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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Well, first thing I would check in the version that is posted online. I used it long ago when the code was new and still classified. We used that and lotran. It was validated for its use, that being estimating the atmospheric losses of a thermal signature for both look up and look down threats.
A look up threat would typically be looking from the ground up to 10K,30K and 50K. 50K was the top of our operational envelop. A look down threat, might look from from 50K to 10K. Code was delivered to us and were under contract to use it.
Possible reasons: Some of the error will be due to the fact that its a band model and a good one would get you within 10% of an LBL. Check the relative humidity settings. I cant recall how that is supposed to be set. It may not handle the strat very well.. line broadening.. not sure because the thing on the web doesnt look like what we worked with.. that was more like what you see in the doc below.
http://www.google.com/url?sa=t&source=web&cd=3&ved=0CCYQFjAC&url=http%3A%2F%2Feastfire.gmu.edu%2Fcourse%2Feos556%2Freadings%2FMod4V3R1Manual.doc&ei=PPVNTuWcF-PWiAKs1_icAQ&usg=AFQjCNGRRhBwGPc2nWTZRLBk7JBBK84Wjg&sig2=qyAe56dIVCE9nCgZQsaYEg
Bottom line. I dunno.
My understanding of ModTran is that it is based on an Air Force program to calculate IR visibility and is not related to any IPCC program. I have used it to calculate what I believe to be raw CO2 temperature sensitivity. I normally use it assuming that 292.993 W/m2 is the amount of heat received from the sun that must be expelled in an equilibrium condition–this is also one of those numbers ModTran likes to produce. I usually leave most of the parameters unchanged from their default states.
So, if I start with a CO2 concentration of 396 PPM, I find that I have to set the temperature offset to 1.48 degrees to get an Iout of 292.993 W/m2 and surface temperature of 301.18 deg K.
Then with a CO2 concentration of 792 PPM, I find that I have to set the temperature offset to 2.4 degrees to get an Iout of 292.993 W/m2 with a surface temperature of 302.1 deg K.
Finally with a hypothetical CO2 concentration of 17,920 PPM, I find that I have to set the temperature offset to 8.04 degrees to get an Iout of 292.993 W/m2 with a surface temperature of 307.74 deg K.
It appears something has changed recently as these results are slightly different than those I obtained about fifteen months ago as follows:
Tropical, Clear Air Default Setting Atm Log2 Gnd Tmp Surface Surface Log2 CO2 (CO2/280) Offset Tmp °K Tmp °C Slope 70 -2.0 -0.81 298.89 26.7 0.9 99 -1.5 -0.365 299.34 27.2 0.9 140 -1.0 0.08 299.78 27.6 0.9 198 -0.5 0.52 300.22 28.1 0.9 280 0.0 0.96 300.67 28.5 0.9 396 0.5 1.41 301.11 29.0 0.9 560 1.0 1.86 301.56 29.4 0.9 792 1.5 2.33 302.03 29.9 0.9 1,120 2.0 2.81 302.51 30.4 1.0 1,584 2.5 3.31 303.01 30.9 1.0 2,240 3.0 3.835 303.54 31.4 1.1 3,168 3.5 4.39 304.09 31.9 1.1 4,480 4.0 4.99 304.69 32.5 1.2 6,336 4.5 5.63 305.33 33.2 1.3 8,960 5.0 6.34 306.04 33.9 1.5 12,671 5.5 7.11 306.81 34.7 1.6 17,920 6.0 7.97 307.67 35.5 1.8 25,343 6.5 8.94 308.64 36.5 2.1 35,840 7.0 10.03 309.73 37.6 2.3 50,685 7.5 11.26 310.96 38.8 2.6 71,680 8.0 12.65 312.35 40.2 2.9Dear Willis,
AFAIK there was a major change of the MODTRAN-database concerning CO2 about 8 years ago, because some of the interaction of CO2 with other molecules was missing before.
Brian W says:
August 18, 2011 at 10:27 pm
You expect me to explain what Hansen means by some random quotation from his paper that has nothing to do with this thread? Why would I want to do that?
I give up, Mister Wizard, how much has the solar constant changed in the last 100 years? Please show your work …
What I don’t get, Brian, is what any of this has to do with the subject of the thread. The change in the solar constant is unrelated to the change in absorption from CO2 doubling, why are you brining it up? The subject is the MODTRAN results and their difference from the IPCC and Hansen’s results.
w.
Why are Hansen’s numbers different? It may matter what Version of MODTRAN you use? Versions prior to around 1999 could not adequately deal with multiple scattering events on the azimuth of the line of sight. Scattering may be inappropriately interpreted as absorption.
Nick Stokes says:
August 18, 2011 at 10:47 pm
Thanks, Nick, but that’s all part of the response to the CO2 doubling. I’m looking at the instantaneous forcing, what occurs when the CO2 doubles but before the system responds. As such, warming and water evaporation don’t enter the picture … which is why I’m looking at instantaneous forcing, to avoid all questions of temperature changes and what they might bring.
w.
Spector says:
August 18, 2011 at 10:52 pm
Spector, again, I’m not looking at how much temperature change corresponds to the forcing as you are commenting on. I’m looking at the instantaneous change in forcing due to the change in CO2, and NOT at the change in forcing.
w.
Laws of Nature says:
August 18, 2011 at 10:57 pm
Does anyone have any actual information on this question?
w.
Robert Clemenzi says:
August 18, 2011 at 9:17 pm
Not true for what I’m looking for. Instantaneous forcing at the TOA (top of atmosphere) is the change in the absorption, which is calculated from the top looking down. When CO2 increases, absorption increases, that’s what drives the whole process.
w.
I am just going to give a dumb laymans perspective here. Why is it when there is about 20 watts of natural forcing relating to perihelion versus aphelion and when it is perihelion there it is the southern hemispheres summer so there is more sunlight and it is stronger directed at the water, we don’t all fry? Wouldn’t there be more water vapour to heat us all up. So to me it seems unlikely even if these what appear to be made up numbers do exist they are less significant than the natural numbers. The other point is that if you are north of the Tropic of Cancer or south of the Tropic of Capricorn the amount of sunlight will vary on average by more than 50% but temperatures vary by much less than this of course. So on alarmists theory as soon as it is your time for summer then with all the many feedbacks you will be in big trouble.
Rob Z says:
August 18, 2011 at 11:11 pm
Thanks, Rob. That’s possible … but the MODTRAN numbers are way, way below both Hansen’s 4.5 or the IPCC 4.1 W/m2. I don’t think that’s some problem with scattering events being interpreted as absorption. This is because if you interpret scattering as absorption, your absorption figures will be too high … but in this case they are too low, much lower than Hansen/IPCC.
w.
Well Willis, the first question I would ask somebody to explain is this. At the mean earth surface Temperature of 288 K the calculated (BB) emission would be 390 W/m^2, and that is indeed the value, that Trenberth gives for the surface emission in his (in)famous earth energy budget.
Of that 390 W/m^2, only 40 W/m^2 escapes to space; 10%. Now using your MODTRAN graph (red jaggie) explain how those holes are taking out 90% and only 10% is escaping.
Also note that the vertical scale is W/m^2 per wavenumber increment; and there’s a whole lot more wave numbers at the right hand end of the graph, than there is at the left hand end, so this rendition grossly exaggerates the low wavenumber energy content, and also the amount of energy removed by the “big” CO2 dip.
Plot the same data on a wavelength scale, rather than wave number, and suddenly the peak moves to where that innocuous Ozone dip is around 1000 cm^-1, or 9.6 microns, and the CO2 hole is now down on the tail. But of course, now that right hand end has more wavelength microns, than the left hand end, so again a distortion occurs.
Howcum, nobody ever seems to plot the integrated energy emission as a function of either wavelength or wave number, so we can see where the energy is really escaping..
Also, does that MODTRAN calculator also calculate the thermal spectrum emitted from the atmosphere to separate it from the thermal spectrum emitted at the start from the surface ?
Oh I forgot, when molecules slough off the surface and become part of the “atmosphere” they suddenly and mysteriously stop radiating thermal radiation.
Willis Eschenbach
You sure are a slippery fellow, but since you ask . The current solar constant is stated as 1366w/m2 correct? My Passive Solar Energy book circa 1979 gives the solar constant as 429.2 btu’s/sq. ft./hr. This is equal to 1353.95w/m2, for a difference of 12w/m2 (rounded). This is a nearly four times difference from 3.2w/m2 and using the IPCCs 3.7w is times still more than three times the amount.
My point is Mr. physics authority is that MODTRANS numbers are so far off they are MEANINGLESS!
Oh and CO2 is a feeble absorber of IR compared to water vapor.
A second question I would ask, is: does MODTRAN calculate the whole cascade of absorption and re-emission from the GHG molecules, or does it take the brain dead “Beer-Lambert Law” approach of assuming that captured photon energy stays captured. In the case of the atmosphere, the thermal emission spectrum, is roughly the same as the surface emission spectrum, so it is a bit difficult (experimentally) to measure the original surface surviving flux (at 70,000 ft or wherever it is observed), and separate it from the atmospheric re-emissions. Of course some people believe that atmospheric re-emissions can ONLY be at those very same GHG molecular absorption bands.
Willis I believe the online version is 1995. If you want to know all the changes they are most likely available in the release notes.
And for clarity HITRAN is the database of molecules.
you can also start here . Satillities work because they use RTEs and you’ll vary often find groups doing comparisions of various models
http://rtcodes.ltdri.org/Description_of_the_codes_simple.htm
GCMs will aslo use band models ( Err i think.. need to check)
GIYF
MODTRAN compare LBL validate
Willis: Modtran is not producing an absorption spectrum; it is calculating the outward infrared radiation that would be seen looking down from 70 km at the nearly-blackbody radiation emitted by the earth’s surface AFTER being modified by absorption by GHG’s AND emission from GHG’s as it passed through the atmosphere. At strongly absorbed wavelengths, >50% absorption can occur in tens to hundreds of meters, so few of the photons escaping to space at these wavelengths were emitted by the earth’s surface. At wavelengths strongly absorbed by CO2, the upward radiation has been emitted on the average by CO2 molecules near the tropopause where the temperature is about 220 degK. The photons emitted by the ground that escaped through the atmospheric window (800-1000 cm-1) have an intensity about right for blackbody radiation at 300 degK with an emissivity a little less than one.
When you add clouds, some (30% ?) of the radiation is emitted by cloud tops and some by the ground (70% ?). Low clouds are much warmer than high ones.
The IPCC’s 3.7 W/m2 is the difference in net outward radiation at the tropopause (not 70 km) for doubling CO2 averaged over the whole global (after stratosphere, but not troposphere, temperatures have equilibrated) taking into account average cloudiness, the vertical temperature profile at various locations on the earth, the varying height of the tropopause etc.
Myhre (1998) is available at http://folk.uio.no/gunnarmy/paper/myhre_grl98.pdf. This paper keeps referring back to results and methodology from earlier papers behind pay walls and certainly doesn’t provide a complete explanation.
RE: Willis Eschenbach says: (August 18, 2011 at 11:16 pm)
Spector, again, I’m not looking at how much temperature change corresponds to the forcing as you are commenting on. I’m looking at the instantaneous change in forcing due to the change in CO2, and NOT at the change in forcing.
I believe the ‘forcing’ is the 292.993 W/m2 energy flow that I keep constant. I am assuming a constant solar forcing energy flow for all CO2 concentrations. This result does not come out of a single run. I set the CO2 concentration and change the temperature offset — 2, 2.5, 2.6, 2.65, 2.63 …– until I reach my standard 292.993 W/m2 forcing level. Then I have the temperature required to force the standard amount of energy into outer space. Once I have a table of temperatures and corresponding CO2 levels, I can use the Microsoft Excel Solver utility to make a polynomial approximation of temperature as a function of LOG2(CO2/280) to get the doubling slope from its derivative polynomial.
Moderators: please give the full text of obscure (ie, American) acronyms.
Eg
AFAIK [as far as I know]
WUWT is a global resource, and acronym usage varies widely.
It is my understanding that the Air Force had good reason to make sure these ModTran calculations were accurate. From a climatological point of view, this is a raw CO2 effect–it does not include effects such as increasing cloud cover as a result of increasing temperature. It only cares with how CO2 and the other gases modeled affect atmospheric transparency.
Hello Willis; The effect of CO2 s to block radiation to space (at the co2 absorption wavelengths) from the surface and replace it with radiation fro the top of the CO2 column at these wavelengths. The plot you give in your article makes that abundantly plain as does the down looking radiation spectrum as taken by the IRIS experiment aboard the Nimbus satellite. Knowing this gives an easy way to calculate the average (or for that matter spot) energy retention due to changes in CO2 as follows.
CO2 absorbs from about 13.4 microns to 16 microns. Integrating Planks law (known for nearly 100 years) over this wavelength range for a 288K black body and a 220K black body yields respectively 49.1 watts/ M2 and 16.7 watts/ M2 .
Without CO2 the surface would not be quite radiating 49.1 watts/ M2 over this wavelength range because of the action of clouds. Clouds also absorb thermal infrared emission from the surface and in turn emit to space from the top of the cloud. Much of the cloud is at an altitude of about 2-3 km and at that altitude the top of the cloud is at about 273K. Integrating Planks law for the above mentioned wavelength range at 273K gives 40.4 watts / M2. Taking the generally accepted figure of 60% cloud cover, the emission to space without CO2 would be 40% from the surface and 60% from cloud tops giving;
49.1 * 0.4 + 40.4 * 0.6 = 43.9 watts / M2 without the intervention of CO2
CO2 reduces Earths emission to space by 43.9 – 16.7 = 27.2 watts/ M2 which agrees well with other estimates. Now the energy retention versus concentration is logarithmic, again widely accepted, and in the case of CO2 at 280 ppm it has been logarithmic for about 9-10 doublings (based on the known absorbance of the CO2 column at 280 ppm of around 2000 abs). Thus each doubling contributes somewhere between about 2.7 and 3 watts / M2.
As I said, this is the average for the whole of the planet and as such it agrees very well with the Modtran data you quote. Its easy for example to repeat the calculation using tropical suface and tropopause temperatures. Doing the calculations this way shows up some VERY interesting issues. Firstly the emission temperature is that of the tropopause so it implies the top of the CO2 column is at the tropopause yet established wisdom claims CO2 is well mixed in the statosphere (there is a significant fraction of the atmopshere above the tropopause). If the stratosphere was indeed well mixed the emission temperature would be significantly higher (look at the emission temp ofr the O3 line at 10 microns whcih does come from high in the stratosphere and you will see what I mean) and the impact of CO2 would then be LOWER.
Also you do not show it but I have some Nimbus data for the Antarctic and it shows a PEAK not a dip at the CO2 line. That means that in the Antarctic the imapct of CO2 is to cool not warm. How come? Well the surface is not much warmer than the tropopause but more significantly the equivalent black body temperature for the surface (the apparent temeprature in the atmospheric window) is only 180K which is far colder than the actual temperature. How is this possible? The only way this can happen is if the surface emissivity is low. That means the emissivity of snow and ice in the thermal IR is nowhere near 1 as claimed by the warmists but is infact less than 0.5 which speaks against the claim of massive positive feeedback if the ice melts. I could go on, there is a huge amount of information that can be derived from this analysis.
Willis,
Does this maybe help?
IPCC AR4 refers to Ramaswamy [2001], which is IPCC TAR, and IPCC TAR states the following:
“IPCC (1990) and the SAR used a radiative forcing of 4.37 Wm-2 for a doubling of CO2 calculated with a simplified expression. Since then several studies, including some using GCMs (Mitchell and Johns, 1997; Ramaswamy and Chen, 1997b; Hansen et al., 1998), have calculated a lower radiative forcing due to CO2 (Pinnock et al., 1995; Roehl et al., 1995; Myhre and Stordal, 1997; Myhre et al., 1998b; Jain et al., 2000). The newer estimates of radiative forcing due to a doubling of CO2 are between 3.5 and 4.1 Wm-2 with the relevant species and various overlaps between greenhouse gases included. The lower forcing in the cited newer studies is due to an accounting of the stratospheric temperature adjustment which was not properly taken into account in the simplified expression used in IPCC (1990) and the SAR (Myhre et al., 1998b). In Myhre et al. (1998b) and Jain et al. (2000), the short-wave forcing due to CO2 is also included, an effect not taken into account in the SAR. The short-wave effect results in a negative forcing contribution for the surface-troposphere system owing to the extra absorption due to CO2 in the stratosphere; however, this effect is relatively small compared to the total radiative forcing (< 5%). The new best estimate based on the published results for the radiative forcing due to a doubling of CO2 is 3.7 Wm-2, which is a reduction of 15% compared to the SAR. "
http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/219.htm
J.
Roy Clark says:
August 18, 2011 at 9:34 pm
[,,,,]
Very interesting comment Roy. Willis will need to keep this thread focussed (good luck Willis), but your thesis deserves a thread of it’s own IMO.
I would consider it a safe assumption that the program in question is not designed to produce a scientific result but an engineering result within the operational parameters of the equipment it was developed to calibrate. Roughly equate that to “horseshoes and hand grenades.” The equipment being calibrated is purposed not to hit the nail on the head but to at least throw the hammer in the general range of the carpenter (the next technological solution in the chain of response).
Just like much of the “climate science” going on right now where people continue to stroke worthless data trying to curry information out of it, I think this “tool” is useless because it is being recalibrated using curried data.
Keep in mind, always, that when you use programs designed for laboratory conditions to attempt a replication of real world data in non-laboratory conditions you are simply generating a larger margin of error. No matter how much you compare your large error to someone else’s large error, they’re still both errors.
I wouldn’t rely on this program or the results it produces either to model or calibrate data because it doesn’t produce a result, it produces a fuzzy-operator for an active calculation system that relies on statistical floatation simply to find anti-noise. The entire system behind MODTRAN was developed to replicate a “predator reflex” in technological form and I sincerely doubt the program you are using has any comparatively useful level of the comprehensive functionality of the still classified versions.
The above typed, keep in mind that the purpose of the discussions here is to back-track into all the science and “science” that has gone into the current arguments for and against man made global climate change. We are here to nit-pick on individual details of the methodology and tools used by the global climate “community” and it is immaterial which side of the tracks you live on.
… The primary issue *I* have with this “model” software is that the earth is not flat and the sun doesn’t turn on and off above it. The production of any “per uniform dimention” data is useless to the furthering of this scientific endeavor for however long the IPCC studies and other papers continue to insist that the only variables are infra-red radiation and greenhouse effect media (gas, vapor, whatnot). This program isn’t designed to produce scientific results, it is designed to pick out a target from background radiation on a moment-to-moment basis.
Where did you get the 1850 285ppmv CO2 concentration from? Research back in 1850 from around Europe, UK, Italy and Germany, puts this level at 490ppmv using the chemical adsorption method used by some today.