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).
Douglas Wise (#591) wrote:
Well, how closely we know how carbon dioxide radiates which have some effects upon the atmospheric calculations. While we have some difficulty following all of the effects, this is actually what is most accurately known as the result of the physics of radiative transfer, and it is quite mature. There are some aspects of the interaction between chemistry and radiative transfer which are perhaps less mature, but that is a different matter. However, as I have pointed out on numerous occasions, we have fairly accurate measurements of the thermal emissions and spectra from carbon dioxide and other greenhouse gases, so we would seem to have a fairly accurate understanding of the radiative transfer which is currently taking place within the atmosphere.
Now with regard to the warming effect within the atmosphere of doubling CO2, obviously something comes out of the models – a climate sensitivity which is not assumed but is instead a product of a given model itself. However, the climate sensitivites which drop out of the models are not our only way of knowing what the climate sensitivity is. We have the paleoclimate record from the past 400,000 years, and it shows rather consistently a climate sensitivity of about 2.8 degrees.
One point, though: climate sensitivity is undoubtedly partly dependent upon the distribution of the continents on the globe as it incorporates other feedbacks. So for example, if you have the continents distributed mainly around the equator, you can have a form of global runaway albedo effect. Once you have passed the tipping point on either side, the climate sensitivity to CO2 doubling will be much smaller. This is largely what made possible Snowball Earth in the remote past – although it was more likely a slushball. But the continents won’t be moving that much in the next 100,000 years, so I believe it is a safe bet that our climate sensitivity will be roughly the same as it has been for the past 400,000 years.
Douglas Wise (#591) wrote:
Well, outside of the partial local thermodynamic equilibria and the non-local thermodynamic equilibria (both of which are fairly narrow phenomena – although as we have seen from some of the papers that have entered the discussion, not really that narrow, principally in the mesosphere and above – and this will influence the manner in which energy is dissipated in those layers, and thus atmospheric circulation at those levels which will influence atmospheric circulation in the stratosphere and troposphere), the condition of local thermodynamic equilibrium holds. Where there is a local thermodynamic equilibrium certain other principles hold.
For example, you will have a Maxwellian distribution of kinetic energy, collisions will take place much more frequently that the half-life of reemission (this is actually what determines whether a local thermodynamic equilibrium applies to a given wavenumber – with a million collisions per half-life being enough to more or less insure it), and so the radiative “Planck” temperature will be equal to the kinetic Maxwellian temperature. Under LTE we know that Kirchoff’s law applies, which means that the emission at a given wavelength is equal to the absorption.
As such, moist air convection increases the temperature of the troposphere which increases the amount of thermal radiation which it emits by increasing the Maxwellian temperature and thus the Planck temperature.
Douglas Wise (#591) wrote:
Under LTE the local temperature for any given gas will be equal to the local temperature of the atmosphere itself. Thus the collisions from oxygen and nitrogen are sufficient to insure that the Planck temperature of greenhouse gases have sufficient energy to radiate at the corresponding wavenumbers.
Douglas Wise (#591) wrote:
Under those conditions the radiation wouldn’t be absorbed by the atmosphere, and as such would be immediately radiated into space rather than having some proportion radiated back to and warming the surface.
Douglas Wise (#591) wrote:
Well, what Alastair is describing there is a result of the greenhouse effect – but a bit removed from the LTE that applies within a given part of the atmosphere. In the absence of an atmosphere, or alternatively, in the absence of greenhouse gases, the effective temperature of the earth would be equal to the surface temperature. However, greenhouse gases diminish the efficiency with which the earth is able to radiate thermal energy into space, and as such they diminish its effective temperature – that is, the temperature that the earth appears to have as viewed at a distance. This is in essence the reduction in brightness temperature which he is speaking of, although unlike the effective temperature, the brightness temperature will be different for different parts of the spectra, being dependent upon the emissivity of greenhouse gases for those parts of the spectra as well as their distribution in the atmosphere.
Douglas Wise (#591) wrote:
Well, I am a little less sure in this area, but given the fact that CO2 is described as being strongly self-absorbing in this part of the spectra, I am of the view that it doesn’t radiate directly into space at this wavelength. Strongly self-absorbing would seem to imply that it is able to reabsorb at that wavelength and does so quite well. Nevertheless, much of the radiation with which the upper atmosphere is able to emit at that level is due to carbon dioxide at that wavelength.
Douglas Wise (#591) wrote:
It does sound like a saturation of sorts, doesn’t it? But not quite the same thing. The saturation of which raypierre speaks is a saturation which more or less translates into an opacity at a given wavelength. If the atmosphere is opaque at that wavelength, it doesn’t mean that it no longer emits radiation at that wavelength, but more or less that none of the radiation can get through without absorption and consequent reemission. In a certain sense, this is where the blinders analogy falls down but pinballs in a pinball machine where you are adding bumpers does a pretty good job. I myself however prefer simply to think in terms of the atmosphere becoming opaque at a given wavelength. As it becomes opaque, it becomes less transparent, and any well-defined image at that wavelength would be lost.
Less and less of the energy at that wavelength will make it to space without making a round trip to the ground given the “isotropic” reemission of radiation (although Alastair has pointed out that this is actually an approximation – which nevertheless helps in terms of getting a handle on the process). However, if the atmosphere is completely opaque at a given wavelength, what this will imply is that parcels of energy (think of them as photons) which leave the surface at that wavelength will stand a fifty-fifty chance of making it out of the atmosphere without making another trip back to the surface and further warming the surface.
I hope this helps.
A bit more of an overview of a number of topics, actually, whereas the discussion had become rather specialized as of late. Fascinating stuff, but a little of a diservice to those coming into the conversation later than other participants.
PS to #600 (response to Douglas Wise)
The first sentence should have been, “Well, how closely we know how carbon dioxide radiates will have some effects upon the atmospheric calculations.” From what I can tell, after that sentence I have done a good job of not mangling the sentences. But I was still tuning up at the first sentence, juggling different ways of expressing the same thought.
Hopefully Ray Ladbury, Raypierre or someone else more knowledgable than myself will respond to your comment, but I figure I have done a fair job.
I would like to thank Timothy Chase for his response (#601) to my post of #600.
I accept that climate sensitivity falls out of the models and is consistent with 400000 years of paleoclimate data. Radiative forcing, however, is presumably built into the models. As explained in the iniating essay to this thread, the formula is log based such that increasing atmospheric CO2 has progressively reducing effects. In the previous one million years, atmospheric CO2 concentrations appear to have cycled between 140 and 280ppmv. Raypierre suggests that, at low levels of the atmosphere (say up to 8km – see Barton Paul Levenson’s post#210) the 13.5-17 micron bands are saturated at 300ppmv and above and that further absorption could only take place in the wings. Am I wrong to think that the radiative forcing formula is a mathematical calculation of the extent to which the atmosphere at this level is approaching “optical thickness”? The formula looks quite simple, leading me to suppose that the fact that high altitude CO2 has only half the blocking effect of lower altitude molecules may have been ignored. Clearly, the paleoclimate record for the last 400000 years does not resolve this point because it relates to times when the main CO2 absorbing bands were in the range from unsaturated to approaching saturated.
Timothy’s reply to my question relating to the escape of non radiative thermal energy to space appears to require the presence of ghgs. We have been assured that, without ghgs, the planet surface would be 20-33 deg C colder because all incoming short wave radiation would immediately leave the atmosphere as outgoing longwave radiation. This does not address what would happen to energy leaving the surface in non radiative form. Wouldn’t it be trapped in the atmosphere, having no way of being converted to radiation? (I suspect you’ll tell me this is ridiculously hypothetical and that, in any event, oxygen would convert to ozone and solve my dilemma.) Would it be mad to think of ghgs as a sort of buffering system? It seems they are blankets keeping heat in but nevertheless are necessary to let non radiative energy out of the top of the atmosphere.
Re where Douglas Wise says :
It seems they are blankets keeping heat in but nevertheless are necessary to let non radiative energy out of the top of the atmosphere.
Yes, that is true. It follows that if the atmosphere is saturated wrt. terrestrial radiation (in the greenhouse bands) at the base of the atmosphere, it must also be saturated wrt outgoing long wave radiation at the top of the atmosphere.
The way I visualise it, is that each layer of atmosphere behaves like a sieve removing a certain proportion of greenhouse gas photons, say 10%. The next layer will 10% of those remaining and so on. If the atmosphere is deep enough then eventuall there will be a layer where there are no GG IR photons, and although CO2 is only 4 parts per thousand of the earth’s atmosphere, with the billions of molecules it contains the height of total absorption is soon reached.
Now think of the top of the atmosphere, where the the sieves get wider as atmopheric pressure gets less, but that is unimportant. Just as every photon leaving the surface is trapped on it way up, looking down from the top everywhere you would see a photon escaping. If you add CO2 to the atmophere all that happens is that the radiation you see is emitted from higher in the atmosphere, because the sieves are tighter but still have the same overall effect.
Of course this means that if the incoming solar radiation is greater than this fixed outgoing radiation, then the planet will get hotter. This is what has happened on Venus. It has heated up until the planet’s surface melted and produced huge clouds of sulphur dioxide limiting the radiation at the bottom of the atmosphere to match the OLR, by reflecting away the solar flux.
On Earth the surface warms until water clouds form, and on Mars the planet warms up until huge dust storms shade the planet allowing it to cool.
Of course that is not what Raypierre is saying, but I think it makes more sense :-)
> Wouldn’t it be trapped in the atmosphere, having no
> way of being converted to radiation?
Everything radiates.
I happened to read the publication of ‘herr’ John Koch (as Ångström did and Raypierrre didn’t),
John Koch, Beiträge zur Kenntnis der Wärmeabsorption in Kohlensäure., Öfversigt af Kongl. Vetenskaps-Akademiens Förhandlinger, 1901. N:o 6 p 475-488
Here is his figure 1:
http://home.casema.nl/errenwijlens/co2/Koch_fig1.gif
Douglas, I’m not sure what you mean by saying a high-altitude CO2 molecule will be less efficient at capturing an IR photon. Are you referring to the lower amount of line broadening due to the lower pressure? Or are you referring to the decreased IR flux at higher altitude? To first order, the absorption cross section of a CO2 molecule will be constant regardless of where it is.
As to the absorption of CO2 in the wings of its line, keep in mind that the tails of the line are rather thick, so this is not insignificant.
Can you be more specific about your concerns?
Re #606
Hans,
Thanks for that info. I had not realised that Dr John Koch had published a paper (nor perhaps did Ray.) My only source was the summary by Very of Angstrom’s summary of the work by Koch. They refer to “unpublished researches of Dr. J. Koch.”
In the figure you uploaded, I have translated the X-axis as “Layer thickness in cm” and the Y=axis as “Absorption in per cent” Curve I is presumably with a pressure of 1.0 atm and Curve II with a pressure of 0.5 atm. Would it be possible for you to upload the complete paper, since being over 100 years old it is surely out of copyright?
I appreciate the replies of Alastair McDonald (#604), Hank Roberts (#605) and Ray Ladbury (#607) to my post of #603.
Hank, I believe you were answering my question as to how thermal energy which leaves the surface as sensible and latent heat and quickly reaches the high troposphere gets converted to radiative energy so that it can escape as OLR. Your succinct answer was that “everything radiates”. I take it, therefore, that you are implying that oxygen and nitrogen molecules can spit out photons which can then journey into space. Insofar as I understood Timothy Chase’s response (#601) to the same query I raised in #591, he suggested greenhouse gases would be necessary intermediaries. He actualy said “Under LTE the local temperature for any given gas will be equal to the local temperature of the atmosphere itself. Thus the collisions from oxygen and nitrogen are sufficient to ensure that the Planck temperature of the greenhouse gases have sufficient energy to radiate at the corresponding wavenumbers.”
Alastair suggests that my statement that “It seems that they (ghgs) are blankets keeping heat in but nevertheless are necessary to let non radiative energy out of the top of the atmosphere” is correct.
Ray, you started me on this hare so I’d be grateful for clarification to your statement that “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.” Do you mean any old molecule (including oxygen and nitrogen as Hank seems to be implying) or just ghg molecules as Timothy and Alastair seem to be saying. Please accept that I’m not trying to stir up trouble and there may be no contradictions between you all – it just seems to me to be so with my lack of understanding of physics.
Alastair, I have managed to glean from this thread that your views may not be mainstream. However, to my simple mind, they seem as plausible as others I have read. I believe that you consider that the lower troposhere is already more or less saturated by a combination of CO2 and water vapour (barring the “atmospheric window”). You then argue that extra CO2 will will lead to saturation at a lower altitude, resulting in a lot of ice melt and reducing albedo with consequences worse than forecast by establishment modellers. However, in your recent reply, you seem to suggest that extra surface heating will encourage more cloud. Doesn’t cloud contribute more to albedo than ice? Perhaps there is no need to feel so depressed! The energy balance chart you referred me to seemed to require an outflow of energy of 235 watts/sq m. If you add 102 for that reaching the high troposphere by convection, 67 of solar energy absorbed by the atmosphere and never reaching the surface, 40 from the “window” and 30 from cloud tops, you exceed 235. Obviously extra CO2 will block some of this but extra cloud would add to it. I understand that a doubling of CO2 would make a difference of 3.7 watts/sq m. This doesn’t seem a lot when set against all the other possible variables. However, like you, I believe we should be taking action as soon as possible to reduce CO2 emissions. As a closet Malthusian, I also think we should prioritise measures to stabilise and ultimately reduce the human population. Climate change isn’t the only catastrophic effect our species is producing.
Finally, Ray, you ask me to be more specific about my concerns relating to the radiative forcing formula for CO2. My comment that high altitude CO2 had less blocking effect than low altitude was related to pressure effects. In the original essay, Ray Pierrehumbert discussed the lengths of tubes necessary to block outgoing IR of the appropriate wavelengths. At higher pressures, less IR can slip between gaps. I suspect that the formula for radiative forcing may relate to degree of saturation at any one altitude and not factor in changes in absorptive efficiency with changing pressure (altitude). I accept that this may be regarded as an absolutely unjustified suspicion, coming as it does from a complete ignoramus in the field.
Douglas Wise (#603) wrote:
Not a problem.
Douglas Wise (#603) wrote:
Nope. The radiative forcing due to a given gas will be in large part the result of its distribution in the atmsopheric column, the temperatures and pressures at specific altitudes, thermal radiation coming up from the surface, etc.. This is afterall what HTRAN is all about.
Please see:
I believe this is correct.
Douglas Wise (#603) wrote:
As I said, I don’t believe we use use that formula – and most certainly not for the entire atmospheric column. We use HTRAN and thus line-by-line calculation.
Douglas Wise (#603) wrote:
But we do know of earlier eras with much higher CO2 levels, 3000 ppm or more. The PETM was roughly 55 MYA. And as I have indicated, the actual empirical measurements which are part of HTRAN are quite detailed – as are the calculations. The logarithmic relationship is an approximation, although a fairly good one.
Douglas Wise (#603) wrote:
Evaporation leads to condensation leads to clouds – which are good blackbody emitters in the infrared. But if you are not talking about evaporation, then you don’t have moist air convection and heat will diffuse into the atmosphere much more slowly. At that point, we are speaking of thermal diffusion which is a much slower process.
Douglas Wise (#603) wrote:
In the absence of water vapor and other greenhouse gases, heat would diffuse through the atmosphere much more slowly from the surface, but it will also diffuse to the surface so long as the atmosphere is in contact with the surface. As the surface will emit radiation, it would also absorb heat from the atmosphere. No runaway effect.
Douglas Wise (#609) wrote:
I believe he was being a little too succinct.
Non-ghgs won’t radiate. If anyone were to make this claim, you should ask what would be the spectra? Given Hank’s earlier recognition of the relationship between the line radiation and blackbody (or realistic body radiation) I believe he will agree. But heat would enter and leave the atmosphere by means of the much slower process of thermal diffusion as the result of being in contact with the surface.
Douglas Wise (#609) wrote:
Thats correct. However, even under non-LTE in the upper atmosphere (for example, 15 microns for CO2), collisions will result in greenhouse gases emitting. But with carbon dioxide at 15 microns in the mesosphere and thermosphere, this will be more due to collisions with photodisassociated O (from ozone) than than absorption and reemission.
Douglas Wise (#609) wrote:
They are not necessary given thermal diffusion – the surface will still absorb heat from the atmosphere while the surface emits thermal radiation.
Douglas Wise (#609) wrote:
Since he is speaking of rotational and vibrational states he is speaking of greenhouse gases.
Douglas Wise (#609) wrote:
I believe Hank will agree with respect to non-ghgs not radiating, and I believe that Alastair will agree with regard to thermal diffusion. Likewise, I believe Ray will agree that what he is speaking of is principally moist air convection. The atmosphere is rather stable in its absence.
Douglas Wise (#609) wrote:
It clouds both increase the albedo and are good absorbers of longwave, and thus are essentially blackbodies in that part of the spectrum. The net radiative effect of clouds can be positive or negative – depending upon the type of cloud. As for “nonstandard views” in this area, we have the spectra. Emissions at different altitudes of infrared at various wavelengths which are essentially fingerprints of the gases which emit at those altitudes.
Please see #555 above. I have included 12 links to images and animations of data received by satellites showing exactly what is being emitted at different altitudes.
Alternatively, please see:
AIRS – Multimedia: Videos: Animations
http://airs.jpl.nasa.gov/Multimedia/VideosAnimations/
… for a number of the animations, including a somewhat detailed explanation of how those animations are created from satellite data gathered from over 2000 different channels within the spectra.
Douglas Wise (#609) wrote:
Thermal energy from the atmosphere emits 195 to space, including 165 from greenhouse gases and 30 directly from the clouds (i.e., without absorption by greenhouse gases – given a window). 40 of what the surface emits is not absorbed by the atmosphere but goes directly to space. Convection doesn’t go to space, but has to have its energy radiated into space, one way or another.
For a complete diagram, please see:
The Energy Balance and Natural Climate Variations
http://www.cara.psu.edu/climate/climatechangeprimer-pr4.asp
Douglas Wise (#609) wrote:
What othere variables? Solar variability?
It has been declining since 1960. And since as far back 1880 it would appear that greenhouse gases (taken together) have been the dominant positive forcing – even over solar variability.
Douglas Wise (#609) wrote:
We don’t use the simplified logarithmic formula in the global climate models but do precise calculations which take into account the pressures and temperatures at different levels in the atmosphere – and which even take into account non-LTE behavior.
Please see:
> everything radiates
No, what I meant is that there’s no molecule that’s an energy roach motel, there’s no molecule that is unable to interact by absorbing and emitting photons at some wavelength, not necessarily infrared.
Energy has to leave a planet either by radiation — or by boiling off physical atmosphere. It doesn’t have to leave as _infrared_ photons, per se. It simply has to leave.
Look at Ray Pierrehumbert’s piece on science fiction atmospheres, here: http://geosci.uchicago.edu/~rtp1/papers/BAMS_SFatm.pdf
Pierrehumbert RT 2005: Science Fiction Atmospheres. Bulletin of the American Meteorological Society, 86, 696-698.
He writes:
“… The planetary settings of many other science fiction stories pose interesting questions from the point of view of physics of climate. On Frank Herbert’s Dune, … A completely dry planet with
a habitable temperature range is no problem, at least if one only needs it to remain habitable for a few hundred million years. For example, [a hypothetical] Venus with a pure Nitrogen-Oxygen atmosphere would have a mean surface temperature of around 300K….”
Take the planet out and leave the ball of gas — it won’t suddenly be unable to get warmer or cooler.
Hank, Alastair and Ray,
In my response #611 to Douglas Wise (#609), I had to say that “I believe [you] would agree…” to various points. Whether you agree or disagree, it would be helpful if you said so, in the case of disagreements, said why, and said anything else you might like to say.
However, it should also be pointed out that none of the four of us are climatologists.
Ray Pierrehumbert (the author of the essay) is a climatologist. So is Gavin Schmidt – who I quoted as saying that climate models do not use the simple logarithmic formula but use line-by-line calculation of absorption and emission at given altitudes, taking into account pressure and temperature, and thus spreading. But Ray Ladbury is a radiation physicist, so he has some expertise in this area.
Douglas,
OK. An atom/molecule can only emit a photon when it transitions from a higher energy state to lower one. Since earthshine is strongly peaked in the IR, it can only affect a few molecules–the so-called greenhouse gases. O2 and N2 cannot under normal circumstances radiate because they have no rotational spectrum (no dipole magnetic moment). They can have a rotational spectrum when excited electronically (mostly at high altitude).
At low altitude most of the relaxation of the excited vibrational state of CO2 is collisional, and this can impart energy to other molecules. Note, however, that even though the energy of the vibrational state is high compared to the thermal energy, a small percentage of molecules will have energy high enough to excite this mode. This process is not significant until the density of CO2 is sufficiently low that the IR photon has a chance of escaping–that’s the effective height you are looking at when you look at the 15 micron line. That is my understanding, anyway. Also note that this is not a perfect line–it gets broadened due to pressure, etc. The basic thing to remember is that if there is energy in one mode, it usually finds a way to get shared in the other modes.
In the thermosphere, the O atoms come from O2 (photodissociation and dissociative collisions).
CO2 at higher altitudes will have significantly less pressure broadening, thus the range of wavelengths at which a molecule can absorb will be smaller. OTOH the peak absorption will be higher. The net of this is that: The emission from below on a CO2 line will be broader, the part that is out on the wings will pass through higher altitudes because the absorption lines are narrower. I spent a lot of July going through this
I’m still somewhat confused.
Minor point: I thought O2 and N2 had vibration and rotation energy levels, but unable to fill them with IR absorption because they are non-polar; could only fill them via collision or translation transfer. Is this wrong?
I thought this thread kinda concluded that 1) Planck function blackbody-type radiation comes from a totally different molecular action than IR radiation, which comes from the quantized vibration and rotation energies of polarized molecules (GH gases) — meaning the two ain’t the same thing, and that 2) gases theoretically radiate Planck function blackbody-type radiation (in addition to some radiating quantized IR line radiation), but from a practical viewpoint it is quite restrictive — requiring certain pressures and having “holes” in the spectrum, e.g. Correct or no? [As an aside one of the bothersome things is all (at least that I’ve seen) of the system level development and analysis of the earth’s energy balance use Planck function radiation (T^4, e.g.) entirely between the atmosphere levels…]
My primary confusion (still) is, after all of the above discussion of GH gases not re-emitting much, not absorbing emissions from the same type of GH gas, convection/latent heat rising to heat the upper atmosphere to get IR radiation finally leaving the earth because its really thin at the TOA, etc., etc., where does the over 300 watts/square-meter of IR atmospheric downwelling (absorbed by the earth) come from?
I had written in #611:
Eli Rabett (#615) wrote:
… and then the single O combines with other O2 to form O3. I knew ozone was involved somehow – but I obviously should have checked.
Eli Rabett (#615) wrote:
For those who are interested, it is well worth checking out…
The graphs are non-logarithmic and give you a decidedly different view from the graph above, from my perspective, a decidedly complementary view. You get to see the effects of pressure and temperature in the absorption curve – reminds me a bit of the “stone thrown in a lake” look of the wavefunction of a bound electron, although not quite.
As the center of the curve becomes progressively saturated, the wings take on more of the absorption. Additionally, Eli introduces the reader to some software where those interested can explore things for themselves – and if you care to understand some of the mathematics behind this, there is some of that as well.
Re #613 where Timothy said “Ray Pierrehumbert (the author of the essay) is a climatologist. So is Gavin Schmidt – ”
If you look up Ray Pierrehumbert’s CV I think you will find that he is a physicist not a climatologist. I suspect that Gavin is a mathematician. Not bad professions, but not equipped to handling the chaotic behavior of weather and other earth system sciences, where what you expect does not happen, cf. GWB and Katrina, Rumsfeld and Iraq. Or what you don’t does cf. The Boxing Day Tsunami.
But the big mistake they are making was pointed out by Spencer Weart when he wrote:
Although people did not deny the facts head-on, many denied them more subtly by failing to revise their accustomed ways of thinking. “Geoscientists are just beginning to accept and adapt to the new paradigm of highly variable climate systems,” wrote the NAS committee. And beyond geoscientists, “this new paradigm has not yet penetrated the impacts community”–the economists and other specialists who try to calculate the consequences of climate change.
Ray and Gavin are not geoscientists. I am!
Re #616 where Rob says
“Minor point: I thought O2 and N2 had vibration and rotation energy levels, but unable to fill them with IR absorption because they are non-polar; could only fill them via collision or translation transfer.”
What you say is roughly correct, but O2 does emit microwave radiation due to its rotation. Moreover N2 can transfer its vibrational energy to CO2 molecules, a feature used in CO2 lasers.
and that 2) gases theoretically radiate Planck function blackbody-type radiation
Theoretically gases do not radiate Planck blackbody radiation, but despite that water vapour does!
But theoretically, their radiation depends on their temperature which is their average translational kinetic energy. Acording to the Theorem of Equipartition (ToE) all modes of energy will be equally populated. Thus the average speed in the x direction will be the same as that in the y an z directions. Fine!
But Einstein then says that this means it will be shared equally with the electronic excitation, the vibrational energy, and the rotational energy as well. No collisions at room temperature can induce any electronic excitation so that temperature is absolute zero. It is frozen out. Rotational energy is low, so collisions can induce that and rotational temperature will equal translational (the kinetic and thermometer) temperature.
The vibrational temperature is the key [edit for appropriateness]
The energy of the translational collisions is not great enough for the electronic temperature to match the translational and rotational temperatures. So the electronic energy frozen is out, and so partly is the vibrational temperature.
But it is complicated :-(
Rod B (#616) wrote:
I believe you are right. However, if they can’t absorb, then they can’t emit – because they are nonpolar, and the only way that they will be able to lose vibrational and rotational energy will be by way of collision.
Rod B (#616) wrote:
It is always quantized – the true blackbody is an idealization. But it is also always smeared, where the lines and bands are broadened, getting closer to that idealization. Clouds are much closer, but there are also some solids that are much closer to the quantization we find among gases.
Rod B (#616) wrote:
Dusts and alloys will have holes as well, but if you change the mix in the alloy so that the structure becomes less regular, those holes will tend fill up.
Rod B (#616) wrote:
I would strongly recommend checking out Alastair McDonald’s post #569. It gives you links to two graphs which show the absorption over the longwave due to different molecules. As the graphs show, when you have an atmosphere composed of multiple greenhouse gases, the atmosphere comes much closer to the blackbody idealization than it would if you had only one greenhouse gas by itself. He also points out that the 15 micron on CO2 are generally strongly self-absorbing and rarely involve transitions to intermediate states.
But I would also suggest that even where there are transitions to lower states of excitation, those which are closest to the ground state will also tend to be self-absorbing. Likewise, radiation which is downwelling will, upon reaching the surface will be reemitted with the same spectra as the thermal radiation of the surface, and not necessarily as photons of the same wavelengths as those photons which were downwelling.
First, a minor quibble with Tim’s characterization of my expertise on climate change. I am definitely not an expert, just a physicist trying to parse out some pretty involved physics.
Now to the issue of equipartition, etc. First, people are tending to ignore the fact that we are talking about huge numbers of molecules. Thus, even when we are talking about rare events, they will occur with some frequency. This includes collisional excitation. Even at altitudes of ~10-60 km, with temperatures around 240 K, ~1.8% of molecules have sufficient energy to excite the 15 micron vibrational line in CO2. So, I don’t think it’s really accurate to say that this mode is “frozen out”. Thus, the 15 micron radiation we see from space is actually coming from molecules high in the troposphere where densities are still high enough that collisions (and occasional Earthshine) excite the vibrational mode, and it is ‘thermal’ radiation as well as being quantized. Add in other molecules, and you get additional lines as well as broadening of all lines, and the spectrum of thermal radiation fills in, giving a better approximation of a blackbody specturm.
Alastair says, “Theoretically gases do not radiate Planck blackbody radiation, but despite that water vapor does!”
Then how do you explain the college-level development of earth energy balance using 2-3-4+ atmospheric levels all radiating ala Planck function? How do you explain solar radiation?
Timothy says,
That I’ll accept, even with Alastair’s O2 rotation microwave exception (that proves the rule…)
But I think not at the same level. (Theoretical) blackbody radiation is quantized like everything is at some level of n*h granularity; vibration, rotation, and electronic energy levels are quantized at massively larger steps caused by wholly different molecular action.
I understand. I also think I understand (??) that gases at low density may have far more “holes” than Planck radiation spectrum
Rod B. (#620) wrote:
Planck blackbody radiation is always an idealization.
However, gases will come closer to approaching this idealization at higher partial pressures (due to line broadening) and when they have a larger number of quantum states. Since water vapor is electrically dipolar, its quantum states include those that are pure rotation. It exists at a far greater partial pressures, and as such will experience greater line broadening. Given these two aspects it will come a great deal closer to blackbody radiation in certain parts of the spectra. Solar radiation is a little different since at this point we are talking about ionization. Molecular vibration and rotation would be insufficient to get us into the visible part of the electromagnetic spectra.
However, we are still talking about energy causing less energetic quantum states to enter more energetic quantum states which decay back into the less energetic – with matter consequently reemitting radiation. As such, there really isn’t a fundamental difference between thermal radiation which arises due to ionization and thermal radiation which arises due to strictly molecular mechanisms.
Rod B. (#616) had asked:
I (#618) had responded:
Rod B. (#620) responded:
Well, the “exception proves the rule” doesn’t work very well in physics. But we were talking about electric dipoles and infrared radiation. What Alastair brought up was a little different – as it involves a weak magnetic dipolar moment.
Pleas see:
Incidentally, this can come in handy in the search for life on other planets. If there is highly reactive oxygen in the atmosphere giving rise to this signiture, this suggests that there is a biosphere which has discovered photosynthesis – since oxygen won’t stay for very long at high levels without being constantly replentished by a biological source.
Rod B. (#616) had asked:
I (#618) had responded:
But I should explain a little further. The thread had more or less concluded that blackbody radiation and line radiation are simply two opposite poles along a continueem.
This included myself. See for example #298, see in particular my post #340.
In #340 had written:
This included Hank. Please see comments #339, , #341 above.
This included Ray Ladbury.
Please see #347:
Rod B. (#620) responded:
The different is a matter of degree, not kind. With solids you have more interactions between molecules which make possible a larger number of lines, bands and a larger variety of mechanisms for line and band broadening, but it is still pretty much the same thing, just more of it.
Rod B. (#620) responded:
Think of it this way…
With the interaction between matter and thermal radiation, you have two extremes and a continuem between them. The extremes are idealizations. One is strictly continuous blackbody radiation at all parts of the spectra. Even in the case of the reflective cavity with a small hole, this cannot be achieved – since to achieve the idealization the hole would have to be infinitely small. The other extreme consists of a single line which is infinitely thin at zero pressure and zero temperature – but there is always natural broadening due to quantum uncertainty, if nothing else. Two unachievable idealizations with the actual reality always being inbetween.
I (#618) had written:
Rod B. (#620) wrote:
It adds up, even though it is fairly complicated
Please see:
The Energy Balance and Natural Climate Variations
http://www.cara.psu.edu/climate/climatechangeprimer-pr4.asp
I (#618) had written:
As has been pointed out on a variety of occasions, while CO2 or other greenhouse gases can lose energy due to collisions with N2 and O2, they can also gain energy due to collisions with N2 and O2.
As Ray Ladbury most recently stated in #619:
This is what makes possible LTE conditions in which the Planck temperature, brightness temperature of reemission lines, and Maxwellian temperature are the same. If it were true that higher quantum states decayed to lower quantum states only at the exact half-life with collisions always taking place before , then collisions would prevent any reemission. But half-lifes don’t work that way. Energy is constantly being transfered by collisions, but reemission takes place according to a law of exponential decay where the states have no memory of how long they have existed. As long as a certain percentage are in an excited state at any given time, reemission will be taking place at a certain rate.
Additionally, we have plenty of empirical evidence which demonstrates reemission throughout the atmosphere. Please see my post #555.
Rod B. (#620) wrote:
Whereever it gets reemitted at whatever altitude reemission takes place, it has only two directions it can go: towards the surface or to space. And whereever it gets absorbed, assuming LTE, the same amount must be reemitted. Whatever gets lost to collisions will also be gained – and more will be gained in the troposphere due to heat transport through moist air convection in accordance with the tropospheric lapse rate.
Re where Rod B Says:
7 September 2007 at 10:15 PM
Alastair says, “Theoretically gases do not radiate Planck blackbody radiation, but despite that water vapor does!”
Then how do you explain the college-level development of earth energy balance using 2-3-4+ atmospheric levels all radiating ala Planck function? How do you explain solar radiation?
The college level courses AND all the text books are wrong. That’s why Timothy says “It’s still not adding up in my mind.” And it is why those ideas don’t add up in your mind. That is why you and Timothy are still questioning!
I presume when you say “solar radiation” you are referring to the fact that it is close to the spectrum of a blackbody at a temperature of 5800K. I explain that by saying that the pressure within the Sun is so great that the plasmas are so compressed that they produce atomic vibrations which cause blackbody radiation.
I could even argue that the surface of the sun is in fact liquid, as anyone can see, but then I would be called an idiot so I wont say that. (Don’t bother telling me that you think I am wrong because I already know that.)
However, how do you explain the Fraunhofer lines if gases produce blackbody radiation?
Sorry to be so agressive :-(
It’s because I have hit the bottle :-)
Cheers, Alastair.
OK, let’s go back to the blanket analogy: what greenhouse gasses do is keep energy from escaping from the surface to space. How does the blanket work? By decreasing the temperature difference between you and the air immediately surrounding you, because the energy you lose is proportional to the difference of the 4th powers of your temperature and that of the environment. So as the blanket heats up, net energy transfer to the blanket from the air around you slows and as the air heats up, net energy loss from you slows. And once the inner surface of the blanket heats up to roughly your body temperature, why is the blanket a good insulator? Because energy transfer across it is slow.
In the case of the atmosphere, you have lots of layers, and CO2 impedes energy transit. As the atmosphere heats up, net energy loss from the surface decreases–so there doesn’t have to be any energy transfer to the ground, just less energy lost.
Now as to the transition from line emission to quasi-continuum of blackbody radiation, think about what happens as you go from an atom, to a gas of atoms, to a liquid and eventually to a solid. Spectral lines broaden and coalesce until with a solid you have energy bands–ranges of energy the electrons can have, not just lines. You can see the start of this process in a gas with the broadening of spectral lines by interactions with surrounding molecules. The lines are still lines, and the atoms can only radiate for the same energy transitions, but the energies are a little smeared out. And how many molecules can radiate at those lines depends on how many have enough thermal energy–so it looks like a slice of blackbody just at those wavelengths. As you get more different molecules or more interactions as in a solid, the spectrum looks more blackbody. Finally the closest to a blackbody spectrum you get is emission from a small hole in a cavity. This is because you have not only the interactions of the gas and the walls of the container, but the interactions of the radiation field itself that come to equilibrium.
No, no, no, Timothy. Planck blackbody radiation is not “always an idealization”. A smooth spectrum output is an idealization (99% of the time). But Planck function blackbody radiation (loosely named to include whatever spectrum) is as real as it gets.
Couldn’t wait. Now to read the rest of the post(s).
Timothy (621), you say,
You (and others) are still confusing Planck blackbody radiation with line emission of, say, greenhouse gases. The former is thermal radiation (maybe if they officially called it that a pile of semantic debate could be avoided), generated by its own molecular processes; the latter is not thermal radiation (strictly speaking), does not have anything close to a “continuous” spectrum, and is generated by a different molecular process. I understand water vapor “line” radiation has near continuous quanta in certain parts of the spectrum (a slippery phrase as the spectrum is the crux of the observable difference) and looks a little like blackbody [thermal] radiation. But it ain’t.
You conclude this part of your post with
That’s a paradox nearing an oxymoron. Or, put another way, that’s wrong! Bear in mind the one and only full spectrum independent variable in Planck’s formulation is Temperature (the real kind). No molecular bonding coefficients; no molecular shapes or polarization; no molecular moments of inertia; etc.
I agree “… the “exception proves the rule” doesn’t work very well in physics…” but it makes the point and sounds good [;-) . I like and appreciate your helpful explanation of O2 absorption/emission.
Now, back to your first and main (and wrong…???) point. Hanks reference in 339 is addressing thermal radiation versus quantum physics, but in any case is a cacophonic forum with various folks doing the same debate we are having. It is helpful in this regard: It is clear that there is no fully complete understanding (agreement, really) nor a scientific consensus on the specifics of thermal radiation ICW greenhouse gas emission. Yet this is the cornerstone of global warming. Doesn’t smell right.
Back to the details: again the two radiation types are dissimilar. Gases, liquids, and solids can do and do both. (there’s still a debate on the gases piece.) Solids, the standard for blackbody radiation, also exhibits line radiation ala spectrographic analysis.
People seem to get all hung up with the “blackbody”. It’s true “blackbody” is an idealization (99% as I said) but the radiation is not. The confusion stems from the fact that the matter experiencing “blackbody” radiation usually has an emissivity (or absorption factor) less than 1.0 (leading to “graybody”, and they also can vary with frequency (leading to “graybody II”). But whatever its variance, thermal (blackbody) radiation originated from an entirely different molecular/atomic activity than line emission. Ray felt (347) that since “blackbodies” do not exist (they’re an idealization), blackbody radiation must be the same as “line” radiation. Not true. I’m sorry they chose the term “blackbody” to mean blackbody, graybody, and, theoretically, whitebody, and everything in between including the variance by wavelength. But it is what it is.
Timothy continues,
For the sake of brevity since this is getting quite long — wrong!
Timothy, with Ray’s help, says, in part,
Hey! We’re getting somewhere. I think this says I misread the statements regarding collisions beating emissions by millions-billions to one to mean re-emission is unlikely to happen. But it sounds like you’re saying the re-emission from greenhouse gases will happen. It just has to go through a million-billion-gazillion molecular energy transfers before. Does this mean that re-emission (granted different photon/different molecule) from CO2 will nearly equal its absorption — eventually?
I’m getting a long ways up the hill, but can’t yet make it over the top. By simplistic numbers there is 519 watts/meter-squared getting absorbed somehow into the atmosphere/clouds. Assuming it all eventually gets into a greenhouse gas and all more eventually gets re-emitted: half up and (eventually) out and half down and (eventually) absorbed — that’s 260 down, not the budget’s 324, and 260 up, not 195. Maybe I’m missing something simple, but it still doesn’t totally add up.
Alastair (622): you said “Theoretically gases do not radiate Planck blackbody radiation, but despite that water vapor does!”
I said, “Then how do you explain the college-level development of earth energy balance using 2-3-4+ atmospheric levels all radiating ala Planck function [T^4]? How do you explain solar radiation?”
Then you said, “The college level courses AND all the text books are wrong…. ”
So I say, “Well! That’s a helluva note!!”
It sounds like you now agree gases can radiate ala Planck because the Sun does. Granted it under pretty high pressure; but so what?
I’d have trouble with the surface of the Sun being a liquid, but a good case can be made for much of the interior Hydrogen being liquid — a superfluid at that maybe, if I recall.
You say, “how do you explain the Fraunhofer lines if gases produce blackbody radiation? ”
There is no reason under the Sun (pardon the pun) why a gas (or liquid or solid) can not experience both Planck thermal (blackbody) radiation and line/Fraunhofer radiation simultaneously. A common trait is a molecule absorbing its Fraunhofer line wavelength from its home body’s Planck radiation so the spectrum of the body we see is the continuous spectrum with characteristic narrow blank lines.
Ray (623) you might be on to something that will help lead me out of my dilemma. But something needs clarifying. You say, “…As the atmosphere heats up, net energy loss from the surface decreases–so there doesn’t have to be any energy transfer to the ground, just less energy lost…. ” How does this reconcile with the Earth’s steady state Energy Budget — the old 390 watts surface radiation leaving and 324 watts from greenhouse gases and clouds being absorbed.
Rod B (#634) wrote:
Rod, for an object to be a true blackbody, the absorptivity at all wavelengths would have to be exactly equal to 1. For an object to be a true grey body, the absorbativity would have to be less than 1, but a single constant for all wavelengths.
Anything else is what would be refered to as a realistic body. In this thread, see Bart Paul Levenson #180, Tim #258, Tim #265. In A Saturated Gassy Argument, most definitely check out the exchange between Ray Ladbury and raypierre 154.
Then check out what Ray Ladbury (#623) just said:
As I have pointed out, for this to result in a true black body, the hole in the cavity would have to be infinitely small. Anything else would fail to achieve equilibrium, however small the deviation. You yourself found it necessary to qualify your statement by means of the phrase “loosely named.”
Ray Ladbury,
You are wrong with both the points you are making.
The greenhouse effect is not like a blanket. A blanket is a solid which prevents the warm air escaping and being replaced by cold air. It has a low coefficient of conduction but mainly works by preventing convection. In the atmosphere greenhouse gases do not prevent convection. In fact they participate in it. What greenhouse gases do is absorb the thermal radiation from the surface and pass that energy to the other molecules in the air so warming it. The air at the bottom of the atmosphere is being continually warmed by the greenhouse gas absorption, not by conduction with a warm human body.
If blackbody radiation was produced by the smearing of emission lines then an iron bar would glow green hot. But it would not produce blackbody radiation at room temperature, because those emission lines are frozen out. And solid iron is a homogeneous non polar molecule so it cannot emit rotational or vibrational energy either.
Blackbody radiation is a type of continuum radiation just like Bremsstrahlung. Thermal blackbody radiation is produced by inter-atomic vibrations which only exist in solids and liquids. You really ought to consult your textbooks rather than making up science to fit your preconceptions.
Timothy,
You are correct that a blackbody is an idealisation, but blackbody radiation to which Rod was referring is not. It is also known as cavity radiation, and probably more correctly as thermal radiation since its intensity depends on the body’s temperature. Note that I say body temperature because only solid and liquid bodies can emit it. Gases do not and vapours only emit it when they condense into liquids.
Note, the intensity of line radiation emitted by gases is a function of their concentration. Line width broadening does not change the intensity of a line, only its width. So thermal radiation only applies to blackbody radiation. However, there is a problem using thermal radiation as a term because on earth the thermal radiation is in the infrared. Since the line radiation absorbed and emitted by greenhouse gases is also in the infrared, people tend to think that they are the same thing. They are not as I have explained to Ray above.
Alastair, ever hear of phonons? Electronic transitions are not the only energy states of importance–especially in a solid. Most of your problems occur because you have an incomplete understanding of the physics. In some solids, the charge carriers are not even electrons or holes, but rather bound states of electrons, holes and phonons with an electric charge a fraction of that of an electron. Blackbody radiation is not a continuum phenomenon–as evidenced by Planck’s need to keep the quantum of action nonzero to obtain the blackbody spectrum. Bremmstrahlung is much simpler–the interaction of a fast-moving single charge with a sea of charge. You can find a derivation of it in an elementary E&M text book. Try finding an elementary derivation of blackbody radiation.
Timothy, you just can’t get past the fact that the term “blackbody” as popularly used in talking about Planck functions is an unfortunate misnomer. It doesn’t mean the purist (and more accurate) definition of a true blackbody. It means any body that radiates ala Planck function. I admit I probably confuse things by switching between “blackbody” and “thermal” (which I would prefer), but I’m still using “blackbody” in the popularly accepted sense. So please stop refutting my “blackbody” function description by claiming I’m not using a perfect true blackbody.
Alastair (628), I agree with (but only 92%) and like your explanation. Makes sense. My only area of disagreement is the “only liquids and solids” part. I’m fully aware of the ongoing debate as to whether gases emit Planck thermal radiation. But one has no difficulty finding texts, papers, and forum posts that say they do, and clearly does not have to “make it up”. (One can find as well stuff that says they don’t.)
Alastair McDonald (#628) wrote:
Actually greenhouse gases are predominantly directly responsible for cooling the atmosphere. The exceptions are a small region in carbon dioxide and a wider range of altitudes belonging to ozone.
I pointed this out in #343:
What warms the atmosphere is primarily moist air convection. The greenhouse gases primarily warm the surface, giving rise to moist air convection.
Alastair McDonald (#628) wrote:
If its peak emissions were in the green part of the spectrum, one might say that it is glowing green hot – although if its peak emission were in the green part of the spectrum it would appear white to our eyes – just like stars.
Please see:
Why aren’t there any green stars?
http://www.astronomycafe.net/qadir/q72.html
However, even at room temperature, iron glows in the green part of the spectra – but very, very dimly – in accordance with its brightness temperature.
Please see:
Brightness temperature
http://en.wikipedia.org/wiki/Brightness_temperature
Alastair McDonald (#628) wrote:
Is ozone non-polar given the fact that it consists of three atoms of the same kind? (Yes.) Is carbon dioxide permanently polar? (No.) What about magnetic moments? (O2 has a small magnetic moment which is responsible for its ability to absorb and emit in the microwave region.) The more the different types of excited quantum states that are possible, the more the emission lines.
Any molecule consisting of only two atoms of the same kind cannot be electrically dipolar, although there may be magnetic moments giving rise to microwave emissions. However, all molecules consisting of three or more atoms will have quantized vibrational states – and even though carbon dioxide is not permanently dipolar, vibration results in it being temporarily dipolar such that it is capable of quantized rotational states.
Alastair McDonald (#628) wrote:
Bremsstrahlung radiation is caused by the acceleration of a charge, Alastair.
And in the strictest sense, blackbody radiation exists only as an idealization since absorptivity and emissivity are never equal to 1 for all parts of the spectra. All thermal radiation is realistic body radiation – where the absorptivity varies in different parts of the spectra.
Alastair McDonald (#628) wrote:
Is water vapor incapable of inter-atomic vibrations? Is CO2 incapable of such vibrations? How about ozone? As far as thermal radiation is concerned, liquids and solids and solids differ from gases in the number of different types of excited quantum states which are possible – given the proximity of the atoms to one another and their bonds. But even in the case of dusts and alloys the quantization is often nearly as sharp in certain parts of the spectra as that of the atmosphere.
Alastair, there are some aspects of your motivation which in all honesty I would prefer not to examine too closely – for my own sake. However, your motivation obviously has very little to do with science.
This forum is intended primarily so that people can learn about the science of climatology. So far you have contributed a great deal more to the obsfucation – and in the process contributed to those who would deny that it is even a science even though you are quite clearly capable of learning and making positive contributions. A good case in point is where you deny that carbon dioxide is capable of emitting except near the surface – where absorption in any part of the spectra is already saturated due to water vapor. But there are others. This post of yours is a good case in point. You have been deliberately obnoxious towards Ray Ladbury, a physicist – who was simply explaining some of the actual science.
I do have a few questions regarding your motivation, though.
Why have you chosen to present your “alternative science” here? Why not create your own forum? Your own blog? There are places where you could create your own wiki and blog with 100 megabytes for images and files – where there is no limit on the size of the wiki itself.
Is it because Real Climate appeals to an audience which you would be unable to attract on your own?
When both parties to a relationship benefit this may be described as “symbiosis.” When one party principally benefits to the detriment of the other, the term “symbiosis” no longer applies.
Many thanks to Timothy Chase for taking so much trouble in trying to explain things to me in his posts of #606 and #611. It must be irritating to have to explain things which are straightforward to you but are causing difficulties to a thicko such as myself. I had almost got to the stage of totally accepting your explanations while still being a bit muddled in my mind. You have certainly helped a great deal over certain matters but every explanation prompts me to further thoughts and questions. May I please ask for forbearance and ask for yet more help?
In #601 you wrote: “If the atmosphere is completely opaque at a given wavelength, what this will imply is that parcels of energy (think of them as photons)which leave the surface at that wavelength will stand a 50-50 chance of making it out of the atmosphere without making another trip back to the surface and warming the suface.” What this would imply to me and, judging from Raypierre’s original essay, is that you are actually defining 50% transmittance and not saturation. You referred me to an energy balance diagram which clearly shows that there is very substantially more downwelling radiation than radiation leaving the top of the atmosphere. Your statement seems to me to be incompatible with the data presented in the diagram. If you were correct, 50% would escape at first attempt, 25% at the next and so on – in other words a lot more than is the case according to the diagram.
In my ignorance, therefore, I am still defining opacity as indicating that nothing can get through the lower (possibly saturated or opaque) layers but, having read the thread carefully, I am suggesting exceptions to explain how the radiation escapes as OLR. You may think this extremely naive but I would be interested in your response, so here goes with my list of possible exceptions:
1) “The atmospheric window”- 40W/sq m from surface (not contentious except insofar as it might be somewhat, albeit slightly, narrowed by increasing CO2 – and nobody I have asked has come up with a figure for this).
2) Areas at either edge of the main CO2 absorption band where a combination of CO2 and water (with overlapping absorption spectra) would cause total saturation in the prescence of water vapour but not in a dry overlying atmosphere.
3) Photons in the embrace of CO2 molecules lifted above the water vapour layer by convection.
4) Possibly, photons in the embrace of water vapour released when the vapour condenses to water droplets in cloud. This may be Micky Mouse thinking – what actually does happen when water vapour becomes cloud?
Further sources of potential OLR are, of course, also available. Clouds, as black body radiators, apparently push 30 W/sqm through the atmospheric “window” while there is also 67W/sqm of solar energy absorbed by the atmosphere (not the surface) to work on. Some of the latter will be temporarily interfered with by CO2 but nothing like as much as at lower levels because of much larger gaps between lines and no complementary blocking by water vapour.
I am suggesting that most of the surface radiation remains at low levels of the atmosphere and a substantial part of OLR may never have got to the surface in the first place. I have come to this conclusion based upon my interpretation of discussion in RealClimate on global dimming but my logic might be flawed so please enlighten me if this is the case.
Apparently, aerosols cause global cooling, partly by reflecting solar radiation and partly by absorbing it in the atmosphere and preventing it from hitting the ground. I understand that carbon absorbs solar radiation but what it then does with it ,I’m not sure. I believe SO2 both reflects and absorbs. The shape of the latter molecule must resemble that of CO2 and I take it that it has a dipole moment and therefore rotates and vibrates with the best of them but fancies shorter wave radiation than the ghgs. The molecules presumably also, therefore, re-emit the photons absorbed. If these got to the surface, they wouldn’t cool us – they’d do the opposite. But clouds are black body radiators. Thus, one would expect that clouds might accept short wave radiation ,just as the surface does, and, instead of spitting it back out in its absorbed form, would emit a blackbody spectrum. Then, of course, water vapour also absorbs some solar radiation. If one has a layer of water vapour, energy hitting the top (from above) is much more likely to be bounced out upwards than it is to penetrate the layer and get to the surface, just as rising photons from the surface are much more likely to be returned there than to get through.
Timothy, when I suggested that 3.7 W/sqm (which rightly or wrongly, I understood to be the extra surface energy which would be retained by a doubling of atmospheric CO2 and would thus have to be got rid of at the top of the atmosphere to get back to equilibrium)) was small as a percentage of the total energy flux and when set against other possible variables, you asked me what variables I had in mind. You proceeded to assume that it was solar variability and went on to dismiss this. However, though I might have misled you, this was not what was in my mind. RealClimate’s posts on dimming suggest that, between the 1960s to 1990s, aerosols reduced surface absorption of solar energy by 7W/sqm, nearly twice the amount but in the other direction than is claimed for CO2 doubling. I fully accept that some of this could have been an albedo effect but, I suspect, not the majority. It is therefore clear that switching solar absorbtion from the surface to the high atmosphere has a cooling effect on the surface. I also think it likely that absorption of IR by high level CO2 may be less warming than absorption at low level.
Other variables include ozone at tropospheric and higher levels, effects of clouds and even man’s extra release of surface energy (though I have been led to believe that the last is insignificant).
I would just say that, as a biologist, 3.7 as a percentage of 235 or 390 (whichever you want to choose) is a very small percentage to rely upon to be biologically significant. I can, in this instance, see the logic of the science. It’s the somewhat arrogant precision given to the numbers that worry me. CO2 is undoubtedly a greenhouse gas and its increase is man made. While not a fan of the precautionary principle, I can see that, in this scenario, the stakes are as high as they could be and can thus sympathise with its application.
I (#621) wrote:
Rob B (#625) responded:
A great many others are “victims of this delusion.” Typically they have a strong background in physics. (I would be an exception to that rule.)
Rob B (#625) wrote:
And why do you claim that reemission by water vapor isn’t thermal radiation?
Because you choose not to use the word “thermal”?
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I (#621) wrote:
Actually I should have said “… between thermal radiation which arises due to jumps in electron orbitals (up to higher energy states at absorption and down to lower energy states during emission) and thermal radiation which arises due to strictly molecular mechanisms.”
Not thinking, not clearly…
… never mind.
Rob B (#625) wrote:
No structure, no matter…
… only idealization.
Rob B (#625) wrote:
No problem.
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Rob B (#625) wrote:
There are a couple of important differences.
TeV, although more knowledgable than the fellow he is helping, probably hasn’t obtained his master’s degree as of yet. The fellow he is speaking to clearly understands that TeV is more knowledgable. But despite their general lack of expertise, TeV clearly understands that the closest thing we have to blackbody radiation is quantized, with transitions between discrete states being the cause of both absorption and emission.
Here, on the otherhand, you have people like raypierre, Eli, Ray Ladbury, Bart Levenson…
People with advanced degrees in physics willing to educate those who are less knowledgeable than they are. raypierre has a doctorate, Eli has a doctorate, Ray has a doctorate. Then you have people such as Alastair and yourself who are intent on denying that there is any such thing as expert knowledge in physics. People who have not obtained even the most basic degrees in the area and who are unwilling to acknowledge the expertise of those who have obtained PhDs in relevant fields.
Interesting contrast.
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Rob B (#625) wrote:
When it comes to radiation transfer, I doubt that you would be able to find anyone with a relvant degree (which I presume would include graduate level quantum mechanics) who would fail to acknowledge that the closest thing we have to blackbody radiation in the real world arises from transitions between discrete quantum states.
Do you know of any?
I believe quantum mechanics would be a prerequisite as the result of the quantized nature of electromagnetic energy and the quantized nature of matter.
Don’t you?
Of course, if you wanted to you could deny that matter and energy are quantized. You do have that latitude – if you wish to take it. Or you could simply claim that there is no scientific consensus in the matter. In fact, you could claim that the world is less than 10,000 years old or that the world is flat if you really wanted to – or if you prefer – that there is “no consensus” since not absolutely everyone agrees, and therefore the only objective position is to take is that these are still open questions.
Will you? Which ones?
Why not?
I suppose you could just claim that you can’t stand the stench.
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Rob B (#625) wrote:
When the absorptivity varies within the visible part of the spectrum, the object is typically colored. Would you call something colored grey? When the absorptivity varies over different wavelengths, the object is said to be a realistic body with a realistic body spectrum.
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Rob B (#625) wrote:
Ray is a physicist with a background in radiation transfer. What is your expertise?
Your entire argument at this point turns on the claim “thermal (blackbody) radiation originated from an entirely different molecular/atomic activity than line emission.” Alright, what is the activity?
Vibration and rotation, and in the case of extremely hot gases, liquids and solids, electrons jumping up to higher energy orbitals upon absorption then back down to lower energy orbitals upon emission. It is the same. The only real difference is a matter of degree, principally in terms of the number of weak and strong bonds, the number of different quantum states, the complexity of the broadening mechanisms and the degree of broadening.
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Rob B (#625) wrote:
Actually I said it with Alastair’s help.
Back in high school I had picked up the two-volume Quantum Mechanics by Claude Cohen-Tannoudji, Bernard Diu and Franck Laloe, an english translation of the french. This is what I taught myself from, informally. Then I lost it on a bus while I was in the navy visiting Honolulu. In any case, it had been a very long time and I was rather rusty – so certain pieces which should have fallen into place didn’t – until Alastair used the term “half-life” in relation to reemission.
Then I responded in #500:
So what I wrote was confirmed by Ray Ladbury but I had put it together independently of him.
As I said, it was Alastair who gave me the missing piece.
I learn from others. I put things together. I try to understand.
What are you doing?
Rob B (#625) wrote:
How long is the half-life of a given state?
Roughly that long.
Exponential decay with no memory of how long any given molecule has been in a particular state.
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Rob B (#625) wrote:
Well, let’s look at the diagram I pointed you to:
The Energy Balance and Natural Climate Variations
http://www.cara.psu.edu/climate/climatechangeprimer-pr4.asp
Sunlight coming in is 342 watts per square meter. Reflected solar radiation going out is 77 off of clouds and 30 off of the surface. Outgoing longwave is 235. 342 = 77 + 30 + 235.
What is coming in equals what is going out.
Ok then.
Let’s drop the reflected and just look at absorbed radiation. That’s the 235.
The radiation which is absorbed by the atmosphere before it reaches the surface is 67. That which is absorbed by the surface is 168. 235 = 67 + 168.
What is entering the climate system as thermal energy equals that which is absorbed by the atmosphere from space plus that which is absorbed by the surface from space.
All right.
Let’s look at the radiation being absorbed by the atmosphere. We have 67 coming from space, 24 coming from the surface in the form of thermals, 78 coming from evapotranspiration, then 350 coming from surface radiation – with 40 watts per square meter from the surface passing through the atmosphere without being absorbed. 67 + 24 + 78 +350 = 519. Now once these watts have been absorbed by the atmosphere, where do they go?
165 goes from greenhouse gases to space. 30 goes from the clouds to space. 324 is back radiation to the surface. 165 + 30 + 324 = 519 — which is the same amount of energy being absorbed by the atmosphere.
What is coming in equals what is going out.
Let’s see…
Now we need to look at the radiation being absorbed by the surface.
168 is coming directly from sunlight and another 324 is back radiation (reemission) from the atmosphere. That’s 492.
Now let’s look at what is leaving the surface after having been absorbed. 24 from thermals. 78 from evapotranspiration. 390 from surface radiation (i.e., thermal emissions from the surface). That is 24 + 78 + 390 = 492.
What is coming in equals what is going out.
Rod, it all adds up.
Douglas,
I don’t consider it a bother when someone is genuinely seeking the truth. But even when this is not the case, discussions will oftentimes give me the opportunity to clarify things in my own mind. I knew basically nothing regarding the greenhouse effect back in May – and I am still learning, both in terms of my understanding and in how I express various ideas.
Douglas Wise (#633) wrote:
That statement was wrong, but I wasn’t taking the time that I probably should have in responding to you. Actually what stands a fifty-fifty chance is whether upon emission a photon will be upwelling or downwelling. Upwelling takes it towards space, downwelling towards the surface. But most of the radiation emitted by the atmosphere near the surface will be reabsorbed by the surface – if for no other reason than the fact that the surface is much closer, and a random walk is more likely to make it to the surface than to space – given an initial low altitude.
In fact, the halfway point as far as radiant energy is concerned is roughly 6 km in altitude – although it has been climbing as has the boundary between the troposphere and the stratosphere known as the tropopause. This halfway point is what is refered to as the “effective radiating level” or “layer.” It has the “effective radiating temperature” which is the temperature that the earth would appear to have as seen from a distance given its thermal emissions to space.
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Douglas Wise (#633) wrote:
How would that work?
Your view would seem to imply that reemission never takes place. But if thermal radiation is absorbed, it will be reemitted.
Energy may be lost to collisions, but energy may also be gained by collisions – resulting in reemission. If it is absorbed and reemitted a fair number of times, eventually it will make it out of the atmosphere simply as the result of a process which is essentially a form of diffusion. But there are other added complications which will tend to make things move a little more quickly than this might suggest. You were here for part of it: transitions from higher to lower states of excitation – where at least some of the energy which gets emitted is unlikely to be reabsorbed.
The other effects you mention (e.g., the atmospheric window, moist air convection, the freeing of latent heat) will also undoubtedly make it easier for energy to make it to space, but they aren’t strictly necessary.
In essence, when adding carbon dioxide or other greenhouse gas to the atmosphere, by making the atmosphere more opaque to thermal radiation, we are disturbing an equilibrium in which the rate at which energy enters the climate system equals the rate at which energy leaves the climate system. We are doing this by increasing the resistance to the flow of thermal energy from the surface to space, lowering the rate at which this occurs. So long as the rate at which energy leaves the system is lower than the rate at which energy enters the system, the temperature of the climate system must increase.
As the temperature increases, the rate at which energy is radiated increases – to the fourth power of the temperature, I believe. As such, the climate system will achieve a new equilibrium despite the increased resistance to the flow of thermal energy, but only as the result of it having a higher temperature which compensates for the increased resistance.
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Douglas Wise (#633) wrote:
Sunlight which is not absorbed but which is simply reflected back into space has in a very important sense never made it into the climate system. It isn’t thermal radiation, at least not in relation to the climate system itself. As such, some aerosols will cool the climate system (temporarily masking the effects of added greenhouse gases) by reducing the amount of sunlight which gets absorbed and converted to thermal energy.
When you speak of black carbon, since this has a lower albedo, this implies that more sunlight gets absorbed and added to the thermal energy within the system. Black absorbs light making things hotter. Sulfur dioxide primarily increases the albedo – although the effects may be different depending upon the source of the radiation. While clouds are “blackbody” radiators, this is true primarily in the longwave. If they were blackbody radiators at all frequencies, given the temperature of the atmosphere they would appear black, absorbing all visible light but reemitting almost entirely within the infrared.
Since they are typically a lighter shade than that, they can’t be blackbody radiators in the visible part of the spectrum. As such, while they increase the greenhouse effect with what thermal radiation already exists within the climate system, they also reduce the amount of thermal energy which enters the system by scattering visible light back into space. Two competing effects. Which one wins? Depends upon the type of cloud.
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Douglas Wise (#633) wrote:
How would that work?
The peak emission for the sun is in the yellow part of the spectrum. Relative to this it actually emits fairly little infrared. Since greenhouse gases are more or less transparent to visible light, they do not absorb visible light. So for the energy to enter the system as thermal energy which gets radiated away by greenhouse gases, it has to be absorbed somewhere else. Generally the surface. Maybe black carbon before it has a chance to settle on sea-ice or some glacier. Perhaps a dark cloud. But generally the surface. Although I can think of one important exception: ozone absorbs ultraviolet radiation in sunlight – warming the upper atmosphere.
*
Anyway, don’t worry too much about being new to this.
As I’ve said, I am pretty much in the same boat. As such, I am quite grateful that there exist exceptionally bright and talented individuals like the climatologists here who know a great deal more than I do. In addition to everything else, it means that I have people I can learn a great deal from.
Perhaps, I should clear up something. First, I did not mean to imply that blackbody radiation results from spectral broadening. Blackbody radiation is the distribution that results from the radiation field being in thermal equilibrium. However, photons do not interact with eachother, so the only way the radiation field can come into equilibrium is by interacting with matter in its vicinity. Spectral broadening brings the radiation field closer to blackbody, because it means the emissivity is nonzero over a larger proportion of the spectrum.
Ray,
I looked up phonons on Wikipedia and it seems to me that they are the atomic oscillations in the crystal lattice of a solid that are generating the blackbody radiation.
Spectral broadening does produde a continuum spectrum over the band, but does not correspond to a Planck curve. The 15 um band has a spike in the centre and troughs on both sides.
Rod,
It is true that many (all) climatology say that greenhouse gas emissions depend on temperature just like blackbody radiation. However any proper science (physics or chemistry) textbook does not make that mistake.
Timothy,
You are correct that ozone is a greenhouse gas, and so is carbon dioxide but iron is not. It radiates continuum radiation and the greenhouse gases radiate line radiation. They originate in two very different ways, as does yet another form of radiation Bremsstrahlung.
However, thanks for pointing out that “But half-lifes don’t work that way.” That has given me the clue I have been looking for. I may not have to pick everyones brains any longer :-)
Alastair, Yes, I know what the 15 micron band looks like. The emissivity is nonzero in that band and in the other absorption bands and zero elsewhere for CO2. The thing is that pressure effects, etc. broaden the line–effectively extending the region where the emissivity is nonzero. What you will find is that if you convolute the emissivity with the blackbody spectrum for the proper temperature, you get the appropriate emission spectrum. This is why you get different effective temperatures for different ghg lines–the emissions are coming from different altitudes at different temperatures.
For those who are interested: a quick cite for the thermal absorption spectra of pure crystals…
Fairly old, but people don’t seem to find pure monoatomic crystals that interesting anymore and focus on the more puzzling multi-atomics and alloys.
#638
Ray,
Yes, the current idea is that the greenhouse gases emit depending on their temperature, but that is not what the physical chemistry books say.
The climatologist say that, since the 15 um band roughly matches the 215K blackbody curve, then that is the temperature of the layer where it is emitting. That is around the tropopause, but I though that we had agreed it came from higher in the atmosphere, i.e. the thermosphere.
Thanks, Timothy, for your post #635 in response to mine (#633).
I think I’m moving on and understanding more. I also think that I probably expressed myself badly in the last post – probably due to the absence of a hen-pecking wife who was away judging gundog tests in Germany and hence unable to monitor my whisky intake.
I tried to suggest that I defined opacity (or saturation)of a particular waveband as indicating that nothing (in terms of that waveband) could get from the surface to the top of the atmosphere. You effectively (but more politely) suggested that this was rubbish and indicated that I must therefore be denying the existence of re-emission.
When I first read the essay initiating the thread, I accept that I assumed that photonic energy grabbed by a ghg molecule would be converted to non radiative thermal energy. I now accept that a chain of events occurs which leads to a variety of outcomes which involve local heating of the atmosphere, re-emissions of photons of longer wavelengths than those absorbed in the first place and a limited number of re-emissions of photons of the original wavelength.
Because of my acceptance of re-emission, I have a slightly different mental picture of CO2 waveband saturation which I will explain thus (I’m not allowing for convection because I think it might complicate matters): As CO2 concentration increases, gaps between lines in the central area of the band start absorbing relevant IR and, furthermore, the two edges of the bandwidth start coming into play as well. Re-emission occurs (half up and half down). As this is repeated for every ascending layer of atmosphere, an ever smaller proportion of photons will get through until the number is so vanishingly small as to be effectively zero (=100% absorbance or 0% transmittance). I believe that I have interpreted Raypierre correctly when I suggest that this is the situation that obtains for CO2 in the 13.5-17 micron part of its absorption band. However, I don’t think you agree with my interpretation and cannot quite understand why.
I accept that one can identify OLR in the 13.5-17 micron band and it is therefore necessary to explain where it’s coming from. I don’t necessarily see this as a problem. In attempting my explanation, may I use the concept of the “effective radiating layer” that you raised in your post? I believe OLR brightness temperatures differ for different IR wavelengths (depending upon which ghgs are interfering) and give an indication of the mean temperature (and thus mean altitude) at which emission has occurred. Alastair has previously given me a cite that demonstrates this (and Ray has confirmed it in post #638). Alastair says that anything coming direct from the surface to space should have a brightness temperature of 300K. The closest we get to this is approximately 290K for IR travelling through the atmospheric window. This is made up of 40/70 from the surface and 30/70 from cloud tops. OLR of wavelengths which are fully saturated by water vapour in the lower atmosphere show a brightness temperature of 275K. (4/7 x 300 + 3/7 x 275) roughly equals 290 – as one would expect.
Water vapour can become cloud and cloud is a blackbody radiator in the IR range. I would suggest that a good proportion of the total solar energy (67W/sqm) absorbed by the atmosphere ends up in cloud and will be emitted over a wide wavelength range unimpeded by water vapour. Its range will also include the 12-18 micron bandwidth which will be somewhat impeded by overlying CO2. At the top of the water vapour layer, even without conversion to cloud, IR of wavelengths normally absorbed by water vapour might find escape to space on re-emission easier. This would certainly be the case if the water vapour column was somewhat higher that the length required for saturation, implying that any downwelling radiation would experience much more interference than upwelling. It is thus plausible to rationalise that most OLR in the water vapour absorbing range is emitted at the wet/dry interface of the atmosphere at a brightness temperature of 275K.
The question remains as to where the OLR in the CO2 absorbing range is coming from, particularly that in the 13.5-17 micron part of the band. I have already suggested that some will come from cloud (indirectly via atmospheric solar absorption) and, though the atmosphere may be CO2 saturated from 13.5-17 microns from the ground up, its top slice won’t be. Some IR will therefore get through gaps. I would also like to suggest that photons absorbed in the 8-13.5 bandwidth might result in re-emissions of others in the 13.5-17 micron range. Finally, non radiative thermal energy high in the atmosphere, having arrived by convection, will stimulate CO2 to spit out some photons. It seems clear that all this must take place somewhere between the cloudtops and the top of the atmosphere, giving a brightness temperature of only 215K.
I think it might be tempting providence to write QED at this stage. In any event, I am not trying to suggesest that the modellers are wrong, only trying to rationalise the mechanism by which the effects they predict could become manifest.
I would like to deal with one other issue where you seemed to disagree with a statement I made. I suggested that switching solar energy absorption from the surface of the planet to the atmosphere would have a cooling effect (on the surface). You asked how that would work. I think it self evident but I also think you may have misunderstood what I was attempting to infer, partly because I somewhat misguidedly linked it in with global dimming which I now wish I had never mentioned because it was something of a red herring. I wasn’t meaning “cooling” as such. I think we’d probably agree that 235 Wsqm hitting the surface would produce more warming than if only 168 Wsqm of solar energy did so with the residue being absorbed in the atmosphere. If you don’t agree, I’ll give an explanation for my thinking. You yourself said that it was obvious that IR leaving the surface was much more likely to be re-emitted back there than to escape to space and that is, indeed, self evident. By the same token, I would argue that IR emitted from cloudtops would receive less interference by going up than down and thus a higher proportion would go to space than to the ground. Same logic as you were using (I think).
I am now happy that I have got my mind sorted and can square what I understood Raypierre to be saying with what the rest of you have been telling me. If you don’t agree with my rationalisation, please let me know. I appreciate that all those who have answered my previous questions know a great deal more about the subject than I do. Nevertheless, you all emphasise that you are not professional climatologists. Given that this website was purportedly designed to explain the science to the uninformed layman, it seems unfortunate that there has been next to no feedback on this thread from the professionals who initiated it.
Timothy, I’m still working on what hopefully is a cogent continuation of our debate (partial). I tried an earlier intro, but it was less than cogent and the moderators rightly canned it.
One sticky point we have, I think, is semantics: There is a process that generates a nominally continuous spectrum of electromagnetic radiation with intensities that vary with wavelength. This radiation stems from the acceleration/collision of free electrons and the translation type vibration of whole atoms and molecules, and is a function of the thermal temperature of the body doing the radiating. It is not caused by electrons jumping among intra-atomic energy levels, though that too causes EM radiation; it is not caused by the relaxation of molecular energy states tied to the bond vibration or the rotation of molecules, though that too causes EM radiation. The free electrons and/or whole atoms or molecules can likewise absorb EM radiation. Now this radiation, which Planck stewed over, tends to have a pure continuous spectrum but for a number of case-by-case reasons often does not make it — sometimes missing by a lot. Further this radiation depends on the material(s) making up the body and is also neither a perfect radiator nor absorber, though some things get within a hair, a bunch get real close, then others miss it a mile, and everything in between. “Missing” means that, by the theory that eventually came out of Planck’s, et al, work, some bodies radiate less energy at a particular wavelength than their temperature would indicate (which can be viewed as either a decrease in expected radiation intensity, or and “effective” decrease in temperature of the body.)
This type of radiation is commonly called “blackbody” or “graybody”, which admittedly are very loose terms bordering on misnomer, not conducive to detailed scientific discourse, and terms you don’t accept. So that we can talk on the same wavelength (bad unintentional pun), what would you like to call this particular type of radiation/absorption?
Anyone else have a view?