By raypierre , with the gratefully acknowledged assistance of Spencer Weart
In Part I the long struggle to get beyond the fallacious saturation argument was recounted in historical terms. In Part II, I will provide a more detailed analysis for the reader interested in the technical nitty-gritty of how the absorption of infrared really depends on CO2 concentration. At the end, I will discuss Herr Koch’s experiment in the light of modern observations.
The discussion here is based on CO2 absorption data found in the HITRAN spectroscopic archive. This is the main infrared database used by atmospheric radiation modellers. This database is a legacy of the military work on infrared described in Part I , and descends from a spectroscopic archive compiled by the Air Force Geophysics Laboratory at Hanscom Field, MA (referred to in some early editions of radiative transfer textbooks as the "AFGL Tape").
Suppose we were to sit at sea level and shine an infrared flashlight with an output of one Watt upward into the sky. If all the light from the beam were then collected by an orbiting astronaut with a sufficiently large lens, what fraction of a Watt would that be? The question of saturation amounts to the following question: How would that fraction change if we increased the amount of CO2 in the atmosphere? Saturation refers to the condition where increasing the amount of CO2 fails to increase the absorption, because the CO2 was already absorbing essentially all there is to absorb at the wavelengths where it absorbs at all. Think of a conveyor belt with red, blue and green M&M candies going past. You have one fussy child sitting at the belt who only eats red M&M’s, and he can eat them fast enough to eat half of the M&M’s going past him. Thus, he reduces the M&M flux by half. If you put another equally fussy kid next to him who can eat at the same rate, she’ll eat all the remaining red M&M’s. Then, if you put a third kid in the line, it won’t result in any further decrease in the M&M flux, because all the M&M’s that they like to eat are already gone. (It will probably result in howls of disappointment, though!) You’d need an eater of green or blue M&M’s to make further reductions in the flux.
Ångström and his followers believed that the situation with CO2 and infrared was like the situation with the red M&M’s. To understand how wrong they were, we need to look at modern measurements of the rate of absorption of infrared light by CO2 . The rate of absorption is a very intricately varying function of the wavelength of the light. At any given wavelength, the amount of light surviving goes down like the exponential of the number of molecules of CO2 encountered by the beam of light. The rate of exponential decay is the absorption factor.
When the product of the absorption factor times the amount of CO2 encountered equals one, then the amount of light is reduced by a factor of 1/e, i.e. 1/2.71282… . For this, or larger, amounts of CO2,the atmosphere is optically thick at the corresponding wavelength. If you double the amount of CO2, you reduce the proportion of surviving light by an additional factor of 1/e, reducing the proportion surviving to about a tenth; if you instead halve the amount of CO2, the proportion surviving is the reciprocal of the square root of e , or about 60% , and the atmosphere is optically thin. Precisely where we draw the line between "thick" and "thin" is somewhat arbitrary, given that the absorption shades smoothly from small values to large values as the product of absorption factor with amount of CO2 increases.
The units of absorption factor depend on the units we use to measure the amount of CO2 in the column of the atmosphere encountered by the beam of light. Let’s measure our units relative to the amount of CO2 in an atmospheric column of base one square meter, present when the concentration of CO2 is 300 parts per million (about the pre-industrial value). In such units, an atmosphere with the present amount of CO2 is optically thick where the absorption coefficient is one or greater, and optically thin where the absorption coefficient is less than one. If we double the amount of CO2 in the atmosphere, then the absorption coefficient only needs to be 1/2 or greater in order to make the atmosphere optically thick.
The absorption factor, so defined, is given in the following figure, based on the thousands of measurements in the HITRAN spectroscopic archive. The "fuzz" on this graph is because the absorption actually takes the form of thousands of closely spaced partially overlapping spikes. If one were to zoom in on a very small portion of the wavelength axis, one would see the fuzz resolve into discrete spikes, like the pickets on a fence. At the coarse resolution of the graph, one only sees a dark band marking out the maximum and minimum values swept out by the spike. These absorption results were computed for typical laboratory conditions, at sea level pressure and a temperature of 20 Celsius. At lower pressures, the peaks of the spikes get higher and the valleys between them get deeper, leading to a broader "fuzzy band" on absorption curves like that shown below.
We see that for the pre-industrial CO2 concentration, it is only the wavelength range between about 13.5 and 17 microns (millionths of a meter) that can be considered to be saturated. Within this range, it is indeed true that adding more CO2 would not significantly increase the amount of absorption. All the red M&M’s are already eaten. But waiting in the wings, outside this wavelength region, there’s more goodies to be had. In fact, noting that the graph is on a logarithmic axis, the atmosphere still wouldn’t be saturated even if we increased the CO2 to ten thousand times the present level. What happens to the absorption if we quadruple the amount of CO2? That story is told in the next graph:
The horizontal blue lines give the threshold CO2 needed to make the atmosphere optically thick at 1x the preindustrial CO2 level and 4x that level. Quadrupling the CO2 makes the portions of the spectrum in the yellow bands optically thick, essentially adding new absorption there and reducing the transmission of infrared through the layer. One can relate this increase in the width of the optically thick region to the "thinning and cooling" argument determining infrared loss to space as follows. Roughly speaking, in the part of the spectrum where the atmosphere is optically thick, the radiation to space occurs at the temperature of the high, cold parts of the atmosphere. That’s practically zero compared to the radiation flux at temperatures comparable to the surface temperature; in the part of the spectrum which is optically thin, the planet radiates at near the surface temperature. Increasing CO2 then increases the width of the spectral region where the atmosphere is optically thick, which replaces more of the high-intensity surface radiation with low-intensity upper-atmosphere radiation, and thus reduces the rate of radiation loss to space.
Now let’s use the absorption properties described above to determine what we’d see in a typical laboratory experiment. Imagine that our experimenter fills a tube with pure CO2 at a pressure of one atmosphere and a temperature of 20C. She then shines a beam of infrared light in one end of the tube. To keep things simple, let’s assume that the beam of light has uniform intensity at all wavelengths shown in the absorption graph. She then measures the amount of light coming out the other end of the tube, and divides it by the amount of light being shone in. The ratio is the transmission. How does the transmission change as we make the tube longer?
To put the results in perspective, it is useful to keep in mind that at a CO2 concentration of 300ppm, the amount of CO2 in a column of the Earth’s atmosphere having cross section area equal to that of the tube is equal to the amount of CO2 in a tube of pure CO2 of length 2.5 meters, if the tube is at sea level pressure and a temperature of 20C. Thus a two and a half meter tube of pure CO2 in lab conditions is, loosely speaking, like "one atmosphere" of greenhouse effect. The following graph shows how the proportion of light transmitted through the tube goes down as the tube is made longer.
The transmission decays extremely rapidly for short tubes (under a centimeter or so), because when light first encounters CO2, it’s the easy pickings near the peak of the absorption spectrum that are eaten up first. At larger tube lengths, because of shape of the curve of absorption vs. wavelength, the transmission decreases rather slowly with the amount of CO2. And it’s a good thing it does. You can show that if the transmission decayed exponentially, as it would if the absorption factor were independent of wavelength, then doubling CO2 would warm the Earth by about 50 degrees C instead of 2 to 4 degrees (which is plenty bad enough, once you factor in that warming is greater over land vs. ocean and at high Northern latitudes).
There are a few finer points we need to take into account in order to relate this experiment to the absorption by CO2 in the actual atmosphere. The first is the effect of pressure broadening. Because absorption lines become narrower as pressure goes down, and because more of the spectrum is "between" lines rather than "on" line centers, the absorption coefficient on the whole tends to go down linearly with pressure. Therefore, by computing (or measuring) the absorption at sea level pressure, we are overestimating the absorption of the CO2 actually in place in the higher, lower-pressure parts of the atmosphere. It turns out that when this is properly taken into account, you have to reduce the column length at sea level pressure by a factor of 2 to have the equivalent absorption effect of the same amount of CO2 in the real atmosphere. Thus, you’d measure absorption in a 1.25 meter column in the laboratory to get something more representative of the real atmosphere. The second effect comes from the fact that CO2 colliding with itself in a tube of pure CO2 broadens the lines about 30% more than does CO2 colliding with N2 or O2 in air, which results in an additional slight overestimate of the absorption in the laboratory experiment. Neither of these effects would significantly affect the impression of saturation obtained in a laboratory experiment, though. CO2 is not much less saturated for a 1 meter column than it is for a 2.5 meter column.
So what went wrong in the experiment of poor Herr Koch? There are two changes that need to be made in order to bring our calculations in line with Herr Koch’s experimental setup. First, he used a blackbody at 100C (basically, a pot of boiling water) as the source for his infrared radiation, and measured the transmission relative to the full blackbody emission of the source. By suitably weighting the incoming radiation, it is a simple matter to recompute the transmission through a tube in a way compatible to Koch’s definition. The second difference is that Herr Koch didn’t actually perform his experiment by varying the length of the tube. He did the control case at a pressure of 1 atmosphere in a tube of length 30cm. His reduced-CO2 case was not done with a shorter tube, but rather by keeping the same tube and reducing the pressure to 2/3 atmosphere (666mb, or 520 mm of Mercury in his units). Rather than displaying the absorption as a function of pressure, we have used modern results on pressure scaling to rephrase Herr Koch’s measurement in terms of what he would have seen if he had done the experiment with a shortened tube instead. This allows us to plot his experiment on a graph of transmission vs. tube length similar to what was shown above. The result is shown here:
Over the range of CO2 amounts covered in the experiment, one doesn’t actually expect much variation in the absorption — only about a percent. Herr Koch’s measurements are very close to the correct absorption for the 30cm control case, but he told his boss that the radiation that got through at lower pressure increased by no more than 0.4%. Well, he wouldn’t be the only lab assistant who was over-optimistic in reporting his accuracy. Even if the experiment had been done accurately, it’s unclear whether the investigators would have considered the one percent change in transmission "significant," since they already regarded their measured half percent change as "insignificant."
It seems that Ångström was all too eager to conclude that CO2 absorption was saturated based on the "insignificance" of the change, whereas the real problem was that they were looking at changes over a far too small range of CO2 amounts. If Koch and Ångström had examined the changes over the range between a 10cm and 1 meter tube, they probably would have been able to determine the correct law for increase of absorption with amount, despite the primitive instruments available at the time.
It’s worth noting that Ångström’s erroneous conclusion regarding saturation did not arise from his failure to understand how pressure affects absorption lines. That would at least have been forgivable, since the phenomenon of pressure broadening was not to be discovered for many years to come. In reality, though Ångström would have come to the same erroneous conclusion even if the experiment had been done with the same amounts of CO2 at low pressure rather than at near-sea-level pressures. A calculation like that done above shows that, using the same amounts of CO2 in the high vs. low CO2 cases as in the original experiment, the magnitude of the absorption change the investigators were trying to measure is almost exactly the same — about 1 percent — regardless of whether the experiment is carried out at near 1000mb (sea level pressure) or near 100mb (the pressure about 16 km up in the atmosphere).
Hank, re your 515 referring to Alastair’s 222, etc. Maybe this got settled and I missed it, but I’d like to review my understandings and ask if everyone or anyone agrees.
1) So-called blackbody radiation (that which follows Planck’s function) is fundamentally different from gas “line spectrum” emission, though both are affected by quantum stuff. One derives from heat creating temperature and getting applied to (mostly) near-free electrons. The roaming and accelerating electrons are responsible for blackbody radiation and follow some distribution (other than Boltzman, maybe?) that does emit a continuous spectrum, for all practical purposes. Gas radiation stems predominately from the quanta energy levels of atomic vibration within a molecule and molecular rotation. This does not produce a continuous spectrum but discrete wavelength emission (though there are certain limited bandwidths where it looks close from the packing of the lines and spreading.) There is some gas emission/absorption at higher frequencies that relate to electronic/ionization energy levels, but this too is discrete.
2) Gasses can emit some so-called blackbody radiation in addition to the “line emission” described above.
3) Equipartition is the tendency of a molecule to equitably distribute and transfer its carried energy among translation, vibration and rotation levels. This is one way that vibration, say, can lose or gain energy. Another is (maybe indirectly) via molecular collisions. Another is IR, mostly, radiation emission and absorption. I say “tendency” because equipartition is a little loosey, has some quanta factors and follows distributions that, in part, are a function of temperature, so is somewhat quantized and non-linear.
4) I would think that a cold CO2 molecule is more likely to (re)emit from vibration, and maybe even rotation to some extent, because equipartition’s dependence on temperature makes it easier for a hot molecule to keep its vibration levels filled.
Hank,
I described one way of testing by using an infrared thermometer from a balloon, but you seem more interested in disproving my ideas than understanding them :-(
Assuming that you are living in sunny California, (you will have to adapt the following experiment if you live elsewhere) park you automobile in the middle of a parking at midday and place a thermometer inside it in a shaded spot. Then with the windows closed and uncovered, wait for two hours and read the thermometer.
Repeat the experiment the following day, but this time leave each window open with a gap of 1 cm (half an inch.) This should make little difference to the amount of glass surrounding the car. If it is true that it is the glass reflecting the infrared radiation back into the car which causes the warming, then both runs of the experiment should give the same temperature reading.
In fact it is the air that absorbs the radiation, and by allowing the hot air to escape the amount of heat retained within the car is greatly reduced, and so also is the temperature reading.
Douglas Wise,
OK, here’s my take on your points and questions–and remember, I’m just a dumb physicist, not and expert on atmospheres. OK, first, the 15 micron absorption line for CO2 is rather broad, and its shape is influenced by the surrounding gas. Also, the lifetime of the vibrational state is quite long, so as long as gas density is at least moderate, the chances of relaxation by collision or other de-excitation process is fairly high. A re-radiated photon can excite another CO2 vibrational state, but there aren’t that many re-radiated photons. Think of it this way: A CO2 molecule in an excited vibrational state has a higher energy than the vast majority of its neighbors. Since it is colliding with and interacting with those neighbors, energy must flow from it (and from the vibrational mode) into its neighbors translational motions. If the gas were hotter, more energy would flow into the vibrational mode from the collisions.
1) As CO2 concentrations increase, the wings of the CO2 absorption line get broader, so, yes the window does get a very small bit narrower.
2) The deviations from a “blackbody” (or really gray-body) radiation curve are largely due to absorption by ghgs of outgoing LWR.
3)Heat transported by convection gets up to the upper troposphere and heats the air there. The warmed air collides, exciting rotational and a few vibrational states. These high-altitude molecules then radiate in the bands where they can radiate. OK, now think of yourself on the space station looking down measuring the light coming from Earth. The light you see in the “windows” comes mostly right from the warm surface, but the LWIR comes from above the cloudtops for the wavelengths corresponding to water absorption features, and for CO2, from even higher than that. And because the upper troposphere is cold, there are “holes” in the spectrum compared to what we’d expect if all the radiation came from the warmer surface. Those holes represent energy, which has gone into heating the atmosphere and ultimately the surface.
4)There’s not that much intensity for sunlight in long wavelengths.
Make sense?
re 546, 539: AEBanner (546) says, “…the process you described depends upon the absorption cross sections of the molecule for photons of different energies, and so is a matter of probability. The absorption lines adjacent to the 14.99 micron line for CO2 are somewhere between one and two orders down, if I remember correctly. So the chance of the transition happening twice ( in any “chain” ) would be between a factor of 10^2 and 10^4 down.”
I agree with the theory but question your numbers (though not smart enough to explicitly re-calculate). The chance of molecule 2 absorbing is less after molecule 1 absorbed at the same discrete frequency because the energy “pool” in that frequency is lessened — for a while. This is just following standard absorption/optical depth theory. It is not lessened just because molecule 1 re-emitted a photon; molecule 2 sees only a “pool” of photons and has no way of knowing their origin, nor does it have a way of selecting a photon emitted from the surface over an exact replica that happened to come from another similar molecule. Your probabilities appear too similar to the repeated coin flipping example.
In any event (whichever probability is correct), this does not say that molecule 2 CAN NOT absorb a photon emitted from molecule 1 of the same species, which others are contending.
AEBanner,
I figure you might like this. Here are images composed from data showing infrared emissions from CO2 in different parts of the atmosphere – plus a few additional links for finding more…
From #322
The first figure on this page (figure 23) shows the observation of CO2 reemission in the 15 μm from the mid-troposphere as observed by NIMBUS-4 IRIS (graph from a textbook published in 1976)
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From #323
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#325
The following page has java animations of satellite data for thermal emissions from various gases, including co2 at 14.1, 13.4 4.45 μm bands in the upper- and lower-level atmosphere temperatures at various times of day.
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From #323
Measurements from Mars with its lower temperatures and pressures:
http://starryskies.com/solar_system/mars/spirit/atmosphere01.html
The CIMSS Realtime GOES Page
http://cimss.ssec.wisc.edu/goes/realtime/grtmain.html
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#325
Here the elevation of land being determined by co2 absorption strength:
From #343, #352
Hank Roberts provided the following in #533
I have provided the following in #384 and #509:
Chuck Booth provided this on a different thread:
… and in response I gave him the following:
Radiation & Climate: Major Projects
Line-by-line calculation of atmospheric fluxes and cooling rates 2
http://www.aer.com/scienceResearch/rc/m-proj/abstracts/rc.clrt2.html
Timothy, I said (541), “…The fact that you believe very strongly in what you profess and even feel you have solid evidence to back up your assertions does not warrant a scientific forum casting out or shunning all who don’t readily subscribe to your dogma…”
Frankly I think your rebuttal post (542) says pretty much the same thing, though not exactly, and with considerably more erudition! You say you’re saving lives while Alastair is asking dumb questions and (implied) ought to be canned.
No, I do not approve attacks on integrity. But claiming someone is wrong in his analysis, even if “him” is an eminent scientist, is not an ad hom attack on integrity.
History is filled with scientific (and other) advances stemming from folks of a different field. Questioning someone smarter than me, per se, does not make me a crackpot, even if I don’t have a union card. (That would mean I can maybe only question 2% of the population!!)
And you conclude by saying, in effect, you have no desire to jettison Alastair (or whoever)… as soon as he agrees with me.
I don’t have “a problem” with the attitude. I simply think it inappropriate and unhelpful.
PS to #355
The last is a line-by-line calculation the net local direct cooling or heating effect of various greenhouse gases. Not quite the same thing as the thermal images/data, but I thought that I would include it.
Douglas (545), these are good fundemental questions and comments. I’m going to offer my 2 cents worth, though my credentials are likely less than almost everyone else.
Absolutely. This is the primordial concept. The mother that gets it all going.
True, though there are different estimates between 10-20% with 10-12% being most likely
re 3) GHG radiation is isotropic. I do not believe re-emission has to be at a longer wavelength, though it is more likely. [Others will disagree] I do believe CO2 absolutely can absorb a photon (re)emitted by another CO2 molecule, though the re-emission and 2nd absorption is likely at a different wavelength than the 1st absorption. See other posts. [Others disagree]
This is right, though one or two folks disagree and are part of an ongoing set-to. Down-welling IR absorption is 140% of the earth surface plus atmospheric solar absorption.
5), 6), and 7) are all accurate
I believe not except on a limited and insignificant scale at the edges due to line spreading. Spreading won’t come close to closing the window with a relatively greatly larger bandwidth. Some here might disagree. (Actually, I don’t fully buy the spreading concept, but that’s my burden and you should ignore it.)
I think the gaps clearly stem from the radiation that got absorbed by whatever and not re-emitted (or 1st emitted in some cases) as it tried to get to the top. I don’t think their is any inherent qualitative distinction between greenhouse gasses. Others more knowledgeable might disagree.
I have no credible comment to statements 3) and 4).
Rod B. (#556) wrote:
Rod, check out #555.
You can see how many links we have provided to data showing the reemission of radiation from carbon dioxide at many places in the atmosphere. Actually a few are from water vapor, but not that many – and there were more links. I have also pointed out that we have plenty of data from other sources – most particularly that from planes.
The first time I remember Alastair having acknowledged just one of the links was #532, but even then he didn’t acknowledge that it was thermal emissions from carbon dioxide. And that was only after I pushed hard. The first time he actually acknowledged that they were thermal emissions was #537.
As I said, Hank Roberts (#530) gave us a fairly good quote just a little while back:
“As Patrick Moynihan once said, everyone is entitled to their own beliefs, but not to their own facts.”
We have plenty of data. Not talking theory. Not talking about anyone having to accept a given theory. Just the data. Just the facts. Don’t just take my conclusions because I say so, but at least accept the facts, the data, because it is there.
Rod B (#556) wrote:
This was the most recent attack on Raypierre’s integrity (#505):
But there have also been the wild goose chases where we have been given links that have nothing to do with what he is trying to prove, quotes from material taken out of context which say something quite different from what he is trying to show, the extreme stubbornness with regard to his positions, the recycling of arguments long after they have been addressed multiple times, the rudeness, etc.. I had actually been wondering about his state of mind.
Not any longer.
It is almost as if you two have decided to play “good cop, bad cop.” He takes no offense at all, is perfectly polite and extremely helpful:
Alastair (#548) wrote:
Thank you for the concern, Alastair. It is appreciated.
Alastair McDonald (#550) wrote:
That is an extremely helpful link, Alastair! Makes a great deal of sense, too, given my understanding of quantum mechanics.
You see, Rod?
You take all the offense, he takes none at all. And for the longest time you have been feeding him questions so that he could go on and on about his shifting, incoherent theory. All of the sudden he goes from incoherent and offensive to hyper-polite and extremely lucid.
I am thinking mindgames. Gaslight. Hank suspected as much at various points as well, at least with respect to Alastair.
Seeing something like this before plenty of times among creationists who tried to act extremely dumb just to see how hard the pro-evolution group would work to respond to them. Their way of demonstrating how much brighter they were than the supposedly well-educated pro-evolution people.
Here I think the purpose is different. Haven’t really got a clue. But I can at least see the pattern. Fairly sophisticated, actually.
Re #454 where Douglas Wise Says:
Gentlemen,
I am not sure if this is also addressed to me since it seems that I am regarded as a non-person (N-P perhaps ) here, but it is an interesting post so here is my 2 cents worth.
Statements:
1) At the top of the atmosphere, their is escape of long wave radiation which, at equilibrium, matches in energy terms that entering by solar radiation less albedo.
If more energy enters the atmosphere than leaves it, then it will warm and vice versa. At the top of the atmosphere the only way energy can enter or leave is by radiation, but the balance need not be maintained by a change in outgoing longwave radiation. It can also be achieved by a change in albedo. (This is obviously true but is often ignored.)
2) There is an “atmospheric window” in the wavelength range of roughly 8.5-13.5 microns where there are no blocking greenhouse gases except for trophospheric ozone. This window allows egress of radiation directly from the surface. However, in energy terms, this represents only about 17% of OLR.
Clouds can and do block that window 30% of the time which may be the reason the window is only 10% wide. See energy balance diagram below.
3) Radiant energy absorbed by the atmosphere will be re-emitted in all directions. It will be re-emitted at longer wavelengths than that at which it is absorbed, the re-emitting wavelengths being longer at lower temperatures and pressure. This implies that a species of molecule which absorbs at one wavelength cannot absorb a photon re-emitted by a molecule of its own kind.
This is not correct. Absorbed radiation tends to be remitted within an angle of 40 degrees of its incidence. In general it cannot be re-emitted at a higher frequency that that at which it is absorbed because that implies it has acquired more energy. It can re-emit at the frequency at which it absorbs. That is how a CO2 laser works.
4) Approximately two thirds of the energy absorbed by the surface emanates from energy re-emitted in the atmosphere and only one third is attributable to directly absorbed solar energy.
That is what this energy diagram from the IPCC TAR shows: http://www.grida.no/climate/ipcc_tar/wg1/fig1-2.htm
5) Radiative transfer is a much slower energy transport mechanism than convection such that the latter process gets most of the effluent energy to the top of the atmosphere.
Radiative transfer through the window is at the speed of light, much faster than convection! Moreover, convection stops at the tropopause, the boundary between the troposphere and the stratosphere. But inspection of the TAR energy balance diagram reveals that the combined Thermals and Evapotranspiration which make up the convection are about 100 W m-2 whereas the net long wave radiation is only about 65 W m-2.
6) Radiation leaving the surface has a near blackbody spectrum. OLR does not. There are gaps attributable to greenhouse gases.
The OLR spectrum does not contain gaps as such. There are bands where the OLR is less than that at the surface. However, there are gaps between the lines which make up the bands. The deepest band is the CO2 15 micron band. The widest bands are caused by water vapour.
7) There is little water vapour in the high atmosphere.
True. Above the troposphere, the stratosphere is very dry although it is wetted by contrails from aircraft. Above that altitude, the noctilucent clouds which may be due to water vapour seem to be increasing in frequency. Of course some clouds are formed from ice crystals not water aerosols.
Questions:
1) Will the “atmospheric window” get narrower with increasing CO2? I can picture the possibility of a curtain hanging on the right (long wave) side of the window which may become partly drawn, thus obscuring part of it. I can see this to be particularly likely if energy is leaving a dry surface with little overlying water vapour but less likely over, say, oceans.
What will happen is that the lines will get broader. So they will tend to close like a venetian blind rather than a curtain. However the bands, which you can think of as a curtain, will not get much broader. But see the introduction to this item by Ray Pierrehumbert to get his take on it. It is quite complicated.
2) Ray Ladbury (#478) talks of gaps in the blackbody spectrum in the case of OLR. However, Step 2 of “The CO2 problem in 6 Easy Steps” (Real Climate Archive, August 6th) cites a graph showing IR spectra taken from space. This graph has no explanation. Rightly or wrongly, I interpret it to be showing that Ray’s gaps are attributable to CO2 and ozone but not to water vapour. Is this correct? If so, is it explained by the dry upper atmoshere.
Here is one example of the OLR spectrum http://www.john-daly.com/spectra.gif If there were no greenhouse gases it would look like the 300 K line. If you look to the left of the CO2 band from the wavenumber of 550 cm-1 you can see that the line is at around 275K. This 25 K drop in brightness temperature is caused by water vapour absorption. You can also see that by inspecting this diagram http://origins.jpl.nasa.gov/library/exnps/f4-3.gif It has increasing wave number running right to left whereas on the Guam diagram they run left to right. But it is also calibrated in wavelength so the 15 um (micron) CO2 band can be easily identified and correlated on both diagrams.
3) How does thermal energy reaching the top of the atmosphere by convection become OLR and make good its escape to space?
Although clouds appear white in visible light, they behave like blackbody radiators in the infrared. They are not gases when the form clouds. Clouds that form near the top of the troposphere can radiate to space without that OLR being absorbed in the water vapour bands.
4) Various correspondents suggest that infrared in the near-15 micron range can be demonstrated in the high atmosphere. Could this emanate from re-emissions of atmospherically- absorbed solar irradiation, given that re-emissions from lower level CO2 molecules couldn’t apparently be an explanation?
The GUAM IR spectra shows that there are emissions in the 15 micron band with a brightness temperature of around 215K. It is generally believed that this radiation originated at the surface and 85K of brightness temperature was lost by absorption. This report from the US Air Force http://www.dodsbir.net/sitis/view_pdf.asp?id=DothH04.pdf (first posted by David Donovan) points out that the band does not alter on a diurnal basis therefore it is not due to solar activity.
Well guys, what’s wrong with that?
Re #555 Timothy Chase
Many thanks for this great list of references. It will take me quite a while to work through it.
Re Alastair (#560)
Alastair,
You are brilliant! Obviously a top-notch climatologist. Are you Gavin?
Re #554 Rod B
I simply offered a suggestion to try to explain your problem. Perhaps it is not adequate, but it was the best I could do.
Quote
My only other guesses at this point would be M, J and R. (First initials.)
Timothy (559, et al), you say you buried Alastair in evidence and data and yet he persists, so can/cane him. Exactly what I accused you of. If one is plain obdurate, he simply should be ignored, not banished — though proving being obdurate is somewhat subjective.
I’m not charging to Alastair’s personal defense; nor am I accusing you individually other than as a current example. I’m just countering what I see as a negative non-specific characteristic. I’ve done it before with wholly different players. I believe it valid. I disagree with a couple of Alastair’s main contentions (and have so stated), but have less than expert certainty and simply want to understand what he is saying before I discard it out of hand. And while I have a level of respect and affinity for Alastair, it is no different for what I have for nearly everyone posting here, including you. I don’t think Alastair needs me to run his defenses; if he does he is in a heap of hurt!
I don’t agree that the attack on Raypierre that you cite is a clear violation of “non ad hom” attacks on integrity; though I admit it’s pushing the envelope, and to that end I would disapprove.
Alastair, re-emission is within 40 degrees of the angle of incidence??? How do it do that??
Re #566 That is in the case of a laser which I saw some where. The photon will not necessarily hit the centre of the molecule, so the stimulated photon will not be emitted in exactly the same direction as the stimulating photon.
But if it is a spontaneous emission I expect that it will be in any direction. Since we are mainly considering spontaneous emission, it may be better to ignore that point.
I am not sure myself how the selection rules etc. work, so I am interested in what anyone else turns up. BTW here is a description of a CO2 laser: http://www.laserk.com/newsletters/whiteCO.html
RE #554 Rod B
I don’t think you are looking at the problem the right way. If you refer to the following ref., particularly pages 9 & 10, you will find some help.
http://www.dodsbir.net/sitis/view_pdf.asp?id=DothH04.pdf
This is a joint research paper by Spectral Sciences, Inc., the US Air Force, and Boston College. It is highly authoritative, and immense. Pages 9 and 10 seem to show that an excited Co2 molecule can relax in stages, so that the energy of a particular emitted photon is less than the initially absorbed photons, so making re-absorption impossible. This supports the DoE statement.
Re 568 where AEBanner says:
Pages 9 and 10 seem to show that an excited Co2 molecule can relax in stages, so that the energy of a particular emitted photon is less than the initially absorbed photons, so making re-absorption impossible.
Yes but we are only interested in the v2 15 micron (667 cm-1) band of CO2 for outgoing longwave radiation. See the two spectra:
http://www.john-daly.com/spectra.gif
and
http://origins.jpl.nasa.gov/library/exnps/f4-3.gif
On pages 9 & 10 both the diagram and the table show the 15 um line (01101) dropping straight to the ground state (00001) thus it does not pass through any intermediate levels, and so the emission could and does re-excite another molecule to the 01101 state from 00001.
On page 10 of http://www.dodsbir.net/sitis/view_pdf.asp?id=DothH04.pdf it says:
Around 15 μm, the bands result from a change of one quantum of bending mode v2. These transitions are very prominent in the emission from the earth’s atmosphere, and the sharp Q-branch can be clearly seen in spectra of the earth taken at spectral resolutions of a few wavenumbers from a balloon, rocket or satellite [Stair et al., 1985; Wise et al., 1995]. The reason for this is very simple. The spectrum of a 300 K blackbody peaks near 10 μm. Therefore, there are large numbers of 15 μm photons in the radiation, termed earthshine, emitted by the earth’s atmosphere as well as by the earth itself. Since the 15 μm CO2 band is very strong, its transitions to the ground state and low lying excited states are severely self-absorbed. This fact has important consequences for atmospheric radiation which we will point out as we go along.
I have not found the important consequences further on yet. Does anyone else know what they are?
No, it doesn’t say impossible. It may be accurate for identical temperature and pressure, it may be correct describing a laser. It doesn’t say whether, for example, a higher, colder molecule can absorb a photon emitted by a lower, warmer molecule. I’m not a physicist, logic is a weak reed in dealing with quantum mechanics, words are not much use. You need to get someone who actually can do the math to address this question, and perhaps someone who can do the experiment.
Stating a conclusion because some text “seems to show” one thing isn’t sound.
AEBanner,
Interesting reference. By what these guys say, collisional excitation and relaxation dominate over their radiative counterparts out to ~65 km–well into the stratosphere. Since the temperature profile is increasing with altitude in the stratosphere, I think this implies that LTE is applicable throughout the range where CO2 contributes to the greenhouse effect. True?
Huh.
Rod B (#565) wrote:
Banished?
I would save that for trolls – although that obviously would be a matter for the moderators. In all the time that I have been on Real Climate, I have never considered Alastair to be a troll – as I understand the term.
Ignored?
I will agree with you there – if someone refuses to acknowledge any and all facts and evidence against their position and does so over a long enough period of time. But how much time is an individual decision, and if they genuinely change I would consider what is in the past to be simply in the past. I believe you might understand why.
My reasons for giving people a great deal of latitude, time and chances are also intimately tied to the necessity of judging their words and actions even if I were to always keep my judgements to myself. But I would prefer not to go into the details of that as I prefer people to discover things for themselves – including their personal ethics. I don’t mind sharing principles, but like ships, I prefer to give worldviews a wide berth. A story for another time, perhaps.
In any case, I haven’t had the chance to keep up with the discussions of science that have been taking place and there are a lot of interesting leads to follow up…
AEBanner (#561) wrote:
Well, I am just returning the favor, I believe. I am going to have to make that paper a higher priority than I have managed so far. But there are other things to follow up, too, and I have been a little distracted as of late.
AEBanner (568), I really didn’t see in your ref that re-emission then absorption by the same molecular species is prohibited. They didn’t talk of it (much?) as opposed to other, implied more interesting or significant, exchanges. Maybe it (CO2 to CO2 at certain levels) doesn’t happen much, for a host of reasons. But nothing I read refuted my base level logic: I agree CO2 can relax in stages (which is changing energy levels in steps). But the energy levels available in a CO2 molecule are fixed and invariant, though some more likely to filled than others, sometimes significantly. If a molecule can emit at a discrete energy level (hf), it can absorb at the same level. And to repeat, if a molecule can absorb a photon at energy A, it has no way of telling if that photon originated from earthshine, another molecule, or another CO2 molecule.
I inferred for the ref that it’s possible for a CO2 (or any other specie) molecule to have a greater tendency to emit at certain levels than, for complicated non-linear reasons, its tendency to absorb at the same level. But nothing says it can’t.
Hank said, “…logic is a weak reed in dealing with quantum mechanics, words are not much use. You need to get someone who actually can do the math to address this question, and perhaps someone who can do the experiment.
Stating a conclusion because some text “seems to show” one thing isn’t sound….”
Unfortunately this is very true! More unfortunately we all have to play the cards we’d been dealt.
Re Ray Ladbury (#571)
Local thermodynamic equilibria, partial local thermodynamic equilibria, non-local thermodynamic equilibria, transitions between excited states, etc. Interesting stuff indeed!
According to the paper, for CO2, deviation from LTE begins around 30 km as the average duration between collisions for a given molecule dips below that of one one-millionth relative to half-life. For two vibrationals it starts becoming noticeable by 50 km. For the third around 75 km. The three vibrational transitions come from two to ground and one excited state to the other.
PS to #576
One point that the author of…
http://www.dodsbir.net/sitis/view_pdf.asp?id=DothH04.pdf
… makes is that the deviation from LTE is a gradual one. It is also worth noting that non-LTE has been incorporated into the GCMs for some time now.
Haven’t read the latest posts, but I have a tentative thought that says I have to partially withdraw my vociferous assertions re re-emission and secondary absorption. I have been saying that CO2 (can) drop energy levels in steps and that if it drops to level B and re-emits from that level, that same level B exists in every other CO2 molecule and therefore can be absorbed. This assumes that the dropped energy delta from A to B was transferred to translation or something. But, if the A to B delta energy drop is the energy released with a photon, that photon will not (or hardly ever…) match any fixed energy level in any CO2 molecule, and therefore can not be absorbed by CO2. [slap! Slap!] I don’t know the probability of each, but, as a rough rule, it looks like I was only half right, as were the folks I claimed were totally wrong. Any of this make any sense to anyone?
There have been several comments pointing out that I may have drawn the wrong conclusions from
http://www.dodsbir.net/sitis/view_pdf.asp?id=DothH04.pdf
OK, but the following paragraph comes from the DOE
http://www.eia.doe.gov/cneaf/alternate/page/environment/appd_a.html
Quote
What happens after the GHG molecules absorb infrared radiation? The hot molecules release their energy, usually at lower energy (longer wavelength) radiation than the energy previously absorbed. The molecules cannot absorb energy emitted by other molecules of their own kind. Methane molecules, for example, cannot absorb radiation emitted by other methane molecules. This constraint limits how often GHG molecules can absorb emitted infrared radiation. Frequency of absorption also depends on how long the hot GHG molecules take to emit or otherwise release the excess energy.
Unquote
Yep, but each time you quote it, since I can’t find the statement anywhere else, and since there’s no footnote for it (and every other statement around it is footnoted), and we don’t know when that particular text was revised, I have to wonder whether anyone else anywhere else has ever said that. I’m sure it’s moving up in Google’s hit list a bit since you’ve been reposting it at RC. Seriously, if you do believe it’s important, tracking down the author or someone who can check it for you might be a useful thing to do.
Re #578
I am only saying that CO2 molecules can reabsorb their 15 um radiation. If you look at the spectra, then CH4 only plays a small part in the absorption. However, there are two very important H20 bands and I have no information on whether they are or are not self-absorbing.
It gets more and more complicated, but that is the way quantum mechanics is, so there is no way to avoid it :-(
HTH,
Cheers, Alastair.
Re #579 HR
Yes, I agree.
I do believe it is important, and I have already e-mailed DoE for further information, but sadly without response.
I am still hoping for a reply in this thread from someone who really knows about this stated behaviour of CO2. I’m just looking for the facts.
Another question. Can CO2 in the atmosphere absorb a photon directly into the 01101 energy level, or does the absorbed energy have to go into a higher level first? (re Dothe’s Fig 2)
Re #578 Rod B
I think I follow your logic for transitions between higher energy levels, but I don’t think it applies for transitions directly to the ground state. Does it? Re last sentence in my #582.
re 582, 583: this might seem to simplistic, but if a photon from earth’s blackbody radiation or any gas’s re-emission (or from wherever) exists at the equivalent level 01101, I assert the CO2 molecule will (can) gobble it straight away (subject to the other factors that determine whether a photon is absorbed or not.)
Re #584
OK, but what are the “other factors”?
Re #580 HR
I’m sorry. I seem to have upset you. I’ll try to refrain from quoting THAT PARAGRAPH again.
> what are the “other factors”?
Google has worked up a pretty good tool for parsing natural language sentences into useful searches, I was informed by a programmer recently.
This works:
http://www.google.com/search?q=what+determines+if+a+gas+molecule+absorbs+an+infrared+photon
Re #587
Hank, thanks!
No 1 on that search – “Multiple Photon Excitation” looks like a great hit!
The quantum effect on polyatomic molecules is very complicated and has only recently (in the last decade or two) been investigated. It will be interesting to read what they have to say.
—
Now having started to read it, I do not think it gives an easy answer to the greenhouse effect. There are very many more polyatomic molecules than CO2 and H2O. I will try to read it in the hope that I might learn something relevant to global warming.
Don’t leap at the first hit — it has useful history about what the theorists believed before the laser developers started doing the “impossible” and how the theorists adapted to that. But Google doesn’t come with a “wise and useful hits first” option — I’d suggest using “search within results” at least, and consider using Scholar instead once you get some terms down that Google finds.
AEBanner (585), much is probably beyond my scope but there is a probability function, which in turn might be temperature and pressure affectsd, that helps in determining whether a photon is absorbed or not.
Many thanks to Ray Ladbury (#553), Rod B (#558) and Alastair McDonald (#560) for answering my post of #545. I think my understanding is advancing, albeit in small steps. If I may, I’d like to raise a few further issues.
I asked to what extent one might expect the “atmospheric window” to shrink as a result of increasing atmospheric CO2. ( My analogy of the right hand curtain being partly pulled across was regarded as less pertinent than the venetian blind analogy. However, the latter seems quite inappropriate because the thread was initiated following Raypierre’s observation that the 13.5 to 17 micron band was already fully saturated. The “window” could thus only be partly obscured by increasing absorption by CO2 of radiation in 12.5 – 13.5 range). Ray said the “window” might get “a very small bit narrower” and Rod suggested it would be unaffected “except on a limited and insignificant scale”. Would it be possible for anyone to tell me what this small effect would be as a percentage of the total extra warming attributable to increasing atmospheric CO2. I take it that it would be a small percentage only, with the balance made up from blocking of radiation at higher altitudes. However, if I understood Raypierre’s essay correctly, it would take two molecules of CO2 high in the trophosphere to have the same blocking effect as one at low altitude. This leaves me to wonder whether it is remotely possible that extra CO2 won’t have quite as much warming effect as the modellers assume, albeit plenty for us to be concerned about.
I remain somewhat mystified as to the translation mechanism for non radiative thermal energy into radiative thermal energy. In explanation, Ray said “heat transported by convection gets to the upper troposphere and heats the air there. The warmed air collides, exciting rotational and a few vibrational states. These high altitude molecules then radiate in the bands where they can radiate.” Which molecules does he mean? I understood (rightly or wrongly) that oxygen and nitrogen neither absorbed nor emitted radiation. This is no doubt a facile question but how are radiating photons created from non radiative thermal energy? Suppose, for example, that there were no ghgs in the high atmosphere. I would expect a colder climate but would I be correct? Effluent thermal energy can only escape to space as OLR. How?
Finally, AEBanner’s citing of the DoE quote which suggests that energy emitted from one CO2 molecule can’t be absorbed by another. Rod B descibes this as a “bomb” which, if correct, would blow a hole in AGW theory. I don’t see that this need be the case. Alastair states that “Guam IR spectra (of OLR) shows that there are emissions in the 15 micron band with a brightness temperature of 215 K. It is generally believed that this radiation originated at the surface and 85 K of brightness temperature was lost by absorption”. Surely, this general belief can’t be correct if those believing are of the view that radiation in the 15 micron band goes directly from the surface to space through the “atmospheric window”. If clouds are black body radiators, they would emit radiation at this wavelength which wouldn’t be blocked by water vapour and only somewhat by CO2 because it is fairly unsaturated at this level. Furthermore, I assume that photons absorbed by water vapour in the 5-9 micron band might result in longer wavelength re-emissions in the 13-17 micron band. As far as I am concerned, the DoE statement helps explain the concept of saturation which was meaningless to me before, given that many were claiming that re-emission always occurred at the same wavelength as absorption.
Quite possibly, I’m still holding the stick by its wrong end and I hope that, if so, you’ll enlighten me.
I kind of doubt that multi-photon absorption has that much of an effect on the greenhouse effect. All the searching I tried on it kept on turning up lasers – although there was a declassified document on the atmospheric effects of a nuclear bomb blast released in 1995. I am hoping that the FBI isn’t tracking those who download the document. Then again, having hung around Lenny Flank for so long and being married to someone who protested the first gulf war…
RE #578 Rod B
I think you are on the right lines here. I believe that photon emission is more likely to be collision induced than spontaneous, and some of the energy of the photon about to be emitted could be transfered into kinetic energy of the colliding molecules. So the emitted photon would have less energy than the initially absorbed photon gave to the absorbing molecule. Therefore, the emitted photon might well not have the required energy to be absorbed by another molecule of the same kind.
re 591, “….Therefore, the emitted photon might well not have the required energy to be absorbed by another molecule of the same kind.”
Or, by chance, any kind of molecule?? It would seem a lower energy photon might have a shot at H2O rotation(?) absorption. But we really don’t have a clue, do we? Can’t the re-emitted photon be almost at any (usually lower) energy level?
Rod B (#594) wrote:
If one were talking about a single molecule which is not encountering collisions, then the energy emitted would have to be equal to the energy absorbed by that molecule. However, it wouldn’t necessarily be the same photon. You may have transitions from one excited state to a less excited state in accordance with the principle that Alastair (#548) cited earlier:
It is worth noting, however, that the Franck-Condon principle is only an approximation, although I suspect a fairly good one.
But when we speak of the 15 micron being stongly self-absorbing, I take this to mean that it does not tend to make these transitions to lower excited states. This becomes important particularly at higher altitudes where it strongly diverges from LTE and is largely responsible for the cooling of the mesosphere and the lower thermosphere. This becomes especially important as this influences the circulation of the mesosphere, gaining energy from both nitrogen and O2, but particularly from collisions with photodisassociated O, or at least this is what I get from the abstract for:
New estimation of the 15 micron CO2 band cooling rate of the lower thermosphere
Ogibalov, V., Shved, G., Khvorostovskaya, L., Potekhin, I., Uzyukova, T
34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA., meeting
Incidentally, I would assume that the cooling off of O strongly influences the chemistry – inasmuch as it will permit O to reassociate.
*
As this example indicates and as we have been reminded numerous times before, you can’t treat the molecules as if they exist in isolation. Collisions will happen, and in fact it is by means of such collisions that the greenhouse gases will tend to acquire their energy.. One question which occurs to me is whether such collisions may result in a molecule being knocked from a lower state of excitation to a higher state. In any case, for that proportion of photons which reach the surface after reemission will naturally warm it, and for this reason much of the energy will still be reabsorbed by greenhouse gases, albeit indirectly, either through thermal reemission from the surface, or what is more likely, moist air convection.
*
One other interesting point:
This would further suggest that the weaker emission bands radiate in part due to solar radiation, whereas with these stronger emission bands it is almost entirely due to thermal radiation from the earth’s surface.
Since we’re into the wildly uneducated speculations here, I wonder if “vibration” or “rotation” is quantized or continuously variable. I realize this isn’t’ even poetry, rather doggerel, attempting to describe something in words on the scale of single molecules.
Is it possible one can “spin up” a rotation by firing various-energy photons at it, and as the rotating particles wiggle thir electromagnetic environment, that will then can shake out new emitted photons whenever it happens to hit the right frequency to match ….
My literal blue-sky wishful thought is for some idea that could, with only a catalytic trickle of energy, tune the gases at the top of the atmosphere to highly favor emission of infrared photons (or, heck, I suppose any other kind) at the height where they have a good chance of departing the planet.
Of course that suggests any sufficiently advanced civilization elsewhere could occupy a planet indistinguishable from a cool red dwarf star from the outside.
This is merely an attempt to post something whacko enough to interest the science-fiction-friendly Ray to rejoin us (grin).
Happy Labor Day …
A couple of the easy ones: the molecules in the upper atmosphere heated through convection are predominately N2 and O2. When one collides with CO2, say, it can transfer some of its translation energy to CO2 translation or, as I understand…, directly to the CO2’s vibration or rotation internal energy (dropping the real temperature of the air with the latter process.) Molecules can transfer energy among their translation, vibration and rotation modes — within the same molecule or inter-molecularly. Whether and when this is done is problematic, gets very complex and depends on quantum mechanical pixie dust, among other variables. Only rotation and vibration can absorb/emit LW electromagnetic radiation energy (photons). Rotation and vibration energy levels are quantized, much like electronic quantum levels, though more plentiful.
Re #596 and 597
For quantised molecular vibration states see
http://www.fas.org/sgp/othergov/doe/lanl/pubs/00285799.pdf
and
http://www.umsl.edu/~orglab/documents/IR/IR2.html
Re #594 Rod B
Reference to Fig 2 in
http://www.dodsbir.net/sitis/view_pdf.asp?id=DothH04.pdf
and to the infrared data in HITRAN for CO2 shows that the lowest radiant energy transitions are in the 15 micron region. Inter-molecular collisions can transfer some energy from a photon about to be emitted from a CO2 molecule into kinetic energy of translation, and so the energy of the photon is reduced, so making re-absorption by another CO2 molecule very unlikely (impossible?). This is in line wih the DoE statement.
Perhaps the lower energy photon may be absorbed by a molecule of water,as suggested by Rod B in #594, but this could only occur at altitudes of less than about 4 Km.
> altitudes
There’s not _zero_ water in the stratosphere, there’s enough of an increase to delay the recovery of the ozone layer, for example:
http://www.giss.nasa.gov/research/news/20010417/