Guest Post by Willis Eschenbach
I’ve been messing about with the “Modtran” online calculator for atmospheric absorption. It’s called “Modtran” because it is a MODerate resolution program to calculate atmospheric infrared absorption written in ForTRAN, which calculates the result for each 1 cm-1 wide band of the wavenumber across the spectrum. Not quite a “line-by-line” calculation, but close. Here’s a sample of the input page:
Figure 1. User input page for the Modtran online calculation for infrared absorption. Left side is user input. Upper right graph shows absorption as a function of frequency. The lower right graph shows the GHG concentrations, pressure, and temperature, as a function of altitude. See here for an overview of the model. Click to enlarge
This shows the situation during the subarctic summer, with no clouds or rain.
Along the way, I ran into a curious mystery, one for which I have no answer.
Here’s the peculiarity I found. I decided to see what Modtran had to say about the “instantaneous forcing”. This is the forcing immediately after a change in e.g. CO2 or other greenhouse gas. In Table 1 of “Efficacy of Climate Forcings” , James Hansen et al. say that the instantaneous forcing from a doubling of CO2 is 4.52 W/m2.
So I tested that claim with Modtran using a variety of different locations, with different combinations of clear skies, cloud, and rain. I started by testing every few hundred PPMV increase, to see if the results were linear with the log (to the base 2) of the change in CO2. Finding that they were perfectly linear, I then tested each situation using 375 ppmv, doubled CO2 (750 ppmv) and two doublings of CO2 (1500 ppmv). I noted the absorption at each level, and compared that to the logarithm (base 2) of CO2. That let me calculate the forcing, which is typically given as the change in forcing for a doubling of CO2. Using Modtran, I get the following results:
Figure 2. Instantaneous forcing calculated by Modtran for different scenarios.
Now, this has the expected form, in that the forcing is highest at the equator and is lowest at the poles. The addition of either rain or clouds reduces the forcing, again as we’d expect, except during subarctic winter when some kinds of clouds increase the forcing slightly.
So the mystery is, according to Modtran, the absolute maximum instantaneous forcing from a doubling of CO2 is 3.2 W/m2 in the clear-sky tropics. I can’t find any combination of locations and weather that gives a larger value for the instantaneous forcing than that. And the minimum value I can find is subarctic winter plus cirrus, at 1.57 W/m2. I can’t find any combination giving less than that, although there may be one.
As a result, according to Modtran the planetary average instantaneous forcing from CO2 doubling cannot be any more than 3.2 W/m2, and is likely on the order of 2.4 W/m2 or so … but according to Hansen et al., the real answer is nearly double that, 4.5 W/m2.
So the mystery is, why is the accepted value for instantaneous forcing nearly twice what Modtran says?
Note that the answer to the mystery is not “feedbacks”, because we’re looking at instantaneous forcing, before any response by the system or any possible feedbacks.
All suggestions welcome, except those that are anatomically improbable …
w.
DATA: Excel spreadsheet here. You don’t need it, though. For any situation, simply use Modtran successively for two CO2 values where one CO2 value is double the other, and note the difference in the calculated upwelling radiation. This is the instantaneous climate sensitivity for that situation.
THE USUAL: If you disagree with me or someone else, in your comment please quote the exact words that you disagree with. This lets everyone know your exact subject of disagreement.
NOTE: I see as I finish this that they have an upgraded user interface to Modtran here … the results are the same. I prefer the older version, the graphics are more informative, but that’s just me.
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Well, and I might be wrong, but the theory of CAGW does not rely on CO2 alone. The theory states that increasing CO2 leads to increasing H20 and the increased H20 creates hot spots in the tropics and it’s these hot spots (via more H2O) that do the warming. CO2 by itself can’t. I’ll only add (the good news) that no hot spot has ever been detected and no increase in H2O has been observed.
Perhaps Hansen and company used LOWTRAN, back in the day it was used for quick rough calculations in the Electro-Optical world since even MODTRAN took a lot of computing resources and time (early 80’s). When we wanted/needed very precise results back then we used HITRAN, but the calculations were so expensive in time and computing resources that we kept the transmission results as tables for later use for as long as possible. From what I can recall the results from using LOWTRAN vs HITRAN could be enough to account for the delta you’ve found.
Emission happens typically 1 ns after absorption.
If I understand what you’re saying…
You ‘checked the reference’ on something ‘everybody knows’, and couldn’t replicate it.
“Check your assumptions.”
Interesting discussion around this going on at JC’s site.
Spartacusisfree | April 12, 2014 at 12:11 pm |
Reply to RobertinAz:
MODTRAN is very good at simulating the planet IR balance. This is because it was derived from the correct physics; 160 W/m^2 mean surface thermalisation. Because it’s proprietary, it can’t be ‘altered’. The IPCC climate model heat transfer is a scam, based on assuming the surface IR is at the 396 W/m^2 black body level, plus the convection and evapo-transpiration.
They then put in fake cooling from the top by incorrectly claiming you can apply Kirchhoff’s Law of Radiation to the semi-transparent atmosphere. When the 40% residual extra energy has produced imaginary extra water evaporation and ‘positive feedback’, they offset it in hind casting by exaggerated low level cloud albedo.
The planet has ~0.97 IR emissivity, used in MODTRAN.
Curious George | April 12, 2014 at 2:38 pm |
Spartacus – you seem to be familiar with MODTRAN. Has it been used in climate models? Does it handle clouds? Does it correctly predict daily surface temperature variations – a) in Sahara, b) in Kansas, c) in the Amazonia?
Willis;
In Table 1 of “Efficacy of Climate Forcings” , James Hansen et al. say that the instantaneous forcing from a doubling of CO2 is 4.52 W/m2.
>>>>>>>>>>>>>>>>>>>>>>>>>
I can’t get the link to load, but my understanding has always been that CO2 doubling = 3.7 w/m2 was the generally accepted value. Is my memory faulty or is there some difference between the two references?
[Loads fine for me, it’s the link in the “References” at the bottom of the page I linked to in the head post. -w]
Yes, this is very odd indeed. I look forward to an explanation from Nick Stokes!
Willis,
Isn’t it that the 4.5 W/m2 from Hansen is for CO2 itself in the atmosphere, but that is not taking into account the overlapping bands with water vapour?
I would not rule out the possibility that the issue that you uncovered is known, but ignored by climate modelers. It would be just one more “parameter’ that needs to be “adjusted” to give them the answer that they want. Very similar to the residence time of CO2 in the atmosphere. When no one was concerned about it and it was a purely academic exercise the residence time was around 10-12 years. Once it became a “parameter” it was “adjusted” up to 100 years to fit the IPCC models.
If the atmosphere were 10 meters deep. I might consider modtran predictions meaningfull. Modtran is just another simulation. No one it seems is intelligent enough to define, predict or measure molar absorptivity for gasses.
It is clear that greenhouse gases do slow radiative cooling rate. H20 is usually over 10,000 parts per million n the tropics. Day night termperature ranges increase from the tropics to the poles. That change can be correlated with prevailing dewpoint. It cannot seen in a trace change in CO2.
The idea of radiative forcing seems a violation of the laws of themodynamics whenever heating is part of the discription.
Where are the experts?
NIST it seem cannot not tell us what ratios are for CO2 and H20, but 10.000 is a bit bigger than 400.
http://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Units=SI&Type=IR-SPEC&Index=0#IR-SPEC
http://webbook.nist.gov/cgi/cbook.cgi?ID=C124389&Units=SI&Type=IR-SPEC&Index=0#IR-SPEC
Each graph must be anotated.
Notice: Concentration information is not available for this spectrum and, therefore, molar absorptivity values cannot be derived.
Something has changed in the online MODTRAN calculator. It produces somewhat different results than it used to. Ref. note #2, here:
http://www.burtonsys.com/climate/MODTRAN_etc.html
@Jim s says:
April 12, 2014 at 12:12 pm
Perhaps the word “instantaneous” has a different meaning in your neck of the woods. Please note we are not discussing water vapor feed backs or hot spots in this particular thread. Hansen’s claim of 4.5 W/m2 is the instantaneous value before the feedbacks kick in.
FYI ” . . . Table 1 of “Efficacy of Climate Forcings” , James Hansen et al . . .” loaded a moment ago.
The mystery is indeed deeper. Because the IPCC value for doubling from present is in fact about 3.7. Cited in many papers, including Trenberth and others.
My money is on Modtran.
The GISS zonal maps for instantaneous forcing for 2xCO2 and 1/2CO@ur momisugly are the nearly precise inverse of each other. It’s not obvious to me why removing half of say 350 ppm should have the same effect, inverted, as adding another 350 ppm,
Willis, I love your posts. I would like you to do modran for h20 at x y z dewpoints then take the residue out and feed it to modtran C02 and get .0001C or so feedback.
I know modtran does not work that way. What is the dew point for 1000 ppm of H20.
Somewhere below 0F.
Spread sheet showing h20 ppm vs dewpoint temp.
http://toms.homeip.net/global_warming/SpreadSheets/Goff-Gratch-vaporpressure.sxc
If it is linear, it should be the same thing but I think the doubling is usually taken from an assumed pre-industrial baseline of 280 ppmv.
I’ve also seen the usual figure for unamplified CO2 to be 3.7 W/m2/K.
This MODTRAN/HITRAN model is where I came in to all this climate mess.
Years ago a friend sent be a link to Dr David Archer’s site where he had course notes and an interface to do this sort of simulation on either MODTRAN or HITRAN (can’t recall which).
He was basically pointing out that cutting atmospheric CO2 in half would not make a bit of difference.
Oddly ,since that time he’s turned his coat and now seems to be a confirmed warmist.
Perhaps his V.C. had a word about where grant money comes from !
The main reason for that is in the influence of stratosphere. Radiative forcing is defined as a change in the flux just above troposphere after stratospheric temperatures have reached the new radiative energy balance, i.e., after the stratosphere has cooled. UChicago MODTRAN does not allow for cooling stratosphere. Results calculated at 17 km looking down are closer to the correct ones, but fully correct values require a model that makes the stratospheric corrections.
You don’t actually think Hansen is telling the truth about “instantaneous forcing” do you?
Please continue with your Modtran mystery research. May i suggest that you check heat balance and heat transfer in Modtran. How the energy that is radiated towards space at a certain level reach that level from earth.
Clue: it can not be done by radiation
I have never been able to determine whether this MODTRAN calculation is for a one shot absorption by GHGs or whether it includes ALL of the multiplicity of absorptions and re-emissions, including the isotropic re-distribution that occurs at each and ever re-emission event.
And if they are assuming a log base 2 behavior, that means they believe Beer’s Law applies, and it doesn’t because Beer’s law presumes the absorbed photons stay dead, and are never re-emitted at any wavelength.
Consequently, I have no confidence in any such calculations.
I also know what the logarithm function is; so a CO2 doubling is 280 > 560 ppm CO2 and it also is 1 > 2 ppm CO2, or one molecule > two molecules of CO2 per liter of air.
The “observed” Temperature and CO2 data, track equally well, whether you take the log base 2 of the CO2 or log base 2 of the Temperature, or the log of neither.
Namely they don’t track at all; see Monckton’s last 17 years and 8 months for example, plus Mauna Loa CO2 over the same time frame.
I think the whole idea is rubbish (of CO2 induced warming).
Yes I do believe CO2 captures some LWIR radiation frequencies; so don’t go there. Roy Spencer, and Frank Wentz can explain the whole thing. It’s called the “oceanic effect”
daveburton says:
April 12, 2014 at 12:53 pm
Something has changed in the online MODTRAN calculator.
Indeed, in the current online version refered by Willis, you still can choose between water vapour influence as pressure or relative humidity, but the results are exactly the same…
In another version at the same university, you can’t even choose the water vapour effect anymore:
http://climatemodels.uchicago.edu/modtran/
WUWT?
Hansen estimate does not take into account of overlap between CO2 and WV?
I was curious where global warming was suppose to warm.
Your results results don’t seem to include the idea that arctic was suppose to get more warming
than anywhere else in the world.
Generally don’t think CO2 does much warming and few watts per square meter sort of goes along with what I assume.
As to your question, maybe Hansen simply applied it global vs doing regionally. And I would guess
doing this should distort the results. But I also think by doing it regionally one also distorts the results. So I find it interesting and interest topic to discuss. Likewise, I would interested in comparison between night and day, though the winter and summer sort does this.
It seems to be global warming is entirely about night time and winter increase in temperature, just any greenhouse effect on the Moon would be entirely about night time and polar warming. Or global warming is not going to increase daytime surface temperature on the Moon of around 120 C, but it could dramatically increase nighttime temperature on the Moon.
And on Earth, if one talking about time of when Earth had average temperature of 25 C, I would not expect much increased temperatures in tropics [maybe less cooling at night in tropics] but mostly expect the Temperate Zones to have a significant increase in average temperature. Or it would be tropical in sense one doesn’t get freezing temperatures at night- and therefore tropical plants could grow in these regions- you can grow orange trees in Oregon or even Canada.