Guest Post by Willis Eschenbach
There’s an online calculator called MODTRAN that calculates the absorption of longwave (“greenhouse”) radiation for various greenhouse gases (“GHGs”), and shows their resulting effect. It does this on a “line-by-line” basis, meaning it examines each interval of the longwave spectrum for each greenhouse gas at each altitude, and calculates the resulting absorption by each species given the concentration and the partial pressure of that species. Figure 1 shows the calculation results for the default values of CO2, ozone, and methane.
Figure 1. MODTRAN results. The jagged red line at the top right shows what is not absorbed by the atmosphere. Colored lines in the background are theoretical “no-absorption” curves for various temperatures. The big “bite” out of the middle section of the jagged red line is mainly water vapor absorption, although it overlaps with the CO2 absorption bands in parts of it. The graph at the lower right shows the pressure, temperature, and the concentration of H20, Ozone, CO2, and CH4 at various elevations. The number “Iout” is the total energy that is not absorbed, so as absorption increases, that number will decrease.
That all seems straightforward, it looks the same as in heaps of textbooks … so where’s the MODTRAN oddity?
The oddity arose as a result of my wanting to know more about the doubling of CO2, and the 3.7 watts per square metre of increased absorption that IPCC claims the aforesaid doubling of CO2 is supposed to cause. To find out what MODTRAN says, I put in 750 ppm of CO2 in the top cell, and I had MODTRAN recalculate the Iout. To my surprise, I found that it was 284.672 … which when subtracted from the starting Iout shown above of 287.844 gives us only 3.2 watts per square metre increased absorption (forcing change) for a doubling of CO2.
I had left the “No Clouds or Rain” choice selected, thinking that the biggest change in CO2 would be in clear-sky conditions. So I figured “well, perhaps clouds or rain increase the absorption when CO2 doubles” … but investigating various cloud results showed that was not the case, they all gave smaller absorption changes. My original intuition was correct, clear-sky conditions give the biggest change in absorption for a doubling of CO2.
So I thought, “well, perhaps I’m looking at the wrong region of the Earth”. The other latitude bands available in MODTRAN are Midlatitude Summer and Winter, and Subarctic Summer and Winter. I took nominal CO2 values to represent the CO2 concentration in 1850 (285 ppmv), default (375 ppmv), doubling of 1850 value (570 ppmv) and doubling of the present value (750 ppmv). I figured that would give me two doublings, and let me see if the increases were linear with the logarithm of the number of doublings. I used MODTRAN to calculate the absorption change in each latitude band. Figure 2 shows those results, with the X axis being the number of doublings, and the Y-axis showing the increase in longwave absorption.
Figure 2. MODTRAN and IPCC values for the increase in forcing due to increasing CO2. The forcing change in each region per doubling of CO2 in watts per square metre is shown after the name in the legend, followed by “T=” and the surface temperature.
First thing I noticed is that the lines are all straight. So MODTRAN does indeed show a linear relationship between logarithm of the CO2 increase and the calculated increase in absorption. So no surprise there.
What was a surprise is that none of the other latitude bands had larger changes in absorption than the tropics. I’d thought that since the IPCC says the global average change from doubling CO2 is 3.7 W/m2, that because the tropics were at 3.2 W/m2, somewhere else the absorption must be above 3.7 W/m2 to make the average correct. But that’s not the case. They’re all smaller.
I also thought that the difference in absorption might be due to the different surface temperatures … but Midlatitude Winter and Subarctic Summer have about the same calculated absorption, yet their surface temperatures are about 15 degrees apart.
Note that what I have shown, the change in absorption, is also called the “instantaneous” forcing, abbreviated Fi. There are various other forcings one can measure. Hansen discusses them here (PDF), and gives a value of 4.52 W/m2 for the instantaneous forcing from a doubling of CO2 according to his climate model (Table 1, p. 7). Presumably his model does a line-by-line calculation similar to MODTRAN to figure out the absorption changes.
The IPCC, on the other hand, says:
The simple formulae for RF [radiative forcing] of the LLGHG [long-lived greenhouse gases] quoted in Ramaswamy et al. (2001) are still valid. These formulae are based on global RF calculations where clouds, stratospheric adjustment and solar absorption are included, and give an RF of +3.7 W m–2 for a doubling in the CO2 mixing ratio.
SOURCE: IPCC AR4 WG1 (PDF) page 140
So it appears the IPCC result is not based on a line by line calculation …
And that’s the MODTRAN oddity. Here’s the question:
1. Why does the MODTRAN calculated line-by-line change in absorption range from a low of 1.7 W/m2 to a high of 3.2 W/m2, while Hansen is saying the answer is really 4.5 W/m2 and the IPCC uses 3.7 W/m2?
Since according to MODTRAN (and logic) clouds reduce the effect of a doubling of CO2, and the IPCC (and presumably Hansen) are using all-sky conditions, that makes the IPCC/Hansen figures even farther from the MODTRAN figures.
Please note that (per Hansen) the instantaneous forcing Fi is greater than the adjusted forcing Fa by about 0.4 W/m2. The IPCC is saying that the 3.7 W/m2 is the adjusted forcing Fa, the forcing after the stratosphere adjusts to the change. So their value for instantaneous forcing would be larger, removing the stratospheric adjustment would put it at about 4.1 W/m2. So the IPCC is closer to Hansen’s value for the instantaneous forcing … but it means they’re further from the MODTRAN calculations.
I don’t know what I’m missing here, and I don’t understand these results, so any assistance gladly accepted.
w.
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Spector says
All you have to do to see this is check out the Savi-Weber HITRAN online utility that provides raw spectrum plots:
http://savi.weber.edu/hi_plot/
Please don’t! There is something very wrong with their plots. Use this instead
http://www.spectralcalc.com
Compare the absorption spectra. They are not even close.
Wouldn’t it make more sense to set the sensor altitude at “0” looking up, so that one can see the effective radiation at the surface?
When I do this I see very little if any effect on W/M2 from varying CO2 levels. There is however, a large effect if you change the Ground T offset. So, it seems likely that there is an assumption in the model that as you warm the air it will hold more water, which will increase the GHG effect.
So, unless CO2 warms the atmosphere, causing more H2O in the air, then CO2 has little effect.
Also, when I set the sensor altitude at 0km, looking up, then change the sky from clear to cloudy an interesting effect happens. The W/m2 INCREASES in a cloudy sky as compared to a clear sky.
This is an interesting result, because it implies a possible error in the understanding of GHG. When the sky is cloudy, solar radiation is blocked, but the water vapor in the clouds does radiate back to earth. However, what is the source of that radiation?
The radiation from the clouds is a direct result of the temperature of the clouds, which is a result of the solar radiation that went into evaporating the water from the oceans to create the clouds. Thus, when we count the back radiation from clouds, we need to subtract the energy used to evaporate the water to create the clouds from the energy budget, or we are double counting the radiation from the clouds, which would lead one to mistakenly conclude that we are in for catastrophic runaway heating if the temperature goes up even a little bit.
michael hammer says:
Consider, at sea level 300 ppm the total absorbance of CO2 is about 2000 abs (Heinz Hug).
Well, he used a fairly low resolution (2 cm-1) which reduced the central peak. Using 0.1 cm-1, the peak absorbance is 130,000. BTW many of the water vapor peaks are between 1 and 12 million.
I suggest that you use MODTRAN looking down, and change the height. The minimum CO2 readings are near 17km. As you move up, the central peak increases to a max near 53km (above the stratopause). Since the central peak is lower above 53km, there must still be a significant CO2 absorption above that point.
According to the references I’ve seen, the atmosphere (including CO2) is well mixed up to the mesopause. (Except for water vapor and ozone, of course.) Above that, it is not.
I think the CO2 line at around 14.5 microns is a single extremely pressure broadened line not a forest of close spaced lines
I have looked at the details of the central peak. There are indeed 20 or so strong individual lines (and many more weak ones) which merge into a single broad line at the surface.
Jim D says:
August 19, 2011 at 8:12 pm
Read the thread first. I discussed this some time before your post.
w.
ferd berple says:
The radiation from the clouds is a direct result of the temperature of the clouds
That is true at the tops of the clouds, but not at the bottoms. When the bottom of a cloud receives energy from the surface, it causes the cloud droplets to evaporate. When the “new vapor” moves up a little, it recondenses into a new droplet and emits the heat of vaporization. This physics allows cloud bottoms to effectively reflect the heat from the surface.
This explains why, on cloudy nights, an IR thermometer says that the surface and the cloud bottoms have the same temperature even though the clouds are actually 10°C cooler than the surface. It also explains how clouds keep the surface warm .. they simply reflect the heat back to the surface.
henry@ferd Berple
that was funny, that last comment.
I am also so amused with all these people thinking you can “calculate” that which has never been measured.
you should read this
http://www.letterdash.com/HenryP/the-greenhouse-effect-and-the-principle-of-re-radiation-11-Aug-2011
and this
http://wattsupwiththat.com/2011/08/16/new-paper-from-lindzen-and-choi-implies-that-the-models-are-exaggerating-climate-sensitivity/#comment-723928
and really laugh about the apparent crazy results from my simple observations…
Spector;
As pointed out above, the AF’s concern is to validate and localize a bogey, a target, using IR. It is not intended for “accurate” geophysics.
RE: Robert Clemenzi says: (August 20, 2011 at 10:57 am)
[Savi-Weber HITRAN Plots]
“Please don’t! There is something very wrong with their plots. Use this instead
http://www.spectralcalc.com
Compare the absorption spectra. They are not even close.”
Yes it does look like someone has messed up their data or introduced a mathematical grating effect into the line-plot program, Now that I have made the comparison; I do not recall it looking that bad in the past. Too bad it was a handy little tool.
Pamela: You got it almost right. You need to add, “… and these men folk sitting around the table jawin’ are not farmers, have never been on a a farm, and have never see a tractor in real life.”
Willis, there appears to be a significant disagreement concerning the MODTRAN plots. You have claimed
Then what is the squiggly red line in the top right? AFAIK, it shows net absorption at every frequency … isn’t that an absorption spectrum?
Several people (including myself) are certain that the spectrum looking down is an emission spectrum. You are convinced that it is an absorption spectrum. One side is wrong, and neither side is able to convince the other. I would like to better understand your analysis of this, perhaps I have misunderstood something.
RE: Robert Clemenzi says: (August 18, 2011 at 9:17 pm)
“In the image above, the CO2 absorption near 666 cm-1 is actually the CO2 emission from the -53C tropopause. This is a common error in interpreting that particular image. To be clear, at those frequencies CO2 is totally opaque. More than 95% of the energy from the surface is absorbed in the first kilometer. What is measured from space is emitted from much higher in the atmosphere. The spike in the middle of the CO2 band is emitted from the stratopause, at 47 km. (Get it, the cold part is from 11 km and the warm spike is from 47 km.)”
I note that the temperature of the region of the mesosphere where the sensor is placed, 70 km up, is about the same as the tropopause so that it might be easy to confuse local thermal radiation with that, which might be coming up all the way through the stratosphere from the tropopause without being absorbed. I note that if I set the sensor, still looking down, to 30 km, the bottom of the CO2 hole seems to get slightly deeper. If CO2 cannot cool the tropopause, we must assume that water vapor is likely to be the primary radiant coolant for that region.
I believe the spectrum looking up is the often thought to be mythical ‘back-radiation’ emission intensity spectrum. It still says ‘Intensity W/(m2 wavenumber)’ on the left. I suspect that solar radiation is not included.
Clemenzi at 11:37.
Robert, with the greatest respect, I have to point out that your explanation generates some interesting paradoxes. Your comments (and the claim of a well mixed atmosphere) would imply that emission at the CO2 line centre would indeed come from the mesopause since the absorbance is so high even the low level at the mesopause would have very significant emissivity. Thus the back body temperature at the line centre would be 175K. A bit further away from the line centre where the absorbance is lower the emission would be coming from the stratopause at 270K. A bit further away from the line centrre again it would be coming from down at the tropopause at 220K and then out in the wings it would be coming from progressively lower in the troposphere with a progressively rising black body temperature. Thus the central region of the absorption line should show a black body emission temperature going from 220k near the edge up to 270K fpor a considerable portion and then down in the middle to 175K. Thats one hell of a gull wing profile. Sure the interferometer on Nimbus may not have enough resolution to detail that but when one convolves a rapidly varying signal with a low pass filter the result is a very noisy spectrum. One can see that clearly in the region between about 16 and 25 microns where there are indeed closely spaced water bands and the Nimbus data shows an intermediate black body temperature with considerable noise. The wings of the CO2 peak in the Nimbus data also show considerable detail yet the central region of the CO2 absorption band is remarkably free of noise – a very clean smooth region with an apparent emission temperature of 220K.
Also according to your data most of that central peak should be from near the stratopause which is far warmer so how come the entire smooth central region shows a 220K black body temeprature? Something does not tie up with your rationale.
michael, you are referring to “Nimbus data” and the bulk of this article is on MODTRAN reconstructions. It would be nice to have a link to what you are seeing.
According to NASA, the Nimbus 3 and 4 FTIR instruments had nominal spectral resolutions of 5 and 1.4 cm-1, respectively. That alone may be enough to account for the differences since the lines are about 0.14 cm-1 apart. In addition, the Nimbus data may have been processed (smoothed) before being presented. There is no guarantee that you are seeing the raw data. (This is actually a common problem in all data, not just climate science. For instance, the Hubble images are highly processed before being made public.)
I agree that some of this does not tie up with [my] rationale. I am still trying to put it all together. Please, point out all the paradoxes, they will help us all to think about this in new ways. At this point, I have not seen anything that suggests that the weird shape you suggest is not the actual shape.
“”””” michael hammer says:
August 19, 2011 at 4:07 pm
Spector at 3:23.
Interesting quote you give from Wikipedia however it is one I have considerable problem with. If CO2 is emitting strongly it is also absorbing strongly (emissivity equals absorptivity) and that means that the black body emission temperature as seen from space should be the temperature of the mesopause ie: 175K. But it is not, it is 220K – the temperature of the tropopause hence there is a slight problem.
Note also, any strongly emitting layer of gas will have an emissivity of 1 so the difference cannot be due to low emissivity. This is because if a gas layer emits strongly it also absorbs strongly and that in turn means that at the absorption wavelength it will look like a black body (something that absorbs all energy at that wavelength). “””””
Actually a “black body absorbs ALL electromagnetic radiation at ALL wavelengths from down to but not including DC, up to and beyond the highest of the high energy gamma radiation, and anything that may be beyond that.
DC is excluded since by definition, NO variation of current aka, acceleration of electric charge can occur, so there can be NO EM radiation at DC.
And for the umteenth time, Kirchoff’s Law citing the absolute equality at any and all wavelengths of emissivity, and absorbance is ONLY true for a material that is in thermal equilibrium with an EM field of radiation; which of necessity has to be a closed system, which the earth’s atmosphere is not, and it most certainly is never in thermal equilibrium, since it is constantly assaulted by a highly varying assortment of EM radiation, from multiple sources, all of which have different Temperatures for their sources, and you cannot have thermal equilibrium and multiple Temperatures together at the same time, it flies in the face of the very definition of thermal equilibrium..
Does it occur to anyone reading here at WUWT, that ALL atomic or molecular line/band spectra of any kind, are prima facie evidence of non equilibrium (in the thermal sense). Resonance absorption or emission processes that are the cause of the very absorption bands that the GHG concept is all about, are all non equilibrium processes; they all involve finite lifetimes of the various excited states, and must terminate eventually.
Get over it; the greenhouse gas warming hypothesis has nothing whatsoever to do with Kirchioff’s Law.
Robert Clemenzi says:
August 20, 2011 at 1:25 pm
Thanks. I’m not sure what you mean by the two, Robert, nor what difference it might make. For me it’s like an argument about whether a ladder goes up or down.
Looking downwards from the tropopause, you see what makes it up from the ground through the atmosphere. If that amount goes down, it means that more is absorbed in the atmosphere. So I’m using it to measure the change in what is absorbed.
I don’t know what kind of spectrum it is. The issue is the change in the absorption, not what name you call the spectrum.
At least that’s how I see it.
w.
The concept is absorption lines when looking down, emission lines when looking up. It just means CO2 and other lines look dark against a bright background when looking down, bright on a dark background when looking up.
The IPCC AR4 says the 3.7 W/m2 number varies by 10% depending which radiative model you use, and some climate models have a 20% variation, so I would not take 3.7 W/m2 as gospel, just a mean of the radiation models.
Willis Eschenbach says:
The issue is the change in the absorption, not what name you call the spectrum.
To me, the issue is where the photons come from and what part greenhouse gases play. Only once the physics is understood can anyone begin to investigate what effect a change in CO2 might have. To be clear, between the surface and space, large parts of the IR spectrum are opaque. The way this opacity changes with altitude defines the thermal structure of the atmosphere.
Jim D says
I would not take 3.7 W/m2 as gospel, just a mean of the radiation models.
Henry@jim
So you agree that this “Modtran” is not based on a method that gave rise to measurements that gave rise to actual results?
I wonder – would also like to know how much the models are saying that the CO2 is cooling the atmosphere?
http://www.letterdash.com/HenryP/the-greenhouse-effect-and-the-principle-of-re-radiation-11-Aug-2011
Emission-Absorption
I believe the difference between the emission spectrum looking down near the surface with a sensor altitude of 0.05 km or greater and the emission spectrum of some higher altitude (subtrahend) would qualify as an absorption spectrum. That result divided by the product of the base value (minuend) times the altitude difference would yield the relative absorption per unit distance.
To actually do something like that, you would have to select the ‘save’ option before running the plot. Then you can ‘view the whole output file’ and copy it to your spreadsheet.
Clementi at 4:07 and Smith at 6:44
Let me say first, the red trace in the spectrum shown in the article is an EMISSION spectrum. It is the energy in watts/sqM which the satellite sees looking towards Earth. If it was an absorption spectrum it would be suggesting the absorption in the atmospheric window was high and the absorption at the CO2 line low. The exact opposite of the true situatioon of course. The smooth coloured curves are computations of Planks law for various temperatures assuming a black body wich means a body for which the emissivity is 1. If the emissivity is less than 1 the object will appear colder than it actually is.
Now in response to George Smith – sure a true black body has an emissivity of 1 at all wavelengths however it is perfectly reasonable to consider whether or not an object appears to be a black body at a cetain wavelength which is what I was referring to. Its only a short hand, if you have problems with that please substitute “a body with an emissivity of 1 at the wavelength of interest”. Your point that emissivity only equals absorptivity in equilibrium is simply not correct. Emissivity/absorptivity is a property of the object not of its temperature or the temperature of objects around it. What is true is that the amount of energy absorbed is only balanced by the amount of energy emitted when the object is in thermal equilibrium. That is a totally different proposition. It comes about because the amount of energy emitted depends on both emissivity and temperature whereas the amount of energy absorbed depends only on absorptivity and of course the amount of energy available for absorption. It is true that emissivity/absorptivity both can change with temperature, for example the emissivity/absorptivity of red hot steel can be quite different to cold steel but the two will still match at all temperatures.
Robert – I agree my use of the Nimbus data is probably a little unfair since I have not cited it. My excuse is that I have been doing some work on this recently along the lines of my comments and it occupies my mind. Unfortunately I don’t know how to post a diagram in this blog otherwsie I would send you the plot. I did get it off the web but unfortunately the site seems to have since been taken down. However lets instead look at the red curve in the subject of this thread which is in fact very close to the Nimbus data (except the spike in the middle of the CO2 absorption band was not there). This shows a largely flat bottomed absorption band for CO2 but with a spike in the middle. According to the scenario you were outlining, the absorption at the line centre is so strong that emission to space should be coming from the mesopause. In that case it should reflect 175K and be a downwards spike. Instead it is an upwards spike to a temperature of about 245K. This would be explicable if the band centre absorption was indeed high and corresponded to an emission altitude within the stratosphere but below the stratopause which is at around 270K . Indeed the Modtran plot clearly has enough resolution (judge by the detail in the water bands between 16 and 25 microns) to show up the gull wing pattern I mentioned earlier yet it is not there. This data suggests most of the CO2 is limited to the tropopause or below with a much lower concentration extending some way up into the stratosphere but not to the stratopause. This is entirely compatible with a poorly mixed stratosphere.
If I put sensor altitude to 20 km in Modtran, there is no extra peak around 667 wavenumber..
At 70 km height there is, so CO2 must be net emitter at the stratosphere.From the troposphere there are not coming many 15 micron photons into the stratosphere.You don’t need photons from the troposphere to excite CO2 molecules.There is enough thermal energy in the stratosphere, that collisions between molecules excite CO2 molecules and after that releases photons.That’s why the upper stratosphere is cooling with increasing CO2.
RE Spector: (August 21, 2011 at 12:34 am)
Correction of previous post:
For “That result divided by the product of the base value (minuend) times the altitude difference would yield the relative absorption per unit distance.”
Replace with “That result divided by the base value (minuend) would yield the relative absorption over that altitude range.”
RE: michael hammer: (August 21, 2011 at 3:13 am)
“… This data suggests most of the CO2 is limited to the tropopause or below with a much lower concentration extending some way up into the stratosphere but not to the stratopause. This is entirely compatible with a poorly mixed stratosphere.”
I am not sure that this applies to the above, but the lower square in the MODTRAN tool output seems to show the assumed variation of temperature, pressure, and concentrations of the ‘greenhouse’ gases with altitude. If anyone knows these to be unrealistic then that could invalidate the results of the tool.