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.
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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
Guess the little polar bear cub was spot on, wasn’t he?
But I’m not sure about the reasoning:
I followed the AGW narrative of radiation and “back radiation” or rather “absorption of longwave radiation from earth’s surface by way too many CO2 molecules in the atmosphere leads to dangerous back emission of heat towards the earth – thus killing cute little polar bears”. I’m still not sure about the exact physical processes involved. Can we really say absorption = emission, as proposed above? Which I also had in mind when I wrote:
Shouldn’t a picture in the CO2 absorption bandwidth be black?
Jeff, you said:
But what would the actual image from your camera look like? Brighter at the surface area due to 1 degree higher surface temp and on top a little larger due to higher emission altitude? This would indicate an increase in the net flow. Which can’t be happening. (And that’s why I said the image must be dimmer when it is larger.)
So maybe the surface is not warming but cooling when the radiating surface area is increased? Or when the surface IS warming than the (higher) atmosphere must be cooling? What then about back radiation?
Something needs clarification here, don’t you think?
As I said before: we need to finally get the basics right. Thanks for bringing it up.
Jeff Id says:
January 2, 2011 at 12:03 pm
alcuin,
DeWitt estimated the effect you describe at tAV. It is one of those minor things I mentioned above.
“Nope. 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.”
I was not aware that gases formed and presented two dimensional heat emitting surfaces around objects such as planets.
A 150 m increase in effective emission altitude would be 150 cubic meters not square meters. Looks like someone should take another look at their numbers.
Gases emit in three dimensions not two.
Jeff Id says:
January 2, 2011 at 11:35 am
Wayne,
Certainly if the greenhouse didn’t stop evaporation or convection you would have the same situation as with free air. If it stops convection but not evaporation, the answer is unchanged. I specified the windows as transparent, although you could get into all kinds of creative thinking about which wavelengths it was transparent to.
The problem was simply intended to illustrate that the greenhouse effect doesn’t have a lot in common to do with an actual greenhouse.
Jeff,
Thanks for the reply. I see your point clearly now that you point to the convection/evaporation of the open plant to the one in the enclosure. I agree. Water absorbs a great amount of energy in that process. Touché.
However, still don’t think the 1ºC difference due to co2 is possible as viewed and measured by the TOTAL E/M bolometric camera. That’s not the physics I know for you then would have to get into exactly where vertically you measured that 1ºC difference and at any point you pick will have the problem of what exactly is happening to the additional (1ºC worth) or energy that is being emitted, therefore cooling. A folly it seems to me.
Jeff, your views make sense
michael hammer says:
January 2, 2011 at 12:20 pm
Given the description, we can see the surface of the planet (sometimes) in visible light. Recall that the two planets were just Earth with different amounts of CO2 and economic vitality.
So, build a satellite (thought experiment – you can afford the satellite) with both a radar/lidar rangefinder that measures the height above the planet. Add a high resolution IR sensitive LWIR sensor that determines the angle of the IR horizon (heck, toss in one that measures the visible horizon too). From those you can figure out the emission altitude and some (much?) of the profile of where the emissions really come from.
I don’t recall when or how we figured out that Venus’ surface was hot. Why I was a kid people were hoping it would be somewhat warmer and truly Earth’s sister planet. That wound up being the case at a atmospheric pressure of about 1 bar, but sadly that was still awfully high above the surface.
OTOH, landing on and leaving the Moon, Mars, and many other interesting places is pretty easy. Well, accepting that we can’t make a manned round trip to the Moon any more. Pity.
Michael, why should someone be able to correctly determine from space the surface temperature of any planet that has surface? I’ll grant you it is a desirable feature in a well configured planet that is inexpensive to explore, but there’s no astronomical law that requires it.
Mike D says, January 2, 2011 at 9:14 am:
“It is stunning to me that the “warmer in the greenhouse” answer in poll question #1 did not get 100% agreement. Maybe I read the question wrong, but I have reread it numerous times.
For your next poll you should ask this crowd whether the Pope is Catholic or not, or where do they think bears defecate.”
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The info I read was as follows:
“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?”
Please note that the two plants are both thermally stable (temperature isn’t changing). The best place I can think of where a plant can be thermally stable with unchanging temperature late in the afternoon whilst inside a green house in the Northern Hemisphere is north of the polar circle, say at this time of the year. Both the plant inside and the one outside will be frozen stiff of course but they will still radiate equally (the same) in the IR spectrum. The curious thing here is that there is no sunshine providing shortwave radiation. But as we all know anything with a temperature above absolute zero (0 °K) must radiate –etcetera, etcetera.
As for answers to what seems to interrest you at the moment:
It is great possibility that the pope is Catholic, but he could of course be an atheist. Only he knows. Where bears defecate depends greatly on where they are at the time but generally brown- and grizzly bears do it mostly in the forest whilst polar bears may decide to fertilize a couple of plants whether inside or outside of any green house erected in their Arctic habitat.
Now then, which type of bear did you have in mind?
Well, happily it looks like nobody has so far asserted that the greenhouse temperature is a function of the thermostat in the greenhouse, and that the owners are either trying to grow rare arctic flowers or mesquite bushes from the Mojave desert.
I didn’t read all the comments, so forgive me if this has been coveremy comments are repetitive.
The thought experiments make perfect sense, given the assumptions; i.e., that other variables, like clouds and increased evaporation, aren’t involved. The problem with the examples is that they have nothing to do with the real world.
This recent post does a good job explaining the problem with thought experiments that don’t deal with all the relevant variables: http://wattsupwiththat.com/2010/12/28/simple-physics-in-reality-my-feather-blew-up-into-a-tree/#more-30345
Willis Eschenbach has some great comments sabout non-linearity somewhere on this site which also speak to this issue.
Jeff,
Thanks for the reply. I explained my reasoning in my original posts. I am sorry but i find your logic is rather wooly and the whole thought experiment just sloppily worded. There are references to “thermal” radiation throughout your original thought experiment and yet you suddenly jump to discussion of total energy balance and total radiation in your answer. I get the impression that I am not the only physicist who found this wooly. I get the distinct impression that many here do not understand the theory of radiative physics and I’d recommend Liou’s excellent textbook – An Introduction to Atmospheric Radiation.
Ok Jeff Id, I know the happenings in the “Greenhouse theory” are different to what happens in a real green house. If the “Greenhouse theory” is a misnomer then that is because it has gone through some modernizations and alterations (as the science has changed) since it was first thought up and named way back in the early 19th century. What I would like to know is:
1) How can 99% of the Atmosphere be warmed by long wave “back radiation” from a trace gas when Nitrogen, Oxygen and argon are all transparent to the stuff?
2) Is it not more likely that since the “bottom” of the Atmosphere and the “top” of the Earth’s surface are in contact with each other – at all points – heat transfer is taking place first by conduction then convection (evaporation) rather than radiation?
3) Why is it warmer over the top of – than it is below a flame? If heat is radiated should the temperature not be distributed equally in all directions? If it is warmer above the flame does that not point to the fact that convection is a much better form of energy transport than radiation?
4) Why is “back radiation” always depicted as being “one directional” only? (The full force, say 333 W/m² is always pointed towards the surface but there must also be 333 W/m² emitted in all other directions i.e. towards space)
5) How do “Greenhouse gases” stop or delay convection or radiation in order to warm the Atmosphere or as it is called “trap heat”?
6) And lastly – for now – How can radiation which is an energy transport system warm the planet any more than conduction or convection can?
Let the “Greenhouse Theory” go and consider instead the fact that air movements on this planet are mainly horizontal and for that reason alone adiabatic cooling (and warming) take a lot longer than if they were vertical.
Mike D says, January 2, 2011 at 9:14 am:
“It is stunning to me that the “warmer in the greenhouse” answer in poll question #1 did not get 100% agreement. Maybe I read the question wrong, but I have reread it numerous times.
For your next poll you should ask this crowd whether the Pope is Catholic or not, or where do they think bears defecate.”
Because the measurement is being taken outside the box and is measuring ALL radiation energy, absorbed or transmitted or reflected and including all frequencies within it’s view, that is, you are more measuring the box than the plant inside the box. At equal stable temperatures with the same radiation falling on both cases the outgoing radiation measured *will* be identical, Kirchhoff’s law. (but not if evaporation / transpiration / convection is also in play as Jeff used in his answer, then the two cases are not really stable and identical.)
You are more thinking of a *thermometer within* the box and not what the thought experiment laid out. Yes, the thermometer inside would show warmer, duh.
Derek’s assertion doesn’t follow the thought experiment either for he speaks of the brightness of the plant in the image where as the experiment was using a total E/M bolometric camera integrated perfectly to a single temperature, not simply a portion of the image.
BTW: the crude remarks to marginalize many here are not needed from your shallow and incorrect viewpoint.
You appear to forget biotic mineralization on your two planets. Plants use photons to fix carbon and excrete IR. The end of plant life is either recycling or mineralization. Mineralization is also there as plants enter the soil and are converted to coal.methane or in the oceans becoming oil/methane.
The amount of mineralization, or ‘bottled sunshine’, is proportional to biotic productivity, which is dependent on plant growth conditions. All thing being equal, plants grown in high CO2 will grow faster, grow in more marginal niches and be a better source of mineralization. So the planet with with more CO2 will have more plant life, have more complex biomaterial (which is just another form of informatio) and so will emit less energy.
in response to Jeff at 12:49 and Rick Werne at 12:20;
Jeff I still have to disagree with the reason you give. Sure if the concentration of CO2 increases the top 1 absorbance will be at a higher altitude. But that only means the 14 micron emission comes from a slightly higher altitude. It does not mean there is a mean emission altitude for the Earth which goes up. In fact the Nimbus data shows clearly that there are 3 main emission regions. In the atmospheric window the emission is from the surface. At the CO2 line around 14 micron it is from a region at 220K which means the tropopause (no other point in the atmosphere is as cold as that). For the ozone emission at 10 microns it is coming from high in the stratosphere at a temperature somewhat higher than 220K. For the water bands below 8 microns the emission is again from a region at 220K which is again the tropopause. The picture for the region above 16 microns is a bit more complex. There are a large numberof narrow lines in this region and the interferometer on Nimbus did not have the resolution to separate them. The result is that it gives a smeared picture intermediate between the surface and the tropopause but with a very large amount of noise (which is how we can be sure it is not emission from some intermediate temperature).
Thus ignoring for the moment the ozone emission band, emission to space comes from the surface and the tropopause with a neglegible amount from anywhere inbetween. Changing green hosue gas concentration changes the line widths and thus changes slightly the fraction from the surface and the fraction from the tropopause. This is absolutely different from saying there is an equivalent emission altitiude which moves up or down. Claiming an equivalent emission altitude suggests that GHG mediation of emission to space is far greater than it actually is and this leads to an overestimation of the impact of rising GHG concentrations.
Because the concentration of CO2 in absorbance is so high, doubling it makes neglegible change even to the 14 micron emission altitude. I realise you will probably diagree with this statement and I would love to debate it with you and show you my data backing up this claim but it is too lengthy to go into here. By the way, paraxodically, one could argue that if the CO2 emission altitude did rise appreciably it would move into the stratosphere where the temperature is higher so emission to space at 14 microns would increase not decrease reducing the green house effect. Actually, more likely it would simply mean the region at 220K at around the tropopause got a bit bigger.
Rick, in answer to your comment – of course there is no law stating we must be able to measure the temperature of a planet from space but the reality is that we can do so and the concept of an equivalent emission altitude would mean that should not be possible. The way we measure it is to look at a wavelength where the surface can radiate to space and measure the emission intensity at that wavelength. Again, look at the Nimbus data (I tried to incude it here but the system does not allow incusion of images try looking for “CDFS lecture 3” on the web a power point file look at slide 4) and one can clearly see the difference in surface emission between a view over the Sahara, the Medditeranean and the south pole yet the CO2 emission temperature is constant at 220K. Incidentally, the south pole plot shows that the emission at the CO2 line is greater than it would be from the surface which means at the pole, the impact of GHG’s is to cool the surface not warm it. A minor issue but interesting non the less.
Jeff Id says:
January 2, 2011 at 12:49 pm
michael hammer says:
January 2, 2011 at 12:20 pm
I left an explanation of why it happens for will. The mean path length and probability of photons being absorbed changes. The probability of escape for each emitted photon in a higher CO2 atmosphere goes down causing the mean emission altitude to shift to a higher position in the atmosphere. The whole warming effect is just this delay in the power flow from absorption to emission causing an energy buildup.
I don’t really enjoy talking much about it because people then assume I’m a big supporter of AGW extremism. I am not, but the basics are real.
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jeff the thing that astounds me is this. people have a choice.
1. accept the basics and focus on all the uncertainty WITHIN climate science.
2. deny the basics and get pwnd in any technical discussion of the basics.
ourson polaire,
The image would be larger and slightly dimmer so no physics are violated. As DeWitt showed above though it is actually within very small fractions of the being perfectly equal.
Will,
“I was not aware that gases formed and presented two dimensional heat emitting surfaces around objects such as planets.”
You should read up on basic greenhouse theory. The atmosphere has a non zero mean emission altitude, again not controversial physics. Due to the random emission direction, the intensity is uniform as though it were a disk facing you. Look up Lambertian emitter for that. You can however think of it as a shell of slightly larger diameter than the planet.
“A 150 m increase in effective emission altitude would be 150 cubic meters not square meters. Looks like someone should take another look at their numbers.”
Will that really just doesn’t make any sense.
Jeremy,
I’m sorry if you found the wording difficult. My day job is an optical engineer so the concepts seemed clear to me but perhaps I’m too familiar. It would have been a complete surprise though if some didn’t find problems with the wording. It is the nature of blogs.
There is no need for thought experiments here, we already have detailed pressure and temperature data for two real planets with greatly differing concentrations of atmospheric carbon dioxide: Venus and Earth. And that data shows that, over the range of pressures in the Earth atmosphere, there is no difference due to their difference in carbon dioxide. There is no “greenhouse effect” at all, in either atmosphere:
Venus: No Greenhouse Effect
Jeff; I note your commment your day job is as an optical engineer. I do research for a multinational spectroscopy company so we should share the lingo – makes things much simpler. A very interesting starting point for an analysis of GHG issues:- it is abundantly documented that the temperature falls with increasing altitude throughput the troposphere reaching a minimum of about 220K at the tropopause. Thereafter the temperature rises again with increasing altitude. This is the case not just in one region of Earth but all over the planet and over the full year.
So the tropopause is a cold region completely surrounded by warmer regions. How is this possible (given the 2nd law of thermodynamics) and what are the implications for the theory of global warming. The starting point is incontrovertible and the implications are profound.
Sorry, that link has a missing colon; the proper link is:
Venus: No Greenhouse Effect
Michael Hammer
“Because the concentration of CO2 in absorbance is so high, doubling it makes neglegible change even to the 14 micron emission altitude. I realise you will probably diagree with this statement and I would love to debate it with you and show you my data backing up this claim but it is too lengthy to go into here.”
Send it by email and I’ll post it at tAV, I hold no opinion as to whether the difference is negligible as that science is still contestable. The only point I make is that the effect exists.
Judith Curry did a post on that recently where they used the 3.7 watts/meter^2 forcing number. I expressed doubts about the accuracy of that number and asked for explanations. Turns out that most of it was pretty weak. The 3.7 W/m^2 is required for the 1.2C. There are a few basic assumptions which sound straightforward enough that one can accept the number but I’m not sold yet. IMO, the jury is still out.
Glass blocks IR so the camera won’t see the plant at all inside the greenhouse but rather it will only image the glass walls and ceiling like a glowing red building with opaque walls. That half of the though experiment is meaningless.
The two planets are more interesting. They will appear to be identical to an IR camera that integrates all thermal radiation. However if you look at the IR spectrum you’ll see a difference in energy distribution by frequency. The planet with more CO2 will have a deficit of radiation at CO2 characteristic absorption wavelengths and that missing energy in those bands will show up as increased energy in all the other bands.
The glass of the greenhouse and the CO2 in the atmosphere are equivalent to lens filters. The camera only sees what the filter allows it to see.
Michael Hammer,
My previous comment was designed around doubling of CO2.
you wrote:
“But that only means the 14 micron emission comes from a slightly higher altitude. It does not mean there is a mean emission altitude for the Earth which goes up.”
Of course any increase in height for any band, increases the average emission height.
also:
“Thereafter the temperature rises again with increasing altitude. ”
I find this line of thought interesting as well but often it runs into more complex discussions that are unsolvable. A similar situation occurs in geologic borehole proxies where heat is apparently removed from subsurface levels.
“jeff the thing that astounds me is this. people have a choice.
1. accept the basics and focus on all the uncertainty WITHIN climate science.
2. deny the basics and get pwnd in any technical discussion of the basics.”
Can you please, once and for all, tell us all just why you have the “truth” here? Is it your background? Is it your religion? What, Oracle?
Will one of the oracles please answer Huffman?
Actually, Oracle Mosher should probably respond to Huffman, since he has apparently assumed the position of the final arbitrer for the physics of AGW. God help us!
Jeff; re your 6:20 posting, I would love to send you the analysis by email but I don’t have an email address. Anthony you have my email address from these posts. Could you please send it to Jeff so he can give me a return address to write to.
Thanks