Abstract: This post describes a new approach to calculating the CO2 greenhouse effect. Instead of calculating radiative transfer from the surface up through the atmosphere to space, exactly the opposite is done. IR photons originating from space are tracked downwards to Earth in order to derive for each wavelength the height at which more than half of them get absorbed within a 100 meter path length. This identifies the height where the atmosphere becomes opaque at a given wavelength. This also coincides with the “effective emission height” for photons to escape from the atmosphere to space. A program has been written using a standard atmospheric model to perform a line by line calculation for CO2 with data from the HITRAN spectroscopy database. The result for CO2 is surprising as it shows that OLR from the central peak of the 15 micron band originates from high in the stratosphere. It is mostly the lines at the edges of the band that lie in the troposphere. The calculation can then show how changes in CO2 concentrations affect the emission height and thereby reduce net outgoing radiation(OLR). The net reduction in OLR is found to be in agreement with far more complex radiative transfer models. This demonstrates how the greenhouse effect on Earth is determined by greenhouse gases in the upper atmosphere and not at the surface.
The CO2 emission height profile for 300ppm and for 600ppm smoothed with a resolution of 20 lines.
This post looks in detail at the emissions to space by CO2 molecules from the atmosphere. The main CO2 absorption band lies at 15 microns. It is composed of hundreds of quantum transitions between vibrational states of the molecule. The reference database for the strengths of these lines is called HITRAN and is maintained by Harvard University[1]. I requested a copy of this database and have been studying it. Fig 1. shows in detail the transition lines within this band and Fig 2. shows the fine detail within the central spike. The line strengths are recorded at 296K in units of cm-1/(molecules cm-2) corresponding to the absorption cross section for one molecule in vacuum.
Figure 1: The fine structure of transition lines that make up the 15 micron CO2 absorption band.
Figure 2: The fine detail of the central spike of lines at 15 microns. It is itself composed of some 30 individual lines. The small peaks are molecular isotopes.In the real atmosphere these lines are broadened due mainly to motion of molecules. This is a rather complex subject but luckily I found a Fortran program [2] which takes as input the line strengths from HITRAN and then integrates them over pressure to derive a net absorption cross section per Mole of CO2. This result is shown in Figure 3. Notice how strong the central peak now becomes with 2 clear side fans of absorption with fine structure.
Figure3: CO2 absorption cross-sections in the atmosphere per mole.To make progress to locate from exactly where IR is emitted to space we need a model of the atmosphere. For this we assume a standard lapse rate of 6.5C/km up to the tropopause at 11 km, then stationary temperatures through to 20 km followed by a linear increase of 1.9C/km in the stratosphere until 48 km above the surface (see fig 4).
Fig 4: Temperature profile and molar absorption cross-section through a 100meter section of theatmosphere.The barometric pressure profile is taken to be
scale = RT/($molar*$g);
P[h]= P0*exp(-h/scale);
The objective now of the calculation is to take each CO2 transition line in turn and then descend from space to find at which altitude the absorption of photons of that wave length within a 100m thick slice of the atmosphere becomes greater than the transmission of photons. We define this height as the transition between opaque and transparency. This is the height at which thermal photons within the CO2 absorption bands are free to escape to space. – the effective radiation height. The absorption rate is simply the molar cross section times the numer of moles of CO2 contained in a 100 meter long cylinder of cross-section 1m^2. A graph of emission heights versus wavenumber is shown in figure 5a for a CO2 concentration of 300ppm in black and 600ppm in red. Fig 5b is a smoothed average over 20 adjacent lines.
Fig 5a :The effective emission height for CO2 in the atmosphere for concentrations of 300 mm (black) and for 600ppm (red). Fig 5b: The CO2 emission height profile for 300ppm and for 600ppm smoothed with a resolution of 20 lines.Note how it is mainly emission heights from the side lines which lie in the troposphere. The emission height of the central peak actually lies in the stratosphere with the central spike reaching up to 25000 meters where the temperatures are actually increasing with altitude. As expected doubling CO2 concentrations rises the emission height significantly but the effect on radiation loss depends on the temperature difference beween the old emission height and the new emission height. Below the emission height, radiation in CO2 bands is in thermal equilibrium with the surrounding atmosphere. This is usually called Local Thermodynamic Equilibrium (LTE). The lapse rate of the atmosphere is driven by convective and evaporative heat loss from the surface, but energy loss to space can only occur through radiation. So the local temperature from where IR photons escape to space determines the radiation flux for that wavelength. The temperatures at the emission heights for CO2 are shown in figure 6.
Fig 6. The temperature at CO2 emission heights for 300ppm and for 600ppm. Note how the central peak actually increases emissions with a doubling of CO2.The effective temperature of the emission height now allows us to calculate the planck spectrum for the CO2 lines. The result is shown in Figure 7.
Fig 7: Calculated IR spectra for 300ppm and 600ppm using Planck spectra. Also shown are the curves for 289K and 220KSo how does this compare with a real spectrum as measured by satellite ? Figure 9 shows a spectrum taken from NIMBUS. There is an overlap with the water vapour contiuum lines below 550 cm-1, which reduces the left shoulder. But apart from that the agreement is really rather good, and in particular note the upward spike at the centre of the line corresponding to emission from the warmer stratosphere. Similarly the flat bottom corresponds to the tropopause at around 216K.
Fig 8: Spectrum taken from Nimbus ii over the equator. The fine structure in the CO2 absorption bite shows the same dependence as predicted within measurment resolution.Finally now we can make an estimate for the radiative forcing due to a doubling of CO2. To do this we first derive the net change in outgoing IR from an increase in CO2 from 300ppm to 600ppm as shown in Figure 9. Note how for the central peak the radiation actually increases for a doubling of CO2 as they emission height lies high up in the stratosphere. This is because temperatures are actually increasing with height.
Next we integrate the change in the radiative flux over all lines in the CO2 band going from 300ppm and 600ppm concentration. The result of this integration works out to be 1.17 watts/m2/sr.
Fig 9. Change in radiance flux shown as 300ppm – 600ppm.However, to derive the net change in OLR we need to integrate this over the outgoing solid angle for photons that reach space. Quoting from Wikipedia.
The integration over the solid angle should be the half sphere of out going radiation. Furthermore, because black bodies are Lambertian (i.e. they obey Lambert’s cosine law), the intensity observed along the sphere will be the actual intensity times the cosine of the zenith angle
, and in spherical coordinates,
.
This then adds a factor 
So finally the reduction in outgoing IR radiation caused by a doubling of CO2 from 300ppm to 600ppm becomes 4.7 watts/m2. This is not far away from the value as calculated by climate models – 3.7 watts/m2 ! This is usually called “radiative forcing”. Note how in the stratosphere the energy loss increases with CO2 concentration. This predicts that the stratosphere should cool, as the troposphere warms. All predictions of warming/cooling are of course based on the assumption that all else remains constant – lapse rate, H2O, clouds etc. The real signature for a CO2 GHG effect would be to observe cooling in the stratosphere where these effects are much smaller.
In the next post I will examine in detail how “radiative forcing” depends on CO2 concentrations.
References
1. http://www.cfa.harvard.edu/hitran/
2. http://home.pcisys.net/~bestwork.1/CalcAbs/CalcAbsHitran.html
Hi Clive,
Great article! 🙂
I have a question. Above figure 6 you state …
“As expected doubling CO2 concentrations rises the emission height significantly but the effect on radiation loss depends on the temperature difference beween the old emission height and the new emission height.”
This doesn’t make sense to me. In my mind, LTE would not be a 2d surface, but a 3d mass of air that will be larger as the thermosphere expands, much like a heat sink that can increase it’s surface much more than in a strictly vertical sense.
Thanks Eric,
Yes, you are quite right. LTE really covers a 3d shell of finite thickness. In my code I assume each layer to be 100m thick. So really the heights I am talking about should be at the centre of each shell and define the temperature of the surrounding atmosphere within 100 m of that height.
I agree that the wording should be changed .
The atmosphere would indeed expand a little if the local temperature increases. I think convection would maintain the lapse rate. However the lapse rate itself could change with an increased relative humidity. These are just the known unknowns.
I think the wording is fine, but that the IR surface expands in 3 dimensions and that the integration procedure would be more accurate if it accounted for that?
Looking at the model, it seems clear that the IR surface/manifold? is much rougher with 600ppm of CO2. This is clearly shown in 5b that the effective emission height is not only higher, but has much greater variance. This also makes sense intuitively to me. Sort of like a puffer fish whose spines would also extend along with his body. http://animals.nationalgeographic.com/animals/fish/pufferfish/
So while the average temperature goes down, the surface area increases greatly.
My mind starts aching when trying to follow the integration procedure, but would it not be possible to account for the increased IR surface area in the integration? Perhaps spreading including a probability with each shell and integrating over the vertical?
You are right – there is a small change in surface area with height but I don’t think that is why the 600 ppm has greater variance particularly in the troposphere. The surface area doesn’t really change that much because the centre of the earth is so far away. 4piR^2 and 4pi(R+dr)^2. I think it may be because the pressure (density) change is exponential.
I will put the code on-line when I have finished and you will see how simple it really is !
Clive
I’d like to look at some code, although I can assure you that it’s not simple (for me at least :)). Thank you for entertaining my questions and for publishing this online. 🙂
Clive, you have written some great posts on GHE and CO2. Did you put the Fortran code online? If so, how can I download it? Could you, or have you, also made your HITRAN input data available? If I use either, I will acknowledge you. My main objective is to make some illustrations I need. All the best.
Clive,
Thanks for devoting the time to do this analysis. It gives me confidence that when we are talking about the “no feedbacks” case we know what we are talking about (no data fiddling involved). If you get some free time you might look at the difference between the “Dynamic Tropopause Potential Temperature” and surface or near-surface temperature. Any relevance for climate sensitivity?
http://climaterealists.com/?id=11197
Regards
Hi Clive,
Perhaps a simple way of looking at the effect of GHGs are by considering each molecule a small IR transmitter/receiver, with a small delay (and chance that energy gained in reception) is given up in a collision w/O2 or N2. Since the direction of transmission is random, the molecules are transmitting toward space approximately 50% of the time, while they are receiving (a briefly storing) energy from all directions (hence 100% of the time). The density of GHGs (not any particular one, because thermal equilibrium ‘shares the energy’ between all species present, not just the GHG active at band for a given event) thus determines the mean free path for IR moving through the atmosphere, hence the number of absorb/emit events, the total delay and hence the energy per unit volume (and temperature via the energy/temperature relation for the species).
In this view, all other things being equal, the temperature must decline with altitude because energy per unit volume declines with altitude (until it is so low that UV – O2/O3 starts to make a significant contribution), given that 50% of emission events in a layer result in ‘permanent’ loss of the energy to a higher altitude and hence ultimately to space.
I’m not sure that worrying about the frequency of the emission to space ultimately matters that much, unless you are trying to match the ‘last look’ NIMBUS spectrum data, for example, because presumably a given quantity of energy will have spent part of it’s trip up from the surface manifested in any number of frequencies, given the range of active frequencies and the thermal equilibrium at each absorb/emit point.
On the other hand, is it possible that a quantum of energy emitted upward at a wavenumber of 680 is trapped there and will be making it’s whole trip being absorbed/emitted at that wavenumber?
As a thought experiment, would it make a difference to the temperature profile of the atmosphere if all of the CO2 in the lower atmosphere were replaced by H2O and CO2 were somehow confined to only the top layers that are not 100% opaque to space (in the present proportions so the NIMBUS data looks the same)?
Thanks for you tremendous help in understanding how this works… I appreciate your insights.
Regards,
Randall
Randell,
Sorry for the delay- I only just saw your comment. Your general analysis is spot on. However I think the frequency does matter because the mean free path for an IR photon depends both on density and absorption cross-section. The cross-section varies strongly with frequency. So for example the quantum line for CO2 at exactly 15 microns has so large a cross-section that the atmosphere is opaque until way up into the stratosphere. Other lines are much weaker and radiate to space at low altitudes. So even if much of the CO2 is thermally excited rather than directly by photons, the energy loss upwards depends on density and frequency.
Regarding your thought experiment. In the lower atmosphere about 2/3 of heat flow upwards is by convection and evaporation, and only 1/3 is by radiation. This proportion changes as you go upwards until eventually at the tropopause ALL energy loss upwards is by radiation. In that sense changing the H2O/CO2 mix may change these proportions and both the height of the tropopause AND by implication the surface temperature will change. The dry adiabatic lapse rate would remain the same although the environmental lapse rate would fall as there is now more water vapor.
regards
Clive
Hi Clive,
I think the statement “energy loss upwards depends on density and frequency” might well be right at the heart of the matter. Consider 2 end-point scenarios, one where frequency does not matter and the other where density does not matter. Also, to make this simple to think about (at least for me), consider that each of these end-point scenarios has an analogous simple electrical circuit, where the voltage corresponds to energy difference, the current to heat flux, and the resistance to specific GHG influence (which is a function of density and frequency). Using this analogy, in the one case, the electrical configuration is like a bundle of vertically stacked resistors (say each 1 meter long, one for each frequency and GHG) that are all tied together so there is no difference in potential at the end of each resistor, and at each elevation, the resistor with the lowest resistance of course carries the most current. Call this the parallel configuration. The corresponding ‘series’ configuration is similar except the only potentials that are equal are at the bottom and top of the stacks. Everywhere else the potentials are determined by the relative sizes of the resistors. Summarizing, one looks like a ‘series of parallels’ (“SOP”) and other is a ‘parallel set of series’ (“POS”) configuration.
I put together a spreadsheet for a 2X2 resistor array in SOP and POS, in order to compare the total resistance calculated and see under what conditions SOP vs. POS makes any difference. Let column 1 be H2O and column 2 be CO2. Let row 1 be the upper atmosphere and row 2 be the lower atmosphere. For this analogy, let’s make the resistances dimensionless by dividing all of them by the ‘resistance’ of H2O in the lower atmosphere, whatever that is, so we can set the ‘resistances’ of upper atm H2O, and CO2 in relation to this value.
I take the ‘conductance’ (the inverse of resistance) for lower atmosphere H2O to be relatively high, given that there is both a lot of water and a lot of IR bands where water is active. Let that have a value of 1. For the sake of this tiny model, set the upper atm conductance/resistance also to 1 (although it might be lower given nothing above to interfere with radiation away… but there is less present).
As for lower atm CO2, the resistance value (inversely proportional to the heat current that flows through this path) must be much higher, given relatively little CO2 and fewer active bands. I set this resistance to 99. (in effect saying that in the lower atm 99% of the heat flow is H2O related, and 1% in CO2 related). In the upper atm, let’s say that 1/3 of the heat flows via CO2, so the resistance is a value of 2? Twice the upper atm H2O value.
Now we can compare SOP and POS and also how sensitive the total resistance is to a changes in upper atm CO2 resistance. In this tiny model, a doubling of upper atm CO2 resistance in the SOP mode from 2 to 4 increases total resistance by 8%, a significant change given how water dominated the model as a whole is. In POS mode, the increase is significantly smaller… .03%. In fact, POS mode predicts virtually no sensitivity to changes in upper atm CO2 resistance, because once energy is ‘in the water channel’ it stays there all the way up.
I think this could be (and probably has been) resolved with lab bench experimentation… it would be along the lines of a test chamber illuminated with each CO2 band at the relevant range of atm pressures, temperatures and CO2 (and H2O) concentrations… looking orthogonally into the test chamber we should be able to see how much of the CO2 band is the result of a quick absorb/emit of this illumination vs. something that looks much more like the full thermal equilibrium (a function only of the temp). I would bet that at higher relative pressures it’s very much a thermal pattern and only at really low pressures does the CO2 illumination band show up strongly. Just guessing though. Anyone out there aware of such lab data?
If my relative ‘resistance’ numbers are nonsense, then the whole exercise is highly suspect. The sensitivity % could be much higher but probably is much lower (for SOP). Please comment if it looks too far off…
Best regards,
–Randall
Why do you choose 100 m as the critical attenuation length for photon emission? As atmosphere is clearly many km thick, photons with 100 m attenuation length would most likely not escape the 100-meter-environment.
It is probably a bit dubious to choose any value as clearly photons are emitted from a fair thickness of air with varying escape probability. However, for comparison, how would the results change if you take, say, 1 km or 3 km as the critical attenuation length? This probably would move more of the emission to the stratosphere.
Second point, I think it is a bad idea to do any averaging of the spectra. At large altitudes, the emission/absorption lines are very slender and the peak-valley variation may have large impact on the emission altitude.
Something in the ballpark of 3 km might actually be fairly good guess for the critical attenuation length as such a change in altitude will cause an appreciable change in pressure and the associated narrowing of lines enhances the escape probability for radiation moving upwards.
Marko, I am choosing a vertical grid with arbitrary 100m intervals. So I don’t think I am assuming anything about the attenuation length, which anyway varies dramatically with atmospheric pressure. The advantage of calculating from TOA downwards is that we know the attenuation length is many times larger than 100m high up. I am calculating the height for each wavelength where more photons are absorbed than are transmitted through. This is the height where for that wavelength the atmosphere becomes opaque for IR photons. This is what I define as the effective emission height.
The trick is to imaging a flux of IR photons going down from space rather than going up from the surface. The result for the effective emission height will be the same.
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Clive,
Better late than never coming across this excellent post and the next one, where I have another question as well.
Fig. 8 shows a spectrum from NIMBUS which corresponds nicely to your calculated spectrum in Fig. 7. The y-axis is labeled radiance which suggests greater emission occurs with 300ppm vs. 600pm. In fact, Fig. 9 defines the change in radiance as 300ppm – 600ppm and the values are mostly positive. This seems counter intuitive unless fewer molecules at lower warmer altitude emit the greater amount of energy.
Can you straighten me out on this? Also, does this analysis account for the average absorption and emission which would occur during a normal 24 hour time frame?
Sorry about the confusion. The difference in figure 9 is a sign change. In fact the radiance is reduced in going from 300 to 600 ppm. What I have plotted is the “radiative forcing” which is the negative of that value !
The whole AGW argument is based on increasing levels of CO2 reducing OLR. This creates a global energy imbalance causing the surface to warm to rebalance the reduction. This “forcing” is equal to the fall in OLR caused initially by increased CO2.
Forcing = – reduction in OLR from top of atmosphere.
This is of course the consensus view of climate scientists, but is it also that of physicists? I read the responses of Jack Barrett and Peter Dietze to criticisms of the Heinz Hug’s “Climate Catastrophe – A Spectroscopic Artifact?” and wonder if the matter is settled. To your point, Barrett would argue “Certainly, at lower temperature [from higher altitude], the collision rate is reduced, but by no means as much as to offset the larger number of radiative molecules.”
I also interested in how transient changes in the lapse rate over 24 hour period is figured into line-by-line calculations converting emission temperature to radiance that appear to be based on a spectral snap-shot. If this is done, I missed it or haven’t got there yet along my path to understanding climate physics. Grateful for any advice on this.
Of course none of the climate models take into account the diurnal changes in the lapse rate between night and day. They just average over that as far as I know. However night time radiative cooling in clear skies must be important and is especially effected by clouds. Clouds are the elephant in the room. They cool the earth during the day and warm the earth at night. The net average effect though on earth of clouds is cooling -22 W/m2. A reduction in global cloud cover of 1-2% would offset all of AGW.
Regarding Barret. As far as I see it CO2 molecules are in thermal equilibrium with N2 and O2 molecules at any given height. Unlike N2/O2 they can also radiate “heat” energy. However they radiate due to collisions with other gases governed by the local temperature.
Hi Clive,
Many thanks for this explanation of the GHE. The “black body surrounded by a shell of greenhouse gas” model was never any better, for me, than a plausibility argument for how the GHE works. So it’s good to find something that is both understandable and seems realistic.
There is a small point I am not sure of. You say:
The objective now of the calculation is to take each CO2 transition line in turn and then descend from space to find at which altitude the absorption of photons of that wave length within a 100m thick slice of the atmosphere becomes greater than the transmission of photons. We define this height as the transition between opaque and transparency. This is the height at which thermal photons within the CO2 absorption bands are free to escape to space. – the effective radiation height.
This clearly is near reality since your calculations reproduce the spectrum measured from space.
However, the “100m thick slice of the atmosphere” seems arbitrary (eg why not a 176m thick slice?). And although such a slice may itself be just about transparent, you might have a significant number of such slices above you, giving something not really all that transparent, through which photons don’t have a high probability of escaping to space.
Would it be appropriate instead to work down to the level at which 50% of incoming photons have been absorbed? So that, if my understanding is right, you’ll have calculated the height above which a majority of outgoing photons escape for good?
I hope my question makes sense.
Dr Best; Thank you for bringing this immense step toward sanity to the discussion. You say:
“The objective now of the calculation is to take each CO2 transition line in turn and then descend from space to find at which altitude the absorption of photons of that wave length within a 100m thick slice of the atmosphere becomes greater than the transmission of photons.”
My naive question is why is there such a large change in emission altitude for the side bands? I would expect the free path to be the same for all of the frequencies in this narrow band. The reason that their amplitudes are small being emitted to space is not because of the thick atmosphere above their emission altitude but their emitted amplitudes are small to begin with.
What am I missing? Or can you point me to something which explains your calculation of fig. 5a.
Thank you so much for your post.
Ron Chappell
I don’t know if this will be of any help to you, but here are some absorption coefficient figures I’ve gathered.
http://bartonlevenson.com/AbsorptionCoefficients3.html
All lines and bands are in microns, all coefficients in m^2 kg^-1. For notes on translation (you probably know this stuff already):
http://bartonlevenson.com/ConvertingAbsorptionCoefficients.html
Thanks so much for the very thorough and thoughtful reply. I’m overwhelmed with confusion. Some seem impossibly large.
arationofreason
Just background:
http://journals.aps.org/pr/abstract/10.1103/PhysRev.41.291
http://www.rmets.org/sites/default/files/qjcallender38.pdf
Callender’s 1938 paper also appears in The Warming Papers, edited by David Archer and Ray Pierrehumbert.
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Clive,
What do you mean by tracking IR photons downward from space? The graphs that I’ve looked at show incoming from space as mostly visible light range and outgoing (upward) as IR.
It is just a thought experiment that allows us to get the same result as doing a complicated radiative transport calculation up from the surface. There are no real IR photons coming from space except perhaps those from the Big Bang microwave background !
Thanks Clive – I’m new to this specific field of study and find your discussions helpful. Do you know why CO2 is (has been) considered nearly saturated when the satellite IR spectroscopy graphs don’t show the near 667 wavenumber (or near 15 microns) as bottoming out? That means the IR detectors are still picking up a reduced amount but an amount nonetheless (40 mW for example in Fig 8). Is the consensus that it is all from upward retransmission from energized CO2? If so, intuitively that would seem a bit much. I understand why the peaks get wider so that’s not my issue. It’s the remaining 40 mW that still reaches the detectors that puzzles me.
That is a very interesting question. The central central lines are already saturated way up into the stratosphere where temperature actually increases with altitude. This means that IR emissions there are actually higher than those lines emitting from near the top of troposphere – hence the spike. As CO2 increases so does the emission height increasing emissions from the central lines producing a GH ‘cooling’ contribution. However the net effect when integrated over all lines is a reduction in outgoing IR and an induced warming effect.
The NIMBUS II data was from early 70s and there is AIRS data from after 2002. The altitudes for both satellites seemed similar (over 400 miles) but AIRS measured tropospheric temps so I’m not sure what that means regarding its altitude.
https://airs.jpl.nasa.gov/data/products
Anyway the AIRS spectrum provided by Pierrehumbert looks almost identical to the earlier NIMBUS II:
https://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf
I confess I still don’t grasp why the 667 dip stops at that level. To me, I interpret this as more CO2 tomorrow could trap even more IR at 667. But on the other hand – why are the NIMBUS II and AIRS graphs so similar when more than 30 years had passed and CO2 levels increased from 325 ppm to 380 ppm?
Without understand this I won’t be able to appreciate what going to 600 ppm will actually do.
I assume you mean the 667 central spike. This will strengthen as CO2 increases but it releases more energy to space, the opposite of trapping more energy!
Basically the atmosphere radiates at different heights and wavelength, but always roughly as a black body with temperature Tlocal. The central spike already radiates from CO2 molecules way up in the stratosphere. The stratosphere temperature increases with height and radiant energy goes as T^4 so more heat is radiated to space from this wavelength as CO2 levels rise. This increases cooling !
You can already see that in the spectra when compared to BB spectra at various temperatures. The spike radiates at 245C whereas the side bands radiate at 220K.
A Black Body!!!! Hardly. Schoolboy error. Is the rest of your nalysis worth reading?
Clive,
I’ve done a little more research. I think the biggest reason the central spike at exactly 667 is a spike is because there is much less CO2 moving perfectly laterally compared to the amount of CO2 encountered with vertical velocity toward or away from the approaching IR (Doppler broadening). That would explain more absorption between 640 -665 and 669-700 than the amount absorbed at the actual 667 wavenumber itself. So the result is more IR at exactly 667 will make it all the way to the satellite detectors.
I reread chapter 4 of David Archer’s Global Warming book. He discusses the Band Saturation effect. Even though he appears to be in the “CO2 is ruining the planet” camp, this section (if I understand correctly) is basically an admission that the levels of CO2 have hit their limit of absorbing any more at 667. He does warn that more CO2 will contribute to additional warming due to available CO2 to absorb near (+ or -) 667 and “broaden” the band even wider. I get that and in a sense don’t technically disagree with it, but the farther you get from 667 in either direction, the harder it is for IR to find a CO2 traveling at that velocity. So if that’s the reason for concern, I can’t see the world collapsing as CO2 moves toward 450 ppm some time in the future.
However, I think a more likely reason for additional warming is something you mentioned in your post: DOUBLING CO2 and BASIC PHYSICS from Feb 4, 2010. There you mention how increased CO2 concentrations could cause the saturation at lower altitudes in the atmosphere. So as CO2 concentrations continue to rise, a higher percentage absorbs increasingly closer to the surface. There may be something to that especially if lower altitude CO2 has a better chance of transferring energy to O2 and N2 while higher altitude CO2 presumably has more time between collisions and has increased chance to re-radiate the energy away as IR before it transfers to other gas molecules to increase kinetic energy. Has someone done the math on this? Could additional warming be simply from continually increasing the ratio of: transfer to kinetic vs radiate to space?
Can you recommend any articles or books that explore this issue?
Can someone check my premise and my math here? I’m studying climate change from a particle physics and quantum mechanics perspective, and want to know if I’m on the right track.
The TL;DR is that the first vibrational mode quantum state of the ground electronic mode quantum state of N2 has more energy than even the highest vibrational mode quantum state of the ground electronic mode quantum state of CO2, and thus vibrational mode quantum state energy preferentially flows from N2 to CO2. The only time CO2 will transfer vibrational mode quantum state energy to N2 is if the N2 is in its ground electronic mode and vibrational mode quantum states, and that’s not likely in the atmosphere for at least the vibrational .mode quantum state.
===============
The average kinetic energy of CO2 molecules at prevalent atmospheric temperature (288 K) is given by:
KE_avg = [1/2 mv^2] = 3/2 kT
… which gives an average thermal energy of 0.03722663 eV and a mean CO2 molecular translational speed of 372.227941 m/s. This thermal energy is equivalent to the energy of a 33.3283159 micron photon. You’ll note the thermal energy is LESS THAN the energy necessary to excite a CO2 molecule’s vibrational mode quantum states. So one would simplistically assume that the opposite applies, that the vibrational mode quantum state energy of CO2 is greater than the translational energy of N2 or O2 molecules (which would be approximately the same as calculated above, due to the Equipartition Theorem) and therefore a photon-excited CO2 molecule will de-excite via a thermalizing collision with N2 or O2, thereby raising atmospheric temperature… except that assumes N2 and O2 are in their vibrational ground states, it neglects the energy in vibrational mode quantum states of N2 and O2.
The wavenumber of any transition is related to its corresponding energy by the equation:
1 cm-1 = 11.9624 J mol-1
667.4 cm-1 = 667.4 * 11.9624 / 1000 = 7.98 kJ mol-1
The Boltzmann Factor at 288 K has the value exp(-7980 / 288R) = 0.03609 which means that only 3.6% of the CO2 molecules are in the lowest excited vibrational mode quantum state {ie: v21(1), bending mode}. These are the molecules that form the lower energy state for the next higher transitions which have an even lower population.
The v2 vibrational (bending) mode quantum state of CO2 in its ground electronic state requires ~0.08279 eV or a ~14.98576 micron photon (per VR Molecules Pro molecular modeler). The first vibrational mode of N2 has quantum energy of 0.14634 eV, more than enough to activate CO2’s first vibrational mode quantum state upon collision. Thus, given that the Equipartition Theorem indicates that the thermal energy of both molecules is similar, during a collision the vibrational mode quantum state energy of a vibrationally-excited N2 molecule will flow to the CO2 molecule, not the other way around.
This, of course, assumes that N2 in the atmosphere is vibrationally excited to at least its first vibrational mode quantum state. And a good percentage of it is…
———-
https://www.osapublishing.org/DirectPDFAccess/5CCF1401-BEE0-71DE-4128232482B99888_303623/oe-22-23-27833.pdf?da=1&id=303623&seq=0&mobile=no
http://adsabs.harvard.edu/full/1964IAUS…18…19D
Vibrationally Excited Molecules In Atmospheric Reactions
“It follows from the solar ultraviolet intensities quoted by Watanabe and Hinteregger that the production of N2* through Eq. 21 will be of the order of 10^10 cm-2 sec-1. Most of the N2* will be in low vibrational levels.” {Comment: That’s 10,000,000,000 per square centimeter per second)
———-
We can again use the Boltzmann Factor to determine the excitation population of N2. While the N2 molecule is IR-inactive due to no change in magnetic dipole, it is Raman-active:
N-N stretching at 2744 cm-1 (3.64431 micron)
1 cm-1 = 11.9624 J mol-1
2744 cm-1 = 2744 * 11.9624 / 1000 = 32.825 kJ mol-1
The Boltzmann factor at 288 K has the value exp(-3282.5 / 288R) = 0.087738 which means that 8.77% of N2 molecules are in the N-N stretch excited state.
When the molar mass of any gas is divided by the density of that gas at 1 atmosphere and a temperature of 288 K, the value 23.633 L/mol is obtained.
So when looking at any 23.633 liter volume of the atmosphere, there will be one mol of N2 and one mol of CO2, when assuming that CO2 is a well-mixed gas.
The mol of N2 in that 23.633 liter volume will contain 32.825 kJ of energy, whereas the mol of CO2 will contain 7.98 kJ of energy.
{{{ 32.825 kJ / mol > 7.98 kJ / mol }}} Energy always flows from a higher-energy density to a lower-energy density regime.
Given that CO2 constitutes 0.041% of the atmosphere (410 ppm), and N2 constitutes 78.08% of the atmosphere (780800 ppm), this means that 14.7969 ppm of CO2 is excited, whereas 68505.8304 ppm of N2 is excited. This is a ratio of 1 excited CO2 to 4629 excited N2. You’ll note this is 2.43 times higher than the total CO2:N2 ratio of 1:1904, and 167 times more excited N2 molecules than ALL CO2 molecules.
As you can see, the number of excited N2 molecules swamps the number of excited CO2 molecules, and on a molar volume basis, N2 contains much more energy than CO2 at the same temperature and in the same volume of atmosphere. Hence, energy flows FROM N2 TO CO2.
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surely no net flow will occur from N2 to CO2 at equilibrium. the Boltzmann factors tell us the proportions of excited CO2 for a equilibrium temperature 288 and the proportion of excited N2 for the same temperature any transfer from N2 to CO2 is balanced by transfer the other way, that’s what the Boltzmann distribution means.
eddiebanner@outlook.com
Hi Clive
Thank you again for your kind response to my post “Carbon Dioxide Absorption Power and the Greenhouse Gas Theory”. It seems that I placed this in an inappropriate thread, and that this one would be much better, so I should like to re-post here if you would please allow it. I am trying to reconcile the ideas in my post with those in your excellent paper above, but I need some help here. Please would you let me know the value you, and the climate models, use for the energy of a 15 micron photon, so that I can compare it with the value I have used, which is 1.3252*10^-20 Joule.
Global warming is certainly happening and much has been written about the Greenhouse Gas effect and it’s claimed warming of the Earth’s surface. The ideas have been based on the ability of molecules of carbon dioxide in the Earth’s atmosphere to absorb infrared photons of 15 micron wavelength, but very little, if anything, has been published about the power which can be handled by the atmospheric carbon dioxide. Nevertheless, GHG advocates claim a “radiative forcing” of about 2 Watts per m2 at the Earth’s surface. The following calculations show that this GHG theory cannot be correct.
Consider a standard column of the Earth’s atmosphere, based upon an area of 1 square metre of the Earth’s surface.
The number of molecules in this column (1) is 2.137*10^29
So at the current concentration of carbon dioxide, 400ppm, the number of molecules of carbon dioxide is (400*10^-6 )*(2.137*10^29 ) = approx 8.5*10^25
From the HITRAN database (2), the ability of the CO2 molecule to absorb a 15 micron photon is given by its absorption cross-section, which is 5*10^-22 m2 per molecule. (Note that this database gives the value in cm2 ).
So, in an area of 1m2 the number of molecules required to absorb 1 photon is 1/(5*10^-22) ; that is 2*10^21 CO2 molecules per m2
But there are 8.5*10^25 molecules of CO2 in the column.
So the number of photons which can be absorbed is (8.5*10^25) / (2*10^21)
= 4.3*10^4 photons per m2
Now, the energy of a 15 micron photon (3) is 1.3252*10^-20 Joule
So the energy absorbed by all the CO2 in the column = (1.3252*10^-20) * 4.3*10^4 Joule
= 5.7*10^-16 Joule per m2
This process can be repeated many times per second because the excited CO2
molecule can release its energy by collision with any molecule in the atmosphere, ready to absorb another photon of the right energy. The mean free path in air at atmospheric pressure (760 torr) is about 0.1 micron, and the molecular velocity is 465 m.sec-1, and so the mean time between collisions is about 2*10^-10 second. So the process can be repeated about 5*10^9 times per second.
Therefore, the maximum power which the carbon dioxide (at 400ppm) can handle is (5*10^9)*(5.7*10^-16) Joule per second per m2, that is approx. 3*10^-6 Watts.m-2
Whereas the Greenhouse Gas theory requires about 2 W.m-2 , which is about 700,000 times the power available. This seems to show that the Greenhouse Gas Theory is not valid.
References
(1) http://www.theweatherprediction.com/habyhints3/976/
(2) http://vpl.astro.washington.edu/spectra/co2pnnlimagesmicrons.htm
(3) https://www.pveducation.org/pvcdrom/properties-of-sunlight/energy-of-photon
Eddie,
Yes, but I think you are forgetting how the convective/latent heat energy flow changes to compensate. This is impossible to calculate but works a bit like a pressure cooker. Heat will escape to space via IR radiation from the surface, top of clouds, H2O and CO2 through whatever is the most efficient. It certainly doesn’t all escape via CO2 IR to space. Only a small fraction in the 15micron band is affected by CO2. Any so called “trapped” heat by doubling CO2 is thermalised and escapes throughout the black-body spectrum. Just that fraction emitted by CO2 rises in height to colder levels.
Clive
Clive
Many thanks for your helpful reply. Food for thought.
Eddie
Clive
Many thanks for your helpful reply. Food for thought.
Eddie
Clive,
A remark on the model that you used to compute the reduction in outgoing radiation at an
increase of CO2 from 300ppm to 600 ppm:
The emission height from the side lines of the spectrum in your model lie well in the troposphere, whereas measurements from the NIMBUS-4 satellite suggest that they lie in- or close to the tropopause (see for instance Fig 9 from
https://pdfs.semanticscholar.org/f9d1/415588a2ee3d6e28e82c290962b8d8d6a453.pdf ).
Outgoing IR Spectra are shown from the Sahara, the Mediterranean and the Antarctic. In the CO2 window most of the side line energy lies around the 220 K Planck line corresponding to an emission height in- or close to the tropopause.
Would these observations from the satellite not lead to a lower estimate of the reduction in outgoing radiation?
It is further remarkable that, while the altitude of the tropopause varies strongly with latitude, that
all three satellite measurement indicate an emission height close to these altitudes.
How can this be explained?
Thanks for your efforts furthering understandings.
I’m getting back to Marcus’ comments from 6th August 2014: With a cross section of 300m2 per mole, 400ppm C02, exp(-10000m/7000m) thinned air at 10000m altitude, I get around 20m free path length and I can’t imagine how the photon would manage to come down to that altitude from external. Wouldn’t it have to pass through roughly 3000m atmosphere equivalent to the density at 10000m?
Correction and sorry: 30m2 (not 300m2) per mole. Free photon path length = (0.0224m3/mole)/(30m2/mole * 400E-6) =1.87 m on ground level at 400 ppm CO2. Applying /exp(-10000m/7000m), hence multiplying by 4.1, the free path becomes 7.80m at altitude 10000m. So the cross-section peak at e.g. 650 cm-1 in figure 3 itself should hinder the photon to arrive at 10000m from external above. The mechanism needs to be more complex and it looks as if effectively smaller cross sections need to be dominant. Looking at the fast varying spectrum and thinking of changing peak shapes with different altitudes, there are probably enough opportunities left over for that. Nonetheless, it would be nice to have an idea about the typical effective cross-sections and height changes involved when changing CO2 concentration changes the radiation force.
I still have a problem with the analysis concerning what the radiative emission of any slab of air would be. N2 and O2 neither absorb nor emit IR radiation. In the absence of any IR absorbing gas the Blackbody radiation B(288K,wl) from the ground would escape unchanged to space. With 350 ppm CO2 the absorption bands of CO2 at 15micron would completely remove the IR from the ground emission in a window around 15 micron, absorbing, and reemitting it in random directions to be reabsorbed etc. collisional deactivation of the vibrationally excited CO2 would add to warming in the lower atmosphere. So this radiation is not available to stimulate emission nor cause absorption in the upper atmosphere. So how does emission from CO2 at high altitude arise? from collisional excitation? but low pressure reduces the probability and low temperature reduces the Boltzman factor for population of the first vibrationally excited state from about 3% at ground level to 1% at 11KM. (so 1% of about 10% due to the pressure drop ). The atmosphere is not in equilibrium, but in a steady state with a static temperature profile does that mean we can ignore detailed balance arguments about rates of collisional activation and emission or can we have a flow of energy from gas KE out into emitted radiation?.
If your picture of upper atmosphere emission contributing to the forcing so allowing an increasing effect with CO2 pressure even long after saturation of the absorption bands has occurred when no further effect might be expected ,then the vibration rotation spectra observed from space should be different from in the lab, with alteration of the relative intensities of lines due to the different sampling depths. I wonder if this is observed.
There is an element of bootstrap in the greenhouse effect. It depends on convection to generate a lapse rate without which there could be no greenhouse effect. The atmosphere is really just a huge heat engine. During the day the sun heats the surface (land and ocean). Warm moist air expands and rises setting up a lapse rate up to the tropopause. However you need greenhouse gases for convection to occur in the first place and this determines the height of the tropopause. Radiation escaping to space cools the earth. Some goes through the IR window and the rest mainly through emissions from H2O and to a lesser extent CO2.
CO2 molecules get excited by local collisions or IR photon absorption and emit IR photons randomly. The key point though is that at any given height the gas is in local thermodynamic equilibrium at some temperature T for that height defined by the lapse rate. Now we use the fall in density with height to calculate the probability that an emitted CO2 photon escapes to space. This depends on wavelength cross section. Satellite detectors measure the IR wavelength spectrum and agree with the calculations based on HiTRAN. The central spike in the spectrum is emitted from the Stratosphere whose temperature rises with height so increases in CO2 actually cool the Stratosphere !
At night the surface cools through direct IR window and convection slows and can even invert in winter. Then we get Frost and Fog ! The cycle then repeats.
Antarctica is interesting as it has a 6 month night-time – hence very low temperatures.
Yes I now agree you have captured the essence of the physics. I looked at the more detailed radiative-convection models which come to the same conclusions after a lot more detailed calculation!
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Hey Clive, in the CO2 doubling scenario, if heat can’t be released until a higher altitude in the atmosphere, wouldn’t the trapped heat warm that layer and the layer above it, where the heat is escaping? Would there be a change in the lapse rate?
Hi Ray,
Heat released in the atmosphere comes from the surface. Hot air rises and cools as it expands setting up the lapse rate in the troposphere. The tropopause marks the height where radiative heat loss to space dominates and convection stops. The temperature at the tropopause is basically the effective (radiative) temperature of the Earth – 255K(-180C).
If we add more CO2 so the effective radiation height increases and by implication so does the tropopause. Since the moist lapse rate remains unchanged. the surface temperature must rise to compensate for the increased convection height.
I think it is best to ignore CO2 radiative transfer from the surface because convection and thermodynamic equilibrium dominate until we reach the effective radiation height for CO2. Likewise the idea that back radiation warms the surface is a red herring.
Back radiation is a red herring with the add on that clouds back radiate through the IR-window.
Embarrassing to have to ask: but why don’t the CO2 molecules, excited by IR photons, give up some of that energy to oxygen and nitrogen molecules through collision? If everything above 0K radiates energy, would some of that energy transferred by collision then be radiated away at the appropriate wavelength for O and N? (I say only some of the energy transferred as some would presumably contribute to convection)
No embarrassment needed because you are correct. Excited CO2 molecules quickly thermalise absorbed radiation from the surface with nearby O2 and N2 molecules so that each level reaches thermal equilibrium. Convection of air dominates heat loss upwards until a level is reached where the atmosphere thins enough until IR photons can radiate freely to space thereby balancing the daily incoming solar energy.
Clive – I must say that I had not expected an answer, yet alone so quickly. Thank you. So I will push my luck with another potentially dumb question that may well demonstrate the danger of a little knowledge….. If excited CO2 molecules quickly thermalise absorbed radiation with (the far more numerous) O2 and N2 molecules, then these CO2 molecules have greatly reduced their own “extra” energy from the absorption of the radiation and greatly reduced the potential for emission, absorption and reemission of photons by CO2 molecules all the way from there up through the atmosphere until they can escape to outer space. Thus, at TOA CO2 will no longer be accounting for a significant proportion of the radiation lost to outer space and the effect of doubling its level has become correspondingly small. I know that this can hardly be so when you look at charts of radiation leaving the TOA but it would be good to know where I have gone wrong. I also realise that water vapour has been ignored but I am trying to understand the theory at the simplest level for myself before adding complicating factors. So it is all a bit basic I fear.
this is one of the best explanations of CO2 importance in the upper troposphere that I have seen. Thanks!
Well I see I am 9 years late to the party, but this is an outstanding post. May I ask for some detail on how you went from Fig 3 (which I independently confirmed) to Figs 5 (where my calculation gives something different). You clearly generated Figs 5 with some computer code, but I didn’t find it here. Thank you.
The aim is to calculate the height in the atmosphere where CO2 photons escape to space – the effective emission height. We know that pressure reduces exponentially with height. Therefore the mean free path for reabsorption of photons also increases. We can simplify the calculation by assuming local thermodynamic equilibrium. So at each height air molecules are thermalised at the lapse rate in the troposphere and increase in the stratosphere.
HITRAN gives us the cross-section for CO2 absorption/emission so we can calculate the height for each line where the mean free path is > top of the atmosphere. Thus is the effective emission height for that line wavelength. I loop over all emission lines and thermalise the results due to line broadening. The result is Figs 5.
I found some PERL scripts that I used on HITRAN
first calculate cross-sections
and second calculate temperature dependence
I hope this is readable !