-distribution of energy through collisions between molecules. The “simple” visualization is thus too simple. The temperature of a gas IS the average kinetic energy of the gas molecules, this is something Roger Pielke Sr. has pointed out a number of times and for many years.
Stephen Richards – that link that “disproves” theory of AGW – well for mine it is based on an incorrect understanding of the 2nd law and how it relates to warming. I see you beat me to my post at 2:06 though – damn moderation lag:)
Jer0me says:
March 29, 2011 at 12:43 am
Can we have one with the number of different GHG molecules represented proportionally for reference? We might not be able to see the CO2 one among all the others, though….
Like this one?
http://i55.tinypic.com/a76tw.png
( In proportion: N2-green; O2-gray; Ar-brown; H2O-aqua (2.5%); CO2-red (400 ppmv) )
What a lot of nonsense. Here is how it really works http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html
“The atmosphere radiates because it has a finite temperature, not because it received radiation”
Further to cal’s points, I don’t understand how CO2 and H2O are able to re-radiate in the way described.
We are told by the AGW crowd that much of the expected (and so far undetected) warming occurs in the middle atmosphere, where the air temps are pretty low (-10C to -30C, up where the planes live).
If a warm surface emits a photon that is captured at this height, and conduction results in almost instant passing on of the captured energy to surrounding gasses (primarily N2) then how do the H2O and CO2 molecules regain enough energy from the comparatively cold gasses to re-emit photons of similar energy?
Surely they can only re-emit much lower energy photons, of similar energy levels to the black-body curve at -20C.
So far the best write up.
However, there is no mention of how much the re-emitted photon helps to warm the earth, and it is only a conjecture that it really does. For does it not hold true that if the re-emitted direction is random above a sphere the, in this case outer, space around it is the most likely direction?
How much of this is theory, and how much observed, in the real atmosphere? Can we see it happening? Can we devise an experiment or observation which would work in the field? Personally, I’ve been struggling with radiative theory, I’ve seen stuff from all sides, and plainly theorists cannot agree. So what do we KNOW?
Hank: thanks for your example. Although I am not a physicist, I thought that at the end in step 7 by Glickstein a little miracle occurred. If we have a perpetuum mobile working a tiny tiny bit, we can use that for heating our homes at least. Perhaps the next winter I will do your painting trick.