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Post by nautonnier on Jun 6, 2009 16:18:49 GMT
glc writes "If you have a reasonable grasp of integral calculus you should be able to follow the article which will hopefully provide a basic understanding of the radiative transfer equations used in the MODTRAN program." We are talking at cross purposes. My problem with understanding is not how MODTRAN works. I am sure it does an excellent job of solving real problems. From my limited knowledge, estimating the radiative forcing of CO2 is not a real problem; it is a hypothetical problem. According to the definition of RF in the TAR Chapter 6, we find "at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values” This to me is a purely hypothetical situation, where high level temperatures are allowed to adjust, but low level temperatures stay constant. Do you agree that this is a hypothetical situation, and if not, why not? And if it is a hypothetical situation, how can MODTRAN, which is designed to solve real problems, solve a hypothetical one? This is my problem with much of this - invented cases like stable 'slab' atmospheres assumed to make the maths workable are then used as if they represented a real-world case. Of course there is no negative feedback if "tropospheric temperatures and state held fixed at the unperturbed values". The oceans are also presumably considered totally static in this model. So simplification of the maths and computer processing time reduction takes precedence over logical concepts and realism in the models. All this seems to be OK when there is a degree of agreement between what the models say is going to happen and what does happen, but when the results continue to diverge at some stage even the most stubborn scientist should think to check on the assumptions and simplifications - but we have a new breed called climatologists who will only accept falsification after more than <name a figure thats high enough> years of divergence.
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Post by socold on Jun 6, 2009 16:47:14 GMT
You have to hold the tropospheric temperatures constant to calculate radiative forcing, because you aren't after the climate response to the forcing but the forcing itself.
Ie if you want to know the radiative forcing from a 2% increase in solar output then you must calculate the energy inbalance caused by the 2% increase in solar otuput that reaches the earth. You can't allow the troposphere to warm up due to the 2% increase or else the inbalance will be lost.
I don't know why they allow for stratospheric temperatures to readjust to radiative equilibrium, but no doubt there is a good reason for this.
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Post by nautonnier on Jun 6, 2009 17:05:58 GMT
You have to hold the tropospheric temperatures constant to calculate radiative forcing, because you aren't after the climate response to the forcing but the forcing itself. Ie if you want to know the radiative forcing from a 2% increase in solar output then you must calculate the energy inbalance caused by the 2% increase in solar otuput that reaches the earth. You can't allow the troposphere to warm up due to the 2% increase or else the inbalance will be lost. I don't know why they allow for stratospheric temperatures to readjust to radiative equilibrium, but no doubt there is a good reason for this. So as JimG has said you are using a hypothetical world not a real one - and therefore the results of the calculations while perhaps interesting results, cannot be taken as relating to the real world. In the case that you state (solar output increasing) then I would expect to see _negative_ feedback mechanisms of increased cloudiness in particular convective weather increasing albedo and carrying heat up from the surface. Similarly in the AGW case the calculations are based on an instantaneous increase in CO 2 but maintaining the troposphere unperturbed. In this imaginary world you can more simply calculate what is called radiative forcing. However, back in the real world the figures from the unreal world remain - unreal. An interesting mind game and mathematical exercise but without application in the real climate. That has not stopped climatological modelers from using these unreal values, but it does appear to have prevented their models from accurately modeling the real world.
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Post by jimcripwell on Jun 6, 2009 17:49:46 GMT
nautonnier writes "That has not stopped climatological modelers from using these unreal values, but it does appear to have prevented their models from accurately modeling the real world."
Thank you for your support. Let me go back to my original question to glc, and see what you think. Myhre et al 1998, merely state that they have used 3 radiative transfer models to estimate a value for the radiative forcing for a doubling of CO2. My whole scientific instinct screams to me that radiative transfer models are completely unsuitable to estimate radiative forcing. However, my physics is not good enough to show that this is so. I could easily understand a paper by Gunnar Myhre that explains why radiative transfer models are suitable to estimate radiative forcing. This paper, so far as I have been able to find out, does not exist. I believe that because such a paper does not exist, therefore the numbers quoted for the radiative forcing for a doubling of CO2 cannot be trusted. nautonnier, do you agree with me that a failure by Gunnar Myhre to write a paper which proves that radiatvie transfer models can be used to estimate the radiaitve forcing of CO2 means that the numbers quoted are meaningless?
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Post by glc on Jun 6, 2009 19:37:57 GMT
JimC, Nautonnier
I think you might be getting hung up on the RF definition. It's true that if we perform a Modtran run which calculates the change in forcing due to a doubling of CO2, say, then we are effectively instantaneously dumping ~300 ppm of CO2 into the atmosphere while keeping tropospheric temperatures and other factors constant. It's the same with laboratory experiments, i.e. we can transmit radiation through different concentrations of gases.
Obviously this doesn't happen in the real world. As CO2 is added troposphere temperatures to the change in RF will respond which - may invoke other feedbacks (both positive and negative). You are using the feedback uncertainty to cast doubt on the whole basic theory. Sure - things might not pan in a totally predictable fashion but there's no reason to think that the ~3.7 w/m2 (~1.2 deg C) is not a reasonably robust result.
Ignoring feedbacks can either of you explain why you think the result of the 'instantaneous' calculation will be appreciably different to the result from the gradual increase.
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Post by jimcripwell on Jun 6, 2009 20:09:52 GMT
glc writes "Ignoring feedbacks can either of you explain why you think the result of the 'instantaneous' calculation will be appreciably different to the result from the gradual increase. "
I still maintain you have got this the wrong way round. It is not up to us to show that radiative transfer models cannot estimate radiative forcing. So far as I am concerned, the shoe is on the other foot; it is up to Myhre to show that radiative transfer models ARE suitable to do the estimation.
So far as my reading tells me, radiative forcing reacts with all four ways in which energy is transmitted through the atmosphere; not just radiation. glc, do you agree with this, or am I wrong?
Also, how can one be "too hung up" on the definition of radiative forcing. If that is the definition, then that is that. One needs to estimate the radiative forcing according to it's definition, or else, once again, show good reason why not adhering to the definition yields the correct answer.
So far as I am concerned, all this discussion shows is what I have suspected all along. Radiative forcing is all smoke and mirrors.
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Post by icefisher on Jun 6, 2009 20:29:47 GMT
You have to hold the tropospheric temperatures constant to calculate radiative forcing, because you aren't after the climate response to the forcing but the forcing itself. Ie if you want to know the radiative forcing from a 2% increase in solar output then you must calculate the energy inbalance caused by the 2% increase in solar otuput that reaches the earth. You can't allow the troposphere to warm up due to the 2% increase or else the inbalance will be lost. I don't know why they allow for stratospheric temperatures to readjust to radiative equilibrium, but no doubt there is a good reason for this. The point being made Socold is arguing for a temperature increase from a forcing without considering the feedback isn't a real world situation. A stew is made of many ingredients. Pointing out one ingredient may not tell you anything about how good the stew is going to taste. Remember Newton's Third Law. "For every action there is an equal and opposite reaction." One needs to observe what that is before somebody can do a real world calculation of the effects that will be realized. Some of the folks around here without any evidence whatsoever are convinced that more CO2 forcing will eventually lead to a warmer planet after natural variation plays itself out. The flaw in that argument is that feedback doesn't play out, its a reaction and in the real world could provide complete balance. . . .or there could be minor or major timing differences. Ultimately AGW plays on fears of the unknown without really any empirical information whatsoever to suggest there is anything to worry about. In fact, there is probably a lot more empirical evidence that a little warming is good than bad and very little empirical evidence we will even get any warming.
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Post by jimcripwell on Jun 6, 2009 22:49:21 GMT
Let me philosophyse again. When I first startesd trying to understand AGW, it was obvious to me that one could never directly measure how much temperatures would rise if you added more CO2 to the air. So, somewhere, there had to be a way of end-running this problem. It was also obvious that there are 4 ways in which energy moves in the atmosphere; conduction, convection, radiation, and the latent heat of water. What I looked for was a graph showing the "greenhouse effectiveness" of CO2 as a function of it's concentration. When I read the TAR, I came across the concept of radiative forcing (RF), which I took to be a measure of the greenhouse effectivensss of CO2, taking into account all 4 ways energy moves through the atmosphere. And in Myhre et al, I found the graph I was looking for.
Now what always worried me was that the way Myhre estimated RF seemed to only take into account the radiative term. As I have explained many times, I dont pretend to understand RF; I just assumed that the scientists who advised the IPCC understood it, and that it was a valid measure of greenhouse effectiveness.
Now, from this discussion, I think I can see that what Myhre and glc are talking about is NOT RF. What it is, I am not sure, but it seems to be some measure of how increased levels of CO2 affect the radiation term in the atmopshere, neglecting the other 3 ways energy is moved. In which case, it cannot be a true measure of greenhouse effectiveness. If this is correct, then the whole basis on which the IPCC has built it's case, is just plain wrong.
Comments?
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Post by glc on Jun 7, 2009 0:08:20 GMT
So far as my reading tells me, radiative forcing reacts with all four ways in which energy is transmitted through the atmosphere; not just radiation. glc, do you agree with this, or am I wrong?
Jim
You're not necessarily wrong, but the other "ways" don't matter (as far this issue is concerned) . Ultimately incoming solar energy is balanced by the emittance of outgoing LW radiation. The earth receives, on average, 235 w/m2 of solar energy. In order to maintain equilibrium it emits 235 w/m2 LW radiation. Some of this is emitted directly from the surface - some from different layers of the atmosphere. In the CO2 band this is at higher altitudes. How energy reaches these layers is irrelevant.
The key point is: How much energy enters earth's climate system and how much energy leaves earth's climate system. and the only way it leaves is by radiation.
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Post by glc on Jun 7, 2009 0:32:11 GMT
The point being made Socold is arguing for a temperature increase from a forcing without considering the feedback isn't a real world situation.
No. This is ridiculous. Your whole post is introducing uncertainties which simply don't exist. I'll try and think of a way of getting the point across in a simple, easy to follow manner, but first can you (and Jim and Nautionnier) answer this question.
Do you accept that the increase in CO2 atmospheric concentrations will impede the flow of energy from the earth's "climate system" to space.
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Post by dmapel on Jun 7, 2009 1:18:40 GMT
icefisher: "The point being made Socold is arguing for a temperature increase from a forcing without considering the feedback isn't a real world situation."
glc: "No. This is ridiculous. Your whole post is introducing uncertainties which simply don't exist. I'll try and think of a way of getting the point across in a simple, easy to follow manner, but first can you (and Jim and Nautionnier) answer this question.
Do you accept that the increase in CO2 atmospheric concentrations will impede the flow of energy from the earth's "climate system" to space."
You are angry and frustrated. But you are being a little bit too aggressive here. Steve will be upset by that.
Look, they are just asking you to show your work. Unless you went to one of those fancy free-form schools in Marin County, you must have been required by your arithmetic teachers to show your work. If you guys want us to bend over and take a multi-trillion dollar reaming to allegedly save the planet, you should at least be willing to show us the calculations.
If it will get you started, I will accept that the CO2 molecules are doing the best job they can to impede the flow of energy from the earth's climate system to space. And I thank them for it.
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Post by socold on Jun 7, 2009 3:37:17 GMT
You have to hold the tropospheric temperatures constant to calculate radiative forcing, because you aren't after the climate response to the forcing but the forcing itself. Ie if you want to know the radiative forcing from a 2% increase in solar output then you must calculate the energy inbalance caused by the 2% increase in solar otuput that reaches the earth. You can't allow the troposphere to warm up due to the 2% increase or else the inbalance will be lost. I don't know why they allow for stratospheric temperatures to readjust to radiative equilibrium, but no doubt there is a good reason for this. So as JimG has said you are using a hypothetical world not a real one - and therefore the results of the calculations while perhaps interesting results, cannot be taken as relating to the real world. In the case that you state (solar output increasing) then I would expect to see _negative_ feedback mechanisms of increased cloudiness in particular convective weather increasing albedo and carrying heat up from the surface. Feedback comes after the forcing. Afterall if there is no forcing there is nothing for the feedback to act upon. There are instant doubling experiments and gradual doubling experiments, not just the first. Again the radiative forcing is distinct from the climates response to it.
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Post by dmapel on Jun 7, 2009 5:12:22 GMT
soclod,
jimcripwell asked for an explanation on why radiative transfer models are suitable to estimate radiative forcing. Seems simple enough.
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Post by socold on Jun 7, 2009 5:19:51 GMT
soclod, jimcripwell asked for an explanation on why radiative transfer models are suitable to estimate radiative forcing. Seems simple enough. Well no it doesn't. That sounds like it could possibly be an essay question at degree level climatology. I don't think anyone here has the expertize to answer it.
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Post by jimcripwell on Jun 7, 2009 11:10:43 GMT
socold writes "Well no it doesn't. That sounds like it could possibly be an essay question at degree level climatology. I don't think anyone here has the expertize to answer it. "
I will openly acknowledge my physics limits me, and I dont understand radiative forcing. However, I can read what is written, and it ought to be self-consistent. The IPCC has defined radiatve forcing. Some people have estimated the numerical value for a doubling of CO2. Others have used this value in other computer models to estimate what will happen to world temperatures in the future. Is it too much to ask that all this work be done on the basis of the same set of rules and definitions? No-one has convinced me that any of the numbers associated with the radiative forcing of CO2 have been estimated according to the IPCC definition. However, very few people seem to think that this matters. Why is beyond me. But how can we be sure that the people who use these numbers in, for example, GCMs, have used them according to the IPCC definition; or do they just invent their own definitions as they go along?
Let me ask a few simple questions, which doubtless dont have simple answers.
1) Has anyone estimated the value of the radiative forcing for a doubling of CO2 according to the IPCC definition in Chapter 6 of the TAR? If so, what is the reference?
2) When the value of the radiative forcing of CO2 is used in other computer programs, is is used in accordance with the IPCC definition?
3) Is the greenhouse effectiveness of CO2 merely a functrion of the radiation balance of the atmosphere, or do conduction, convection and the latent heat of water also play a role?
4) Is it possible, in practice, to estimate how much world temperatures will rise in the future as levels of CO2 increase, purely from the radiative properties of CO2?
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