UPDATE: Jeff provides his answer below
Guest post by Jeff Condon (reposted by request from The Air vent)
Derek has been in a war with ScienceofDoom over the what appears to be Planck radiation. I’m actually not sure of his position because it doesn’t make sense yet to me but he left a thought experiment on the thread which could make for some interesting discussion. Some will find it pretty easy, while I bet others will get all tied in knots over it. As a suggestion, taking a thought experiment to an extreme is often a good way to identify a preferred design path or to understand differences in similar situations. I will give the answers in the coming days, they are already written so I can’t back out but as you consider them I’ll warn that this post is not about the subtleties but rather about the bulk differences.
I’m going to paraphrase Derek’s experiment below and then add another of my own. If it’s not the exact same as his it doesn’t matter the idea is still interesting.
For our experiment assume we have a bolometric camera for measuring emitted thermal radiation as an image. IOW a cool toy which in this case happens to detect all EM wavelengths with perfect sensitivity. To be clear, the camera integrates to measure the radiative emission temperature of the object.
We have two plants, one is contained in a transparent box (greenhouse) the other in open air, both thermally stable (temperature isn’t changing). We take an image of the two plants in the early afternoon on our fancy camera, what do you find in the image?
Derek asserts that the greenhouse plant will be warmer and therefore brighter, but lets continue this experiment further.
For our second experiment we have two thermally stabilized earths, one which has today’s CO2 and one which has 2X today’s level. All other conditions are identical and for some quirk of Id-ian physics, they orbit one right behind the other around the sun such that we can observe them simultaneously on our fancy camera. Now the CO2 of the higher concentration planet will block some of the emitted radiation creating the AGW greenhouse effect so the planet has a 1C warmer surface temperature. (For this thought experiment basic physics are required, planets are stabilized and I’m going with a 1C estimate chosen at random). Since it’s my universe, I’m staying on the warm Earth with a functional economy (right side) and sending all the vegetable eating enviros over to the cold economically devastated one on the left. haha- too fun, I probably should stick to the science for this though.
Now from a distant point we observe our otherwise identical worlds using our amazingly fancy camera. What would our camera reveal?
I’ll give the answers to these with supporting explanations tomorrow or the next day depending on how much fun people are having.
============================================================
1/2/2011 Jeff’s answer:
Ok, so the point of this thought experiment was to engage the public in a consideration of the differences between the greenhouse effect and an actual greenhouse. Most here already know that the name itself is a misnomer, but by considering the physics of what is going on we can better understand the argument and better present our opinions on the subject. The majority of the answer below was emailed to Anthony yesterday before he ran the post at WUWT with the note- just to make sure I can’t back out!
I know everyone is wondering what my answers will be to the two greenhouse situations, we’ll see how many will be convinced to change their opinions – or insist that I change mine 😉 . It turns out that both problems are fairly straightforward when considered from an engineering standpoint. In the thermally stabilized systems of the example where temperature is not changing, energy into the system is equal to energy out. We’ll cover the greenhouse vs free air plant situation first. Both plants receive the same energy but the ability to remove heat from the system is limited in the greenhouse plant through convection and evaporation. So in the case of the free air plant, although it is receiving the same energy it has 3 methods of cooling: convection, evaporation and radiation. In the case of the greenhouse, we can consider evaporation and convection negligible so the only option to release the energy is through radiation. Since our camera is only measuring radiation, and since both plants must emit the same energy they receive, the free air plant radiation will sum like this:
Measured EM radiation = Energy in – convection energy – evaporation energy
the greenhouse will sum like this
Measured EM radiation = Energy in – zero convection energy – zero evaporation energy
Therefore the camera will show the greenhouse plant as warmer (brighter) than the free air plant. This holds true even if we include non-zero convection and evaporation for the greenhouse because they are still reduced values requiring a higher EM emission to balance the energy equations.
So now we have the situation where we have two planets, one with more CO2 than the other. We know that CO2 absorbs certain outgoing wavelengths of light. We also know energy in is equal to energy out for both planets. Although this is called the greenhouse effect, it is actually quite different. For both the high and low CO2 planets, the only available cooling mechanism is EM radiation. All the energy coming in has to escape by EM radiation to space, so the equations balance like this.
Measured EM radiation = Energy in
That’s it really. Both planets will measure exactly the same to our camera yet one has a higher surface temperature. The reason this works is that the average energy emission altitude has gone up, allowing a warmer surface yet the net flow is the same.
Caveats: Now I warned that some will get tied in knots over the nuance of this example. There are all kinds of subtleties of the situation which cause minute differences in the planet example. For instance, increasing CO2 will increase the albedo to incoming light, reducing reflected energy and we get a microscopically higher energy in and therefore were our camera of perfect accuracy we could measure a very slightly higher measured radiation from the warmer planet. If this is your explanation, we are in agreement. There are other details as well, but in bulk the answers are A – greenhouse plant is brighter, and B – both are the same.
I read several comments which got the right answer, Carrick was the first to write the correct answer in the comments at tAV, although he kept the answers subtle enough that people had to read it carefully. If you were one who got them both, congratulations. If you are unconvinced by my explanations, ask away and I’ll do my best.
Jeff
Jeff Id says January 4, 2011 at 4:53 pm :
O H Dahlsveen says January 4, 2011 at 3:38 pm: “Jeff, I would still like to know the scientific explanation/answer to my oft repeated question: How are gases like Nitrogen, Oxygen and Argon warmed if they cannot absorb IR radiation?”
Jeff: “Well, who said they couldn’t? I see IR in the oxygen band! See fig 2. It sounds like you’ve been reading Will’s blog.” (Jeff sends me a link which I promised to look at later, as a quick look took up all the time I could spare right then.)
OHD: Well, I have for a long time been of the opinion that they can, as the few air-molecules present as high up as the Thermosphere are said to be N² and O² I have of course got no idea whether they are exited/warmed by short-wave IR, gamma or by x-ray radiation. But The IPCC and other “Greenhouse specialists” have given me the impression that they cannot be warmed by either incoming short wave IR or by out-going long wave IR.
So if 99% of the atmosphere (Nitrogen78%+Oxygen21%+Argon0.9%) cannot be warmed by radiation and at the same time heat exchange between the top of the surface and the bottom of the atmosphere by conduction is so unthinkable these days that it is never even mentioned, I feel I have got to ask: How is atmospheric heating accomplished? As to ignore conduction between the two is to ignore The Zeroth Law of Thermodynamics. (Unless there is a vacuum between the surface and the atmosphere that I don’t know about).
And yes, I have been reading Will’s comments but I did not know he had his own blog. So – no, I haven’t.
Jeff: “One of the things I’ve done at tAV is to try and make things as simple as possible so I’ve made a few posts like this. BTW, Figure 2 of that post should convince every person on the planet that CO2 warming is real. What it shouldn’t do is convince people that it warming is big, dangerous, bad, slightly measurable, significant, or in any way not good. Those points are separate issues.”
OHD: “We may be talking or writing at “cross purposes” here. – If you are not trying to convince me, and others, that warming (by CO²) is big, dangerous, bad, slightly measurable (or measurable at all), significant, or in any way not good. Then we are more or less in agreement as I have no problem with understanding that certain gases, especially Water vapour, has better heat retention capacities than others. And I do understand that the planet is “warmer over all” i.e. warmer on the night side and cooler on the day side than it otherwise would be. However only an atmosphere that can stop or delay both convection/evaporation and if you please radiation can hope to have a “greenhouse effect”
Jeff: Well, as an optical engineer I’ve worked this math often, and were you to even slightly consider chucking this simple stuff out (as you suggest), you should chuck every incandescent light out of your window. I mean right out the window b/c it turns out that if the math is bad, incandescent lights must not function.
OHD: The Governments are chucking the incandescent light bulbs out for us in any case Jeff. It is not the maths that is wrong. The maths get the same answer irrespective of what the atmosphere is made up of. Irrespective of whether the planet is the size of a pea or the sun, Whether it is completely dry or 100% water.
But as you mention the incandescent light bulb, the filament is maybe as close to a “blackbody” as a star is but what do you think cools the bulbous glass bit of the bulb? Why is the air above the bulb warmer than the air just below it when the bulb is lit?
When I was at primary & middle schools (1947 – 1954) we were told by our teacher that a “doubleglazed” light bulb had once been made, i.e. glass vacuum – glass vacuum – filament. When the bulb was lit, the inner glass soon melted. (This was done as an experiment to prove that in the atmosphere, conduction/convection was stronger than radiation.) – I have never found any references to this experiment, but then again I never thought I had to as by the time I was a mechanical engineer (1964) those principles had not changed. The interest in global warming (that had just stopped) was as high in those days as now, the difference was that then they were still looking for an explanation for “The Ice Ages” (At the time The Milankovitch Theory” was out of the window as summer in the Northern Hemisphere was thought to take place just as the earth was at its furthest point from the sun in its eliptical track. As long as the orbit stayed the same size the shape of it should not matter enough for a big freeze. (But that’s another story)
So to be fair, as a mechanical engineer I have never come across a thermo-dynamical problem that has been in need of Planck’s Law. Even when I briefly worked for the “Nuclear Power Industry” the only “extra thing” I had to learn about radiation was how to shield it, avoid it and de-contaminate myself. But then again, I was not designing the reactors.
Jeff: Watts are power not energy, the measure is Joules per second – energy flow. Joules are energy, Joules per second are power. So when you say energy in Watts/meter^2, the term “energy” is incorrect. The Watt unit is actually power.
OHD: I know, but tell that to the IPCC and Trenberth & al
“Jae,
Warmed relative to what? How warm?”
Well, as warm as the surrounding molecules. All the molecules in a given locale are in thermal equilibrium (Local Thermal Equilib. is the official phrase). We don’t have the situation where the HOH and OCO molecules are buzzing around at greater speeds than the N2 and O2 molecules, just because the latter don’t absorb IR. The GHGs don’t just absorb and fire photons, they COLLIDE with neighboring molecules and give them energy (as well as getting energy back from them). IIRC, there are thousands more collisions per microsecond than there are photons being “fired” at STP.
I think that it is all this energy (kinetic and potential) STORED in all these molecules that keeps the Earth “warmer than it should be,” not some “greenhouse effect.” Any greenhouse effects are probably overshadowed–or at least greatly weakened–by convection (lapse rate), just like they are in a real greenhouse when the doors and windows are opened.
It is no simple coincidence that atmospheres on other planets have about the same temperature at a pressure of 1 bar (sea level here) as our planet does. See this article: http://www.ilovemycarbondioxide.com/pdf/Rethinking_the_greenhouse_effect.pdf
OHD, My best guess is that CO2 has an immeasurably small effect on our climate, that doesn’t change the basics of this post.
“Why is the air above the bulb warmer than the air just below it when the bulb is lit?”
Because the glass envelope absorbs the IR and warms up. The air is heated by conduction. If the glass were perfectly transparent to IR a similar effect would occur but over a wider area because Air is more rarefied than glass so the energy would be diffused further from the source.
It’s too bad the government thinks badly of incandescent, they are nearly 100 percent efficient from October until May in my climate.
The point of the post is NOT to convince anyone that the microscopically tiny amounts of CO2 in the atmosphere are bad but rather that the basic effect is real and that it really is not correct to refer to it as a ‘greenhouse’ which actually behaves entirely differently.
My opinion is that if WUWT readers are very clear on understanding the basics, they can make their cases more clearly to the rest of the world. If we stomp around like Will, nobody will or should listen. When I discuss AGW with friends, I say yup it’s a real effect, but it is very much exaggerated, and anyone who tells you they know the answer to how much warming or how dangerous it is, you know you are speaking to an advocate. Nobody really knows.
I do believe that warming is not following mainstream predictions and that there is substantial and growing evidence that CO2 based AGW is definitely not a problem. I also believe that if it were a problem,there is still nothing we can do about it.
[SNIP]
Perhaps if there are things you did not grasp about what I have said, it might be a sensible approach to ask me to clarify them. But I would need you to a bit more specific than to simply accuse me of being “completely indecipherable”.
For someone who purports to be so knowledgeable on the subject, I am surprised that you seem to be completely unaware of the fact that there is no real scientific basis and absolutely zero real word evidence which supports the so called “Greenhouse Effect” hypothesis. On this matter you seem to be seriously lagging way behind the curve Jeff. As a matter of fact the only place the greenhouse effect actually exist is inside the UNIPPC and state funded computer models.
Computer models which in order to model the greenhouse effect, are fed with certain specific kinds of data. One of these, as you may or may not know, is a form of data called convective parameterisation.
What would be the chance of a computer model NOT producing a greenhouse effect if the convective parameterisation was underestimated I wonder ? What happens when you inhibit convection Jeff ? And finally what is the likelihood of the IPCC accurately estimating rather than underestimating the total global atmospheric convection, given the collective personification of a catalog of errors the IPCC have shown themselves to be ? Hmm…. that’s a tricky one!
As for an apology, save it. You are what you are Jeff, what good will apologies be?
[Reply] Sorry about the SNIP, but you should be able to make your points clearly without me needing to. Deep breath time. RT-mod]
JAE,
I agree with everything up to STP. Heat capacity is another matter entirely. It only determines how quickly the gas changes temperature. If the heat capacity of Air were 10x greater than we think it is, it would only affect the reaction time of warming — slightly.
Well Jeff ID I have by now read/studied your blog posting and must confess I am non the wiser.
I can go along with you even to the end of your thought experiment when you say: “So from a few simple concepts, two gasses at the same temp, one transparent the other black (at infrared wavelengths), we’ve demonstrated that different absorption gasses heat differently when exposed to an energy source.”
You have got the two gases (50% CO2-50% transparent) contained in a small canister. Whether one or both gases are heated by the laser is immaterial as they have no choice but to seek equilibrium whilst in the can and they do so just as you describe it.
The words you could have used are “by conduction”.
So let’s take your thought experiment a bit further. Why not ask ourselves what happens to the hot CO2 laden air when it exits the can and joins the rest of the atmosphere? One of the laws of nature (when made easy) says something like this: “Hot air is less dense than the adjacent air and experiences a buoyant force, just like a bubble of air in water”
Questions: How does CO2 get around that law? And if it doesn’t then why does it not form a band at a level above which the rest of the ever thinning atmosphere can no longer support it?
-Ozone (O3) does.-
When CO2 comes out of a chimney it rises straight up into the air with all the other hot gases. But then it cools off and comes down again. Or is blown around in the wind like it normally does. So even when pre-heated the rascal won’t behave any different than any other gas.
-Why is that?-
Jeff Id says on January 5, 2011 at 3:56 pm:
“OHD, My best guess is that CO2 has an immeasurably small effect on our climate, that doesn’t change the basics of this post.”
After reading the above and your further answers to my comments I have come to the conclusion that there are perhaps ‘not such a big difference’ between the ways we look at these things.
Jeff:
“If the heat capacity of Air were 10x greater than we think it is, it would only affect the reaction time of warming — slightly.”
I don’t understand this statement. If you put a bunch of hot rocks in your tent at night, you stay warmer longer. The temperature on Earth is NOT dictated by radiation alone. I hope you read the link I provided. Can you explain the observations presented there (data from NASA) with the GHE concept?
“The specific heat capacity represents the amount of energy required to raise 1 kg by 1ºC, and can be thought of as the ability of a substance to absorb heat. Therefore the SI units of specific heat capacity are kJ/kg K (kJ/kg ºC). Water has a very large specific heat capacity (4.19 kJ/kg ºC) compared with many fluids.”
From: http://www.engineeringtoolbox.com/heat-work-energy-d_292.html
Please see this link. Specific heat capacity is key to understanding atmospheric temperature, as is obvious from the information there.
Jeff says:
“Heat capacity is another matter entirely. It only determines how quickly the gas changes temperature. If the heat capacity of Air were 10x greater than we think it is, it would only affect the reaction time of warming — slightly.”
Well sorry but I don’t buy that, mostly because of what I have read on the subject and also based on that knowledge because it sounds to me like Jeff just made that up. Besides, reaction time of warming is not the point.
Specific heat capacity is the determining factor of the temperature at thermal equilibrium of all gas mixtures. So not only do the most abundant gases dominate but if they also have a higher specific heat capacity then there’s simply no arguing.
The point is can 385 parts per million CO2 determine the temperature of 999,615 parts per million O2 and N2 or is it more likely that 999,615 O2 and N2 molecules in every million will force the 385 CO2 to cool. Particularly considering that O2 and N2 both have higher specific heat capacities than CO2.
See here: http://www.engineeringtoolbox.com/spesific-heat-capacity-gases-d_159.html
I know Jeff is very convincing and all but it has to be said that it stretches credulity somewhat. But the clincher for me has to be the fact that there is zero evidence for Jeff’s claims. Jeff provides no evidence whatsoever and simply expects us all to take his word for it because he knows best.
Our everyday evidential experience tells us that a few hundred warm molecules will be cooled by almost a million cooler molecules, not the other way round.
Sorry if I seem pedantic but I need proof. I don’t have any religious tendencies.
To claim that the effect is so tiny that it probably can’t effect anything, about something that you can provide no evidence for in the first place, is pretty much the same as saying it doesn’t exist at all when all is said and done.
I mean if it’s that tiny of an effect would it not be just as valid to say that the O2 and N2 is cooling the CO2, rather than the CO2 is actually warming the O2 and N2. If it’s that close to call isn’t that what is referred to as semantics?
That is what it seems to have come down to with Jeff’s position here.
But with a concession to all those who have staked their reputation on the fraud, it kind of gets them off the hook, but it gives little comfort to those of us who’s governments seem intent on pushing ahead with Carbon taxes regardless, like here in the UK.
REPLY: take it over here please, you’ve been called out:
http://noconsensus.wordpress.com/2011/01/05/kicking-puppies/
This thread is closed to this argument. – Anthony
Will,
Here you go:
http://noconsensus.wordpress.com/2011/01/05/kicking-puppies/
JAE,
The temperature on earth is dictated by energy flow. The capacity to hold energy just determines the heating and cooling rates caused by changes in the flow.
OHD
“Questions: How does CO2 get around that law? And if it doesn’t then why does it not form a band at a level above which the rest of the ever thinning atmosphere can no longer support it?”
CO2 is very low mass per molecule. If you look at the average velocity of molecules in the atmosphere – speed of sound. It becomes more obvious why separation doesn’t occur. When you combine that with the miniscule 30ms (no reference) between collisions, it’s impossible for gasses to separate by IR absorption in Earths atmosphere.
jeffid:
“The temperature on earth is dictated by energy flow. The capacity to hold energy just determines the heating and cooling rates caused by changes in the flow.”
I hope you know that you are making no more sense than Will!
JeffID: You seem to be ignoring the important parts of my comments to you. Why? I am still waiting for your response to the link I provided above. WHY is the temperature on all the planets so close to ours when assessed at a pressure of 1 bar? Can you explain this with the “greenhouse theory?” Of course not! Somehow,most people cannot think beyond the “box of radiation cartoons!” It is truly wierd!
Jae,
I’m not intentionally ignoring anything. I read the link you gave. My first thought was – cool, my second was, is it true? Then I started considering the nuance.
Gas giants have substantial internal heating. Jupiter is nearly a star. Can we really look at is for an Earth proxy. Saturn has considerable radiation as well. I’m not sure what the meaning is WRT Venus or Mars. Mercury is gravitationally locked. Venus has a super dense atmosphere — consider how strange that is being close to the sun. Mars has a rarified atmosphere. I’ve been into astronomy since I was probably 8 when I accidentally discovered the moons of Jupiter with a spotting scope.
Why is it up to me to answer any of these questions? I wrote a post on the difference between a greenhouse and a greenhouse gas.
“Somehow,most people cannot think beyond the “box of radiation cartoons!”
My points here are no concession to the believers, they are the foundation by which the discussion can progress for all sides.
Jeff,
thank you for deconstructing Will’s “paper” over at tAV. And thank you for doing it so carefully. Otherwise some errors slip through and contaminate your reasoning when you read stuff like this and don’t know all the details.
Maybe you can now come back to my two questions? I think they are to the point of this thought experiment.
1. When the surface of the 2xCO2 planet is 1 degree warmer (and the larger CO2 column is also warmer) which other part of the atmosphere will be cooler and how? Because if not, the image of this planet will be brighter and possibly larger.
2. What happens at the tropopause on the 2xCO2 planet? Why is the tropopause so important?
Thanks again.
Jeff:
You evidently don’t understand the article at all! You, and nearly everyone else, are arguing that a “greenhouse effect” explains any heating that is above the BB temperature. Siddons is arguing that such an effect is not necessary to explain that extra heat, because of the gas law, T = PV/R. V and P depend ONLY upon stored energy (kinetic and potential). Therefore, T has to, also. The article backs up this fact by showing that all planets with atmospheric pressures above 1 bar have about the same T at one bar, REGARDLESS of how much GHGs are present in the atmospheres. IOW, it is impossible to have an atmosphere at a pressure of 1 bar without the temperature being raised above the BB temp (by about 33 C, in fact). He cites pretty powerful empirical evidence from NASA to back that up. There is NO empirical evidence for an atmospheric greenhouse effect! It is very likely, IMHO, that any warming caused by backradiation is immediatelly cancelled by convection–just like in a real greenhouse when the doors and windows are open.
ourson polaire says:
“1. When the surface of the 2xCO2 planet is 1 degree warmer (and the larger CO2 column is also warmer) which other part of the atmosphere will be cooler and how? Because if not, the image of this planet will be brighter and possibly larger.”
I think DeWitt’s answer is best for this one but it may be unsatisfying.
“The luminosity is the same. For 2x CO2 the total power will be spread out over a slightly larger surface area so it will be slightly dimmer, not brighter. And an increase in effective altitude of 150 m (Tsurf + 1 C at a lapse rate of ~6.5 K/km) changes the surface area by 0.005%. If planet 1 radiates 240 W/m2 then planet 2 radiates 239.9889235 W/m2 and Teff is lower by 0.003 K. Good luck measuring that.”
We know that the power out of the ‘stable’ planet must be equal to the power in. Energy must balance so if the 2xCO2 world had a larger emission surface, it must emit cooler or nothing balances. So with a warmer troposphere, how can that happen. The answer is in the lapse rate of 6.5 K/km, which DeWitt was pretty clever to think of this estimate for this calculation. If you increase your altitude by 150meters, you lose about 1C of temperature. By calculating the 1C difference in emission you can estimate how much cooler the emission must be in order to balance the power.
DeWitt found for 150 meters, a net area change of 0.005%, from the power balance he was able to determine the required watts per meter squared of emission such that the planet was stable. Because Watts emitted are are proportional to temp he was able to determine just how much temp change was required to maintain balance — 0.003C cooler.
But now we are talking about a 1C ground level temp increase and a 0.003C change in temp at the emission altitude. Basically the emission surface is microscopically larger than the first order calculations determined so you can think of it as a microscopically more effective radiator. At the average emission altitude of this example though, the atmosphere it is actually (1-0.003)C warmer than it was without the greenhouse gas.
Again, my prayer to the anti-AGW gods is that I don’t believe doubling of CO2 will cause 1C. I don’t know how much it would cause. This was just an example which happens to provide equal irritation to both crowds. The believer crowd finds my number of 1C denial low, others here find it believer high.
JAE.
“You evidently don’t understand the article at all! ”
I don’t see how it applies to this post.
“There is NO empirical evidence for an atmospheric greenhouse effect! It is very likely, IMHO, that any warming caused by backradiation is immediatelly cancelled by convection–just like in a real greenhouse when the doors and windows are open.”
I’m not convinced that there is NO evidence, but don’t disagree that changes in backradiation can be cancelled by convection/evaporation/cloud formation etc. It is just an entirely separate issue.
Jae,
New idea, you are a technical guy, why not do a post on it explaining the highlights for others to read. I’ll carry it at tAV.
jae says:
January 6, 2011 at 5:43 am
“You, and nearly everyone else, are arguing that a “greenhouse effect” explains any heating that is above the BB temperature. Siddons is arguing that such an effect is not necessary to explain that extra heat, because of the gas law, T = PV/R. … The article backs up this fact by showing that all planets with atmospheric pressures above 1 bar have about the same T at one bar, REGARDLESS of how much GHGs are present in the atmospheres.”
The study you reference simply shows that the atmospheres of all of the planets are behaving, approximately, as ideal gases. And you left out one variable – the ideal gas law is actually T=PV/nR, where n is moles (from which mass can be calculated).
For a given mass of gas in an enclosed container you can expect different temperatures to generate different pressures because the density (n/V) is a constant. The pressure must change as temperature changes because the volume available to that fixed number of heated molecules is constant.
What prevents an atmosphere from expanding as it heats up? What is your “container”? At a given pressure we fully expect atmospheres of similar makeup to achieve the same temperature – that’s what the ideal gas law shows. If the atmosphere at 1 bar of pressure on Venus were significantly hotter than Earth’s atmosphere at 1 bar of pressure, the question would be “That’s impossible – what is preventing Venus’ atmosphere from expanding? Where’s the invisible glass ceiling?!”
Think about it from sea level up. If we heated the Earth’s atmosphere a few degrees, the mass of the atmosphere wouldn’t change appreciably (with the mass of the increased energy calculated by m=E/c^2). So you get a hotter atmosphere with essentially the same mass, on a planet with the same surface area. With the same mass on the same area, the pressure of the atmosphere at sea level remains constant no matter what temperature you set your atmosphere to. Since the surface temperature is hotter, with the same pressure, the density of the atmosphere at sea level must decrease. And that expansion is going to happen all the way up.
Good point Steve. The key is that in the ideal gas law, the volume (of the atmosphere) is not fixed. It simply adjusts to the temperature. So the temperature of Earth (surface or any other point in the atmosphere) is not ‘fixed’ by the ideal gas law.
Jeff,
Thanks a lot elaborating on DeWitt’s calculations. Mathematically it definitely makes sense. IF you have 1C warmer surface temp THEN you must have a 150 m higher average emission altitude TO MAINTAIN the radiative balance.
But I’m still not sure about the underlying physics. How heat is transported, radiated and even stored due to GHGs: IF you have a doubling of CO2 THEN you have exactly WHAT?
Wouldn’t that be a nice thought experiment? 🙂
ourson:
There have been many different posts on the physics of the greenhous gas effect (and specifically related to CO2) on many different science blogs and science books, each targeting a particular audience. I found that many of the posts for general audience lack the scientific depth needed to understand the physics, and the scientific papers give full depth but not enough width to understand the bigger picture.
One of the best ‘compromise’ explanations I have ever seen is this post from Chris Colose. It’s got depth (explains the GHG effect in detail) and width (explains context, such as why water vapor has little influence over CO2 induced warming).
http://chriscolose.wordpress.com/2010/02/18/greenhouse-effect-revisited
Please check it out, and let me know if this at least addresses some of your questions regarding the physics of GHG theory.
“But I’m still not sure about the underlying physics. How heat is transported, radiated and even stored due to GHGs:”
Now again, I don’t know the magnitude of the effects but transport and radiation work the same for all gasses. Conduction, convection, phase change and radiation. The concept that heat is ‘stored’ in GHG differently than any other gas would be very strange indeed.
The concept of captured heat, has to do with energy but really the energy is not captured. Instead it is a buildup in a continuous flow of power. As energy flow from the surface to space faces a restriction, the energy concentrates a little before the restriction. Therefore the ‘energy’ need not be stored as new energy is continued to be supplied from the sun. If you shut the sun off, it wouldn’t make much difference which gas you had over your head.