Introduction
MODTRAN is a spectral band radiative transfer code first developed in the late 1980s by Spectral Sciences, Inc. in partnership with Air Force Geophysics Laboratories. It was essentially a higher resolution (1.0 cm-1) version of AFGL's LOWTRAN code which integrated radiation transfers over 20 cm-1 wavebands.The most recent version, 5.2, was released in 2009 and is proprietary. A single license runs $1200, technical support and updates are extra. Too rich for this hacker.
Older versions are in the public domain with the full FORTRAN source code available for anyone to download, compile, and execute on a local machine. Too much hacking for this poseur.
Some smart folks at U. of Chicago have been kind enough to host a web-enabled copy of MODTRAN3 Version 1.3 12/1/95, which sports a simple and mostly intuitive GUI and outputs some pretty pictures:
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| Figure 1 - The graphical output for a model run with default options set. Isn't it purty? |
Updates happen automagically every time an option is changes, and it's wicked quick -- it updates almost faster than I can blink. Even more exciting, the Show Raw Model Output button works as advertized, and pops open a new tab (in Firefox on Ubuntu) with plain ASCII tab-delimited output. Copypasta into a spreadsheet application is then dirt simple -- just the right amount of hackery for this Gnumeric/Excel guru/data junkie.
It simply begs for experimentation. So I did, repeatedly. The number of different things I have thought to do with it exceeds my ability to cram into a reasonably-sized post -- and likely the attention span of any putative audience -- so this post is to be a short introduction of how I use the thing and how it compares to real-world observation.
I should probably quit blathering now and get on with it ...
MODTRAN vs. Reality
The About this model link contains this pretty picture (caption from original) ... |
| Figure 2 - MODTRAN results (red) compared with data (solid black) from the Nimbus 3 IRIS instrument from Hanel et al., 1972). |
... which is like, impressively good. The atmospheric window regions from 8-9 and 10-13 microns follow the theoretical blackbody emission curve between 315-320 K, implying a surface temperature between 42-47 °C or 107-116 °F ... otherwise known as damn hot.
The big dip in the 15 micron region is our friendly neighbourhood CO2 (here at 325 ppmv as it was circa 1970-1972) completely gobbling up outgoing photons from the surface ... plus everywhere between there and about 12 km above terra firma. Why 12 km? The 15 micron absorption band touches the 220 K blackbody emission curve, which is -53 °C or -65 °F. Not only is that bloody cold, it's the "standard" temperature of the tropical troposphere at about 12,000 m, or 39,000 feet above the surface ... a place where only jet aircraft thrive.
"Aha," you say. "220 K is ALSO the temperature of the stratosphere at about 70 km, are we not seeing 15 micron photons from there as well?" Short answer is yes. Long answer is the topic of Part 2 (or 3, depending on how many more digressions I chase) of this series, so hold that thought.
The modelled 10-13 micron window runs more toward the cool end of the range, implying that the retrieval in that band is biased a bit hot, the model a bit cool, or some combination of both. If the model, it could be anything from parametrization/vertical profile issues to oversimplified physics.
It could also be completely wrong physics of course, but as the Air Force presumably first developed this beast with the aim of anticipating what happens to aircraft (and/or missiles) at high altitude/velocity atmospheric penetration, I'm guessing their science/engineering teams weren't full of total cranks.
Overall, fidelity to observation warrants confidence that this transfer code reasonably represents reality.
But I want ...
MOAR MODTRAN vs. Reality
The much cited and apparently excellent A First Course in Atmospheric Radiation by G.W. Petty contains this figure, familiar to many climate warriors: |
| Figure 3 - Clear Sky and Thundercloud spectra from a satellite somewhere over the Western Tropical Pacific circa 1970-72. Credit: G. W. Petty (2004) |
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| Figure 4 - Clockwise from upper left: Standard tropical atmosphere temperature profile (surface = 302 K); MODTRAN vs. observational spectra from 100-1500 cm-1; integrated radiative flux looking up, down and the absolute value of the difference; outbound (upward) radiative transmittance from surface to 70 km between 100-1500 cm-1. |
The Planck distribution curves use the temperature of the standard atmosphere at the relevant level, which are 302 K for the surface and 218.9 K at 70 km. The model says that surface emissivity is assumed to be 0.980, which I take into account (hopefully correctly).
The surface temperature is a free parameter in this implementation of the model, I chose 302 K to fit the observational curve. Default for the standard tropical atmosphere is 299.7 K. The input form asks for this parameter to be adjusted using an offset value, so 2.3 is the value I put there to get 302 K.
The transmittance plot represents the fraction of radiation at a given wavelength hitting the simulated sensor which was directly emitted from the source -- in this case the surface. 1 is the maximum value, which means that 100% of the incident photons were emitted directly from the surface target and arrived unimpeded. 0 is the minimum value, and means that 100% of the incident photons were emitted from something OTHER than the surface.
This is important: 0 transmittance does NOT mean that NO photons at a given wavelength are striking our hypothetical instrument. NOR does it mean that absorption at that particular waveband is "saturated" between the surface and the sensor. It only means that whatever radiation shown in the intensity plot for a given wavelength was emitted by some other layer of atmosphere, NOT the surface.
Transmittance for a skyward-pointing sensor would not make sense -- the only relevant targets would be the Sun or the cosmic background radiation, both of which are at frequencies with little overlap in the terrestrial longwave spectrum which is the sole focus of this version of MODTRAN. Somewhat confusingly, the code still returns transmittance in the data dump for an upward-looking model run, but the values are exactly the same as they'd be for a downward looking run at the same altitude.
Finally, the LW flux figure in the lower left corner is the integrated flux in W m-2 looking up, looking down, and the absolute value of the difference between them. In either case, chunking those values into the Stefan-Boltzmann relationship between radiative power and the 4th power of temperature gives -- NOT necessarily the temperature at a given atmospheric level -- but the so-called "effective" temperature of the radiative flux our simulated spectral sensor is "seeing" through all layers of atmosphere to the target.
Normally we'd expect a sensor pointed toward the surface to return a higher integrated flux value than one pointed skyward, in the case of the above plot this is NOT true. Reason being that a real sensor on the real surface looking up is picking up mostly radiation from lower, warmer layers of atmosphere than one at 70 km looking down, which is seeing more radiation from higher, and therefore (generally) cooler atmosphere. Further instalments of this series will show up/down views from the SAME altitude; however, the balance of plots in this post will hold altitude at 0 km looking up and 70 km looking down for apples-to-apples comparison.
Yes, there are MORE plots for this post, hopefully I'll remember what brevity and concision mean as I describe them.
MODTRAN vs. Sliced and Diced Reality
Ever wondered what an atmospheric absorption/emission spectrum might look like if CO2 were the only major LW-active constituent? Turns out there's an app for that: |
| Figure 5 - MODTRAN CO2 Only, 325 ppmv, surface temperature 302 K, tropical atmosphere. |
Compare to the theoretical blackbody temperatures of 283 and 271 K. Am I really telling you that if we magically knocked out water vapour, methane and ozone from the real system, that the atmospheric temperature would fall 76 K (137 °F), or that the surface temperature would RISE 17 K (31 °F)? (!)
No, that's not what I'm saying. Remember -- what our imaginary sensor is seeing in this model is not a "real" temperature, it's just the temperature of what the target "looks" like based on the incident radiation hitting it.
The case of the flux from ground level looking up does make some sense, without (primarily) water vapour in the atmosphere, there would be significantly less "back-radiation" being emitted from the atmosphere, and much of what the sensor would be seeing is the cosmic background radiation of deep space, which has an apparent ("effective") temperature of about 2.76 K ... really chilly.
The situation from 70 km looking down may be less intuitive, but makes sense (to me) with a little thought: removing all LW-absorbers from the atmosphere except CO2 leaves huge swaths of the spectrum which were previously impeded on their way to outer space open for those photons to now take a straight, unfettered shot to 70 km. The transmittance curve confirms this: practically everything outside CO2's main 13-19 micron absorption band has a transmittance of nearly 1.
Assuming constant solar input, all that radiation once hindered from escaping would rush out like air leaving a slashed tyre, and surface temperatures would fall. Dramatically. How much? MODTRAN doesn't do that estimate for us -- the input parameter for surface temp is 302 K, and it "stubbornly" keeps it there. I'll walk through how to derive the new theoretical equilibrium temperature in the next post in this series.
Notice also that outside the main CO2 absorption/emission band we see some non-zero radiant intensity for the blue "looking up" curve. Those are NOT due to CO2, but rather to the default parameters for CFCs, aerosols, dust and what have you that this implementation of MODTRAN (somewhat annoyingly) does not allow the user to futz with.
Some say that water vapour is the most important "greenhouse" gas in our atmosphere ...
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| Figure 6 - MODTRAN H2O Only, surface temperature 302 K, tropical atmosphere. |
Much ado has been made about flatulent cows, warming oceans releasing methane clathrates from sequestration and thawing tundra spewing methane from suddenly not-permafrost because methane is -- ppm for ppm -- a far more potent GHG than CO2 ...
 |
| Figure 7 - MODTRAN CH4 Only, surface temperature 302 K, tropical atmosphere. |
Last on the list for this section is ozone ...
 |
| Figure 8 - MODTRAN O3 Only, surface temperature 302 K, tropical atmosphere. |
Well, no, probably not, because then you'd have to use more CFC-compressed spray-on sunscreen and then we might get one of them there vicious cycles going. Adding more stuff to the atmosphere to mitigate the consequences of the CO2 we've already put there is a double-down on uncertainty for one thing ... we know what CO2 @280 looked like ... a bit chilly but livable FOR CERTAIN.
Oh look, I digress again ... and I've got, like another 6 plots slated for this post. I'll spare us all and only do 4.
MODTRAN and Alternate Realities
... but hopefully not actual future realities. Seriously, you really don't want to do what I'm about to show you to OUR atmosphere: |
| Figure 9 - MODTRAN CO2 3,200 ppmv, surface temperature 302 K, tropical atmosphere. |
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| Figure 10 - MODTRAN CO2 only 3,200 ppmv, surface temperature 302 K, tropical atmosphere. |
Doesn't look bad you say? For the record, 3,200 ppmv is three doublings of 400 ppmv. The worst case IPCC scenario from AR5 (RCP8.5) only calls for CO2 equivalent (all GHGs and aerosols) of 2,641 ppmv by 2225 or so, and 1,231 ppmv equivalent by 2100. They're of the opinion that even 1,200 ppmv is a Bad Idea. I'll compare their projections for 2100 to what MODTRAN says we'd expect at the same level in a future post.
For now, I'll sign off with something that really isn't within the realm of possibility even if we burned all the oil and coal still in the ground:
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| Figure 11 - MODTRAN CO2 965,00 ppmv, surface temperature 302 K, tropical atmosphere. |
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| Figure 12 - MODTRAN CO2 965,00 ppmv, surface temperature 302 K, tropical atmosphere. |
And ... even if it didn't spread out across multiple spectral bands as shown, impeding the path of photons trying to get to TOA is a simple matter of adding more absorbers. The ONLY limit is how many of them you can pack into a given column of atmosphere, which according to the maths of the Beer-Lambert law is theoretically infinite -- just make the path longer, or put more absorbers in the same volume -- optical depth of a medium does not asymptotically approach any upper value as either of those two parameters increase.
Well done, sir.
ReplyDeleteComments on another post were getting so long, I thought it appropriate to move the conversation here if anyone was still interested, especially since it was totally off topic there. For background start about here:
ReplyDeletehttp://climateconsensarian.blogspot.com/2016/03/the-difference-beteween-fraud-and-farce.html?showComment=1459469063237#c7841738144841038700]
RobH and I were discussing latent heat and its relevance to heat transfer through the atmosphere. He explained how water vapor condenses at a sufficiently high altitude such that the concentration of water vapor remaining above that cloud level has very little radiative effect. I agree with that.
What we still may disagree on is whether the whole process--evaporation, release of latent heat in the cloud level, followed by emission of radiation to space--is a net transfer of heat through the atmosphere. What Rob said was “Heat is transported to the atmosphere through this process but that heat is moving [in] all directions, not primarily up. Maybe there was some semantic misunderstanding during our back and forth discussion. Nevertheless, we ended up with a major disagreement over whether more heat is radiated up or radiated equally in all directions.
Here is my understanding based on what I have learned from the climate science community at large, not primarily skeptics. Near the surface, the upward and downward radiation is nearly the same. It’s not, but let’s assume it is. The amount of radiation absorbed from above equals that from below and the same for emissions. The explanation is because the density of molecules at any elevation close to the surface is so large that an emission will result in an absorption within an equally short distance above or below that elevation. Above the cloud layer, the density is so much less that absorption of upward emissions are greater than absorption of downward emissions. Although emissions remain the same, the net effect is a net amount of upward radiation.
Chic,
DeleteNevertheless, we ended up with a major disagreement over whether more heat is radiated up or radiated equally in all directions.
The following should all be review for you because I know we've discussed it.
Any given layer is going to emit equally in all directions. I think you're getting tripped up over the difference between that and net radiative flux, which is quite directional in the vertical.
On balance, net LW flux is always negative, i.e., outbound. Further away from the surface, the greater the net radiative loss from a given layer. This is expected to be true whether the entire system is at a steady-state equilibrium or not.
I think you're getting tripped up over the difference between that and net radiative flux, which is quite directional in the vertical.
DeleteI'm not tripped up. We're talking past each other. Where did I ever say that emissions in any layer are NOT going to be equal in all directions?
Where did anyone other than you agree with me that more radiation is absorbed from below than is absorbed from above resulting in net upward radiation?
On balance, net LW flux is always negative, i.e., outbound. Further away from the surface, the greater the net radiative loss from a given layer. This is expected to be true whether the entire system is at a steady-state equilibrium or not.
Exactly (although I may have reversed the convention of the +/- sign). Now let's see who else will agree.
The original point I was making was that ocean-atmospheric coupling doesn't change the radiative balance of the planet. It's an ongoing process whereby heat is exchanged between the oceans and atmosphere. Conversely, changes in LLGHG's do change the radiative balance of the planet.
DeleteThe other point I was making was, when IR encounters a radiative gas molecule, that re-radiated energy is radiated in all directions, not just up, nor even primarily up.
Perhaps the issue here is regarding time frame. Additional WV added to the atmosphere through ocean-atmospheric coupling will affect radiative balance (clouds are generally a wash) but the residence time of the WV is short. Over the long term the feedbacks due to WV are going to dependent upon longer term changes in global temperature.
DeleteIt's an ongoing process whereby heat is exchanged between the oceans and atmosphere. Conversely, changes in LLGHG's do change the radiative balance of the planet.
DeleteLet me rephrase that to see if I understand. First I assume you agree there is always a net transfer of energy from ocean to atmosphere including by evaporation. You are saying changes in water vapor concentration in the atmosphere from an increase in evaporation do not change the radiative balance, but an increase in IR-active gases (What is LLGHG?) do change the radiative balance?
The other point I was making was, when IR encounters a radiative gas molecule, that re-radiated energy is radiated in all directions, not just up, nor even primarily up.
Although that is a convoluted way of saying it, everyone agrees with that. Emissions occur equally in all directions.
Additional WV added to the atmosphere through ocean-atmospheric coupling will affect radiative balance...
I'm glad you clarified that, because it seemed you contradicted that in your previous comment.
...(clouds are generally a wash)...
Yes in the sense that cloud feedback being positive or negative is controversial.
...but the residence time of the WV is short. Over the long term the feedbacks due to WV are going to dependent upon longer term changes in global temperature.
The residence time of wv is immaterial since it is constantly replaced by evaporation, but yes the net effect of water and clouds depends on temperature. So are we all in agreement so far?
So are we all in agreement so far?
DeleteI believe so.
The point here is that WV cannot act as a forcing. It's always going to be a feedback, whereas LLGHG's can be a forcing due to their residence time in the atmosphere.
WV is only going to change radiative forcing as a response to temperature change.
Ocean-atmosphere coupling is just a property of heat exchanges, moving heat into and out of the oceans.
OK, this is good. Just a couple things more to clear up.
DeleteBy LLGHG do you mean long lived IR-active gases or LWIR-active gases?
It seems you would agree that if CO2 in the atmosphere remained constant while wv slowly increased that wv would be forcing the planet to heat up? I know the former won't happen any time soon, but suppose for whatever reason temperatures continued to rise. Would you consider wv to be a forcing?
"Ocean-atmosphere coupling is just a property of heat exchanges, moving heat into and out of the oceans."
I have a problem with that. Here is where you referred to me to earlier: http://eesc.columbia.edu/courses/ees/climate/lectures/o_atm.html
"The water vapor leaving the ocean is transferred by the atmosphere eventually condensing into water droplets forming clouds, releasing its latent heat of vaporization in the atmosphere, usually quite remote from the site of the evaporation, thus representing a significant form of heat transfer, [latent] heat transfer.
This is crucial. By what mechanism will most, if not all, of this heat deposited at cloud level NOT get radiated to space?
By LLGHG do you mean long lived IR-active gases or LWIR-active gases?
DeleteLong lived greenhouse gases. CO2, CH4, etc.
Would you consider wv to be a forcing?
By what mechanism could this happen?
This is crucial. By what mechanism will most, if not all, of this heat deposited at cloud level NOT get radiated to space?
When that heat is deposited in the atmosphere it's just like any other form of heat. It's being radiated in all directions. You also have air convection taking place, though.
Ultimately all that heat gets re-radiated out of the atmosphere, but the whole issue revolves around how long that heat bounces around within the atmosphere. More greenhouse gases, the longer the pathway for IR to exit the climate system.
"By what mechanism could this happen?"
DeleteMy point is that water vapor is a much more potent IR-active gas. If clouds had no possible negative feedback, more water vapor would be expected from temperatures increasing. That would be a direct forcing, not a feedback.
"More greenhouse gases, the longer the pathway for IR to exit the climate system."
That would be the case if all IR-active gases behaved like a black body. CO2 is only active in a small window and is much less concentrated compared to wv. What physical law mandates that heat must continue to be bounced around after the sun goes down? What prevents the CO2 above the cloud layer from emitting all the radiation it wants to space? I'll elaborate on this after I finish the SkS article.
If clouds had no possible negative feedback, more water vapor would be expected from temperatures increasing. That would be a direct forcing, not a feedback.
DeleteI don't think so. WV is still acting as a feedback to temperature.
It's worth noting that CO2 is a feedback when changes are occurring naturally. It's human emissions of CO2 that make it a forcing.
What physical law mandates that heat must continue to be bounced around after the sun goes down?
As long as the surface is emitting IR and there are GHG's to impede emission to space, then IR is going to bounce around relative to the amount of GHG's in the atmosphere.
What prevents the CO2 above the cloud layer from emitting all the radiation it wants to space?
IR radiated down from above a cloud layer is going to be absorbed and re-radiated by the WV in the cloud. Right?
"IR radiated down from above a cloud layer is going to be absorbed and re-radiated by the WV in the cloud. Right?"
DeleteYes, that's going to happen. What I am getting at is the atmosphere continues to thin as you go above the clouds and on up. Although the emissions are equal up and down at any given level, there will be more net energy radiated to space than down. This is because all the radiation emitted down comes back up, but not all the radiation emitted up comes back down. This is basically how the atmosphere cools. Nothing new or unorthodox about that.
My goal is to learn how to describe the combination of release of latent heat and subsequent emission of that energy to space with equations. If possible to be done correctly, it should allow mathematically expressing how a change in CO2 concentration effects the amount of radiation emitted to space.
Chic,
DeleteWhere did I ever say that emissions in any layer are NOT going to be equal in all directions?
It was strongly implied by you saying that you and RobH were in disagreement. I see now that you two have cleared up that misunderstanding.
My goal is to learn how to describe the combination of release of latent heat and subsequent emission of that energy to space with equations.
Description of the NCAR Community Atmosphere Model (CAM 5.0)
If the body of the documentation doesn't have what you seek, I estimate that the Bibliography contains ~400 literature citations.
Here's what I don't get, Chic. You seem to accept all the aspects of LLGHG effects. If CO2 is a radiative gas then human contributions of CO2 raising atmospheric concentrations will increase the radiative forcing of the atmosphere.
DeleteCorrect me if I'm wrong, but it sounds to me what you're saying is, it's not clear to you how large a forcing that is.
If that's correct then that should be extremely easy to resolve, since the IPCC states that there is a "very high" level of scientific understanding related to CO2 forcing.
Most skeptical researchers (the few there are left) generally focus on uncertainties in the feedbacks, rather than challenging anything related to forcings.
Rob,
DeleteWhen I first started investigating the climate change issue a few years ago, I asked a question at RealClimate about why the rise in temperature occurring between 1910 and 1940 was about the same as between 1970 to 2000? [This is myth #51 on the SkS website.] My post at RealClimate was censored. This confirmed my suspicion that AGW science was politically contaminated. Then the climategate emails surfaced. That's when I decided to question everything orthodox.
You referred me to the SkS myth #30 as evidence of CO2 sensitivity. I haven't finished it yet, but early on I read:
Studies have shown that these radiative transfer models match up with the observed increase in energy reaching the Earth's surface with very good accuracy (Puckrin 2004).
Puckrin et al. compared simulations of surface radiative fluxes based on one clear-sky cold day in 1996 produced by three climate models with corresponding output from an accurate line-by-line code. But the paper didn't mention any “observed increase in energy reaching the Earth's surface” that I could find. Maybe Dana1981 cited the wrong paper. Otherwise it’s misleading.
So call me a cynic, but I'd rather check these things out for myself than take someone else's word for it.
Brandon,
DeleteThanks. You sent me the link to CAM5 model previously. I'm still going through it.
Then the climategate emails surfaced.
DeleteWhat they heck did you find so terrible there?
I asked a question at RealClimate about why the rise in temperature occurring between 1910 and 1940 was about the same as between 1970 to 2000?
DeleteThat one's pretty well understood, Chic. No one has ever claimed that CO2 is the only forcing acting on climate. From 1910 to 1940 you had less volcanic cooling, greater aerosols from early industrialization and higher solar forcing, in conjunction with increasing man-made CO2. The difference today is since volcanic has been normal, solar falling and a much more rapid rise in CO2 emissions starting in the 1960's.
Chic,
DeleteI'm still going through it.
It's a lot to go through.
Rob,
DeleteThere's no sense in dredging up climategate. It won't bring us any closer to agreement on the science will it? It really is my goal.
"That one's pretty well understood...."
Maybe we needed some volcanic activity in 2000 to explain the pause. I admit I don't know the forcings and models well enough to argue against your explanation.
Still, why do you think I didn't get your answer to my question at RealClimate?
Maybe we needed some volcanic activity in 2000 to explain the pause. I admit I don't know the forcings and models well enough to argue against your explanation.
DeleteGavin Schmidt has done a forcings adjusted representation of model outputs here:
https://climatecrock.files.wordpress.com/2016/01/modeltempschmidt15.jpg?w=502&h=417
Chic,
DeleteThen the climategate emails surfaced. That's when I decided to question everything orthodox.
Similar experience here, with a slight difference. I didn't so much question everything orthodox so much as it marked the beginning of me studying the science in greater depth than I previously had.
There's no sense in dredging up climategate.
Unless this is a carryover from a previous conversation somewhere else, in this thread it appears you were the first to bring it up.
Brandon, I was just sharing my experience. Don't feel you have to jump to anyone's defense.
DeleteRob, I was being cheeky about the pause. I don't need any more work on my plate, but thanks for link anyway.
This comment has been removed by the author.
ReplyDeleteAnother subject also came up on the "The Difference Beteween Fraud and Farce, Reflux" post that is arguable better situated here.
ReplyDeleteIn one comment, I said "No one knows what CO2 sensitivity is."
That was poorly stated and probably caused most of the confusion that ensued over which sensitivity we were referring to. I realize many factors influence global temperatures and I think the catch-all term for that is climate sensitivity. My purpose in commenting here is to learn more about what is known, right or wrong, about the specific effect on the global temperatures due only to CO2 separate from any other factors, which I call CO2 sensitivity. So at one point I asked,
"Do you understand the difference and why it is crucial to investigate the latter separately. I would appreciate you putting my objective in your own words so I know you get it."
My question was answered with a question:
Do you understand that CS is merely a measure of response to radiative forcing, regardless of the source? Solar or volcanic don't have a difference CS. The differences you get are time dependent. Thus my question to you about whether you understand TCS, ECS and earth systems CS.
I never got the satisfaction of knowing that I was understood. But I learned something very valuable. I was unaware of CO2's "emergent property" which means the sensitivity to CO2 could change with time. But does it really? Why would that be?
Chic,
DeleteI was unaware of CO2's "emergent property" which means the sensitivity to CO2 could change with time.
I thought that discussion was about climate sensitivity to CO2 being an emergent property of AOGCMs rather than an input parameter.
I hadn't thought of it before reading your comment, but in the real system, yes I think it's correct that CS (the response to *any* external forcing) is variable due to non-linear feedback processes.
Ice-sheet albedo feedback is the first one which springs to mind, so let's run with that as an example. My expectation is that climate sensitivity to solar variability would be inversely proportional to ice area.
The discussion was mainly about whether climate sensitivity in general (CS) was well-quantitated. I still am not sure RobH recognizes a difference between that fraction of climate sensitivity due to CO2 alone. However, he did refer to the orthodox 1.2 K for a doubling of CO2, so surely he must.
DeleteIs that 1.2 K constant or is it a function of other factors?
"...but in the real system...."
What other system was anyone referring to?
"yes I think it's correct that CS (the response to *any* external forcing) is variable due to non-linear feedback processes."
I think that is likely true, but I'll let you worry about the degree of variability. I'm concentrating on CO2 and how likely it is to drive the temperature change that induces the feedbacks.
CS changes with time merely because (if I understand it correctly) you're measuring relative to surface temperature. TCS doesn't take into account the thermal inertia of the oceans and ice. ECS is what you get after 30-40 years if you paused the changes in radiative forcing. Earth systems is what you end up with after everything balances out over a few millennia.
DeleteSo, in essence, all CS doesn't change so much as what you can measure at any given time. At least that's my current understanding of it.
I realize that CS, TCS, and ECS are all important distinctions. Please realized that a separate issue is the degree to which a change in CO2 produces a temperature change. That has nothing to do with time since the molecule itself doesn't change. I'm interested in how the change in the concentration of CO2 in the atmosphere affects the temperature. Is that clear?
DeleteOn second thought it's not clear to me. The effect of CO2 is continually confounded by the amount of wv present. So any consideration of CO2 sensitivity has to be done while accounting for wv somehow. The subsequent effect of the other feedbacks is another matter. IOW, ice albedo doesn't change the way CO2 to absorbs and emits radiation.
DeleteChic... SkS has a very good explanation of all this that you can read here:
Deletehttp://www.skepticalscience.com/empirical-evidence-for-co2-enhanced-greenhouse-effect-advanced.htm
If you have specific questions related to this I can find out which of the authors at SkS wrote it and get more detailed information.
OK, thanks I'll let you know.
DeleteI finished the SkS citation "How do we know CO2 is causing more warming?" My critique is not intended to change your understanding of all this, but to let you know where I think the missing links are in AGW orthodoxy. This is an outline of Dana1981’s post:
Delete1. Comparison of ULWR spectra show differences in flux for IR-active gases.
2. Assume DLWR spectra differences represent warming due to specific gases.
3. Radiative transfer models predict these energy fluxes with good accuracy.
2. Describe the change in CO2 concentration by the formula dFc = 5.35 ln(C/Co).
3. Define climate sensitivity as dTi = lamda * dFi where Fi represents all forcings.
4. Assume a doubling of CO2 will result in a temperature change ranging from 2 to 4.5 K.
5. Assume dFc = dFi and calculate the climate sensitivity as lamda = (2 to 4.5)/3.7 K/W/m2 = 0.54 to 1.2 K/W/m2.
6. Calculate temperature change expected to date as dT = (0.54 to 1.2)*ln(390/280)/ln 2 = 1 to 2.2 K.
7. Observe only 0.8 K of warming to date.
8. Rationalize that there is 0.6 +/- 0.4 warming to come, at least.
9. Assume the other forcings cancel out, so the radiative forcing from CO2 alone gives a good estimate as to how much further temperature change to expect.
Notice how many assumptions are in that outline. Some can probably be justified, but the assumption at 4 comes out of nowhere. The assumption at 9 is required to justify the assumption at 5.
If readers aren’t confused enough, Dana calculates a best-case scenario by assuming sensitivity is only 1 K instead of something in the 2 to 4.5 K range, which results in a CS of 0.27 K/W/m2.
The point is nowhere in this scenario is there any confirmation of exactly what the sensitivity to CO2 alone is. There is an assumption that sensitivity including feedbacks is 2 to 4.5 and there is an assumption that all feedbacks cancel out.
Why do you think these are assumptions? These are all aspects of climate science established in the available research. Number 4 is not an assumption. That has been the position put forth by hundreds of scientists working on the IPCC reports for decades now. Number 9 is well established and represented in the radiative forcing chart I've posted several times now.
DeleteChic,
DeleteThe point is nowhere in this scenario is there any confirmation of exactly what the sensitivity to CO2 alone is.
I'm going to start asking you this every time I see you doing this from now on: what is your standard of proof? IOW, what *would* convince you? How do you propose to *exactly* determine climate sensitivity to CO2 is? And for the love of Pete, what would you do with that information if you had it?
Rob,
DeleteThese are all aspects of climate science established in the available research. Number 4 is not an assumption.
Now you are back to assertions and arguments by authority again. I will never respond to that. You already stated how you think CS is well-quantitated. The range itself tells me it's not. What have you said that would make me change my mind?
I'm not arguing to be right on this stuff. I'm investigating the details because I don't trust the consensus, especially SkS. No hard feelings I hope.
Brandon,
DeleteWhat is my standard of proof? I did answer it before by saying that laboratory experiments that simulate the atmosphere would be good, albeit problematic even if I had unlimited resources. Would it satisfy you if I simply say that proof is impossible, but the CS range has got to be narrowed down?
"And for the love of Pete, what would you do with that information if you had it?
Open a brewski and say "Praise the Lord, consensus at last!
I will stop commenting until I make some progress on evaluating the CAM5 model equations.
Also I hope to have a post on the GTE ready, if you'll have it and I get it done soon.
Chic,
DeleteI did answer it before by saying that laboratory experiments that simulate the atmosphere would be good, albeit problematic even if I had unlimited resources.
I'd think it would need to be a pretty big apparatus. And there's always the issue of validation. How do we know that it's an accurate model when there's so much uncertainty in how the real atmosphere works? A lot of really bright people have been working on this for decades. When they say that computer modelling is providing useful insight into the actual system, I tend to believe them.
Would it satisfy you if I simply say that proof is impossible, but the CS range has got to be narrowed down?
I'm happy with the concession that proof is impossible. I'm dubious that a narrower CS range would make any difference to you. Too narrow, and you start asking how they got the error bars to be so small. You know your mind better than me, of course. But from the outside this is what it looks like to me:
Me: Theory says a,b,c.
You: Where's your evidence?
Me: Evidence says x,y,z.
You: That could mean anything, where's your theory?
Me: @$%@$^^@$!$%@#$!!
Open a brewski and say "Praise the Lord, consensus at last!
You're killing me here. For one thing, "consensus" doesn't mean "unanimity" to two decimal places. I exaggerate for effect, of course.
For another thing, *is* a consensus that we're having an effect. How much, how soon, and what happens then, not so much. All narrowing down CS gives you is, maybe, an indication of how much time it would take to reach a given temperature level assuming some future (and unknown!) emissions scenario. It tells you diddly-squat what happens at that temperature level.
There's also a consensus on what 280 ppmv looked like, that's really just a matter of history. We know that wasn't catastrophic ... we're here ain't we? It's difficult to play the odds when you don't know what the odds *are*.
Hindsight may not be exactly 20/20 in this case, but it beats the hell out of our present foresight.
I will stop commenting until I make some progress on evaluating the CAM5 model equations.
That could definitely keep a guy out of trouble for a decade or two.
Also I hope to have a post on the GTE ready, if you'll have it and I get it done soon.
I'll be on the lookout for it. Cheers.
I think this may just be what you're looking for...
Deletehttp://folk.uio.no/gunnarmy/paper/myhre_grl98.pdf
Myhre et al 1998, New estimates of radiative forcing due to well mixed greenhouse gases; GLR Vol 25, No 14, pages 2715-2718
The range itself tells me it's not.
DeleteI believe I made this point before, but I'll make it again here since it's pertinent. The entire range for climate sensitivity includes potentially catastrophic outcomes on a business-as-usual emissions pathway. If CS is ~2°C then we have a little extra time, but not more than an additional decade or two. If CS is in the 4.5°C range then we're really behind the 8-ball.
If one argues for CS below 2°C, then they need to also be ready to argue the upper end could potentially be higher as well (where there are much greater uncertainties related to methane releases).
We could spend the next two decades trying to further constrain CS, but to what purpose? What it all comes down to is risk analysis. We already have a strong level of confidence that we have a massive challenge regardless of a precise estimate of CS. What it comes down to is economic impacts. Which costs less? Do you spend now to mitigate the problem or wait and spend later to adapt? Every economic study suggests that mitigating now presents the lowest economic impact.
Chic... An addendum to one point in the previous thread: I got confirmation that CS is, indeed, an emergent property that comes out of modern climate models.
ReplyDeleteYes, I was glad to learn the term and be reminded of the application to TCS and ECS. Do you have any thoughts/references on whether that applies to CO2 sensitivity alone? I'm still not confident you understand the distinction I see. Maybe there is none. I'd like to know.
DeleteWhat I don't get is the idea that you think the direct CO2 sensitivity holds any special information. That's a well defined and well accepted figure. As mentioned before, most prominent skeptics focus on feedback and accept the forcing due to CO2.
DeleteYeah, as if they know any better than everyone else what the results of the inevitable warming will be. The mind boggles.
DeleteOK, one more comment. I mean it this time.
DeleteRob, there are two basic formulas for CS. One is the simple one without feedbacks, like I outlined above. The formula(s) with feedbacks goes something like this: lamda = dT[2xCO2]/(1-f) where f represents the total feedbacks. So if wv feedback is negative, the CS is lower than predicted otherwise. You can read up on it here: http://www.image.ucar.edu/idag/Papers/Knutti_nature08.pdf
From the Knutti paper...
Delete- - - - -
"Ever since concern arose about increases of CO2 in the atmosphere causing warming, scientists have attempted to estimate how much warming will result from, for example, a doubling of the atmospheric CO2 concentration. Even the earliest estimates ranged remarkably close to our present estimate of a likely increase of between 2 and 4.5 °C (ref. 24). For example, Arrhenius25 and Callendar26, in the years 1896 and 1938, respectively, estimated that a doubling of CO2 would result in a global temperature increase of 5.5 and 2 °C. Half a century later, the first energy-balance models, radiative convective models and general circulation models (GCMs) were used to quantify forcings and feedbacks, and with it the climate sensitivity S (refs 9, 21, 27–31). Climate sensitivity is not a directly tunable quantity in GCMs and depends on many parameters related mainly to atmospheric processes. Different sensitivities in GCMs can be obtained by perturbing parameters affecting clouds, precipitation, convection, radiation, land surface and other processes. Two decades ago, the largest uncertainty in these feedbacks was attributed to clouds32. Process-based studies now find a stronger constraint on the combined feedbacks from increases in water vapour and changes in the lapse rate. These studies still identify low-level clouds as the dominant uncertainty in feedback4,5,33.
Requiring that climate models reproduce the observed present-day climatology (spatial structure of the mean climate and its variability) provides some constraint on model climate sensitivity. Starting in the 1960s (ref. 27), climate sensitivities in early GCMs were mostly in the range 1.5–4.5 °C. That range has changed very little since then, with the current models covering the range 2.1–4.4 °C (ref. 5), although higher values are possible34. This can be interpreted as disturbingly little progress or as a confirmation that model simulations of atmospheric feedbacks are quite robust to the details of the models. Three studies have calculated probability density functions (PDFs) of climate sensitivity by comparing different variables of the present-day climate against observations in a perturbed physics ensemble of an atmospheric GCM coupled to a slab ocean model35–37. These distributions reflect the uncertainty in our knowledge of sensitivity, not a distribution from which future climate change is sampled. The estimates are in good agreement with other estimates (Fig. 3). The main caveat is that all three studies are based on a version of the same climate model and may be similarly influenced by biases in the underlying model."
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In other words, as I read it, any potential damping effect has been sufficiently eliminated even though there are uncertainties with low clouds. I know Andrew Dessler did a empirical study on low cloud effects and found that they are most likely positive, not negative.
http://www.nasa.gov/topics/earth/features/amplified-warming.html
This is further confirmed by geologic events like exit from snowball earth events. And it's evidenced by the necessary radiative forcing to get glacial-interglacial cycles over the past million years.
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