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
IN = OUT … or … BOOM.
A tweak to the question that explicitly states that we aren’t interested in the instantaneous changes as opposed to the relative values of the differing equilibrium states would seem helpful.
In the first example the plant is slightly cooler due to the glass absorbing some of the light including some IR, however the glass enclosure is slightly warmer as a result. The volume occupied would contain the same amount of energy as the identical volume for the unenclosed plant (assuming no air movement – perhaps a vacuum is required, but that sucks for the plants).
The two planets will show the same temperature signature as you are not adding any more energy to either planet – so where would any additional heat be coming from?
Don’t you mean ‘subtleties’, not ‘subtitles’?
[Fixed, thanks. ~dbs, mod.]
“… I’ll warn that this post is not about the subtitles but rather about the bulk differences.”
Subtitles or subtleties?
[Fixed, thanks. ~dbs]
The same amount of heat would be leaving each if they are each in equilibrium. But the distribution of EM frequencies would be slightly different. The CO2 planet would have the EM at slightly higher frequencies.
An interesting connection between Common Purpose and Climategate
http://www.stopcp.com/cpclimategate.php
http://www.stopcp.com/cpclimategate.php
Happy New Year.
Ok – had a quick read and tried to get a handle on the basic premise. A bit unfair for a Jan 1st post methinks!
for the first one – it bothers me that there is no mention of transmissivity of radiation through the glass into (or out of, but I’m assuming its irrelevent!) the greenhouse compared to the external plants. The term transparent would imply completely transparent to all radiation (in and out) ? Anyway, I deduce that the thought experiment is basically about thermal convection/conduction restriction within the transparent box? (which of course is the PRIMARY way a greenhouse gets warmer than surrounding (external) air)
On the assumption that transmissivity and reflectivity is meant to be the same for both plants but convection (and conduction) is restricted within the ‘box’ – the plant in the box should indeed appear ‘warmer’ or brighter.
The second experiment describes identical planets at thermal equilibrium but with differing CO2 – but because the second planet is 1C warmer it should reflect that within its EM spectra. Because they are stated as thermally stabilized, and in equal orbits, etc, the radiation in must equal the radiation out but the measured spectra would/should show the 1C surface temp difference. Ergo, I would guess the warmer planet seems ‘brighter’ but the actual temp difference has arisen due to the time lagged equilization of radiation in and out at some time BEFORE the measurement was made and NOT as a result of a difference in radiation in/out between the two planets at the given time of measurement. (in other words, if the measurement was made shortly after the 2x CO2 was introduced on the second planet, it would appear to radiating less (cooler) than the first planet as the atmosphere ‘adjusted’ to equilibrium conditions)
In both cases, radiation in/out equilibrium is supposedly achieved (though the greenhouse box will of course change when the sun goes down!) and this is the critical point. The ‘brightness’ is reflecting only actual temperature (BB radiation curves and all that jazz) differences – not differences in incoming versus outgoing radiation between the objects…
If I am wrong, or missed some subtle point, I am sure its because its Jan 1st and I am hungover! LOL
My question is: what would happen if on these twin earths the normal one had 1 atmosphere but the other had 50 times normal atmosphere?
Kev has missed the point by looking for detail, the two observed systems are the same. Internal temperature is not important to the question, it is a distraction. I think myself that since the systems are at equilibrium, it is not representative of a climate system.
If you are SURE of the answer, let’s turn things inside out.
Let us say that there is no sun – that the warmer planet is warmer due to internal radioactive decay and gravitational collapse. Which planet will be brighter?
Does it matter if the energy is coming externally or internally with respect to radiation from a warm body?
Sean Houlihane says:
January 1, 2011 at 3:16 pm
thanks Sean – I am not 100% sure, but took the clue from this sentence….
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from that I took it to mean that the camera is basically a thermal measuring device, which of course, is exactly what BB radiation is all about.
I agree with your comment regarding a climate system – though I presume that the premise is that so long as radiation in/out isn’t changing significantly, the climate system is irrelevant?
In the first example the greenhouse plant is of course the warmest, because the enclosed area is able to hold onto some heat, to the point where equilibrium is reached between incoming and outgoing energy. There is no “runaway” or “catastrophic” warming – just an initial temperature rise then stability at that warmer temperature. Once equilibrium has been reached, the appearance to the camera would be the same as the outdoor plant: energy in = energy out.
In the second example (assuming these are non-real-world earths, so forgetting about increased clouds and albedo and all that jazz, and assuming that CO2 levels are not rising) I’ll refer you to the same answer. It’s a bit warmer in the earth with the extra CO2, but the view from the camera shows the same brightness for both earths, as the energy out is equal to the energy in (which of course is the same for both earths).
You’re going to tell me I’m wrong, I can sense it…
In the greenhouse the temperature will be slightly higher but not because of greenhouse gases but because of suppression of convection. That’s how greenhouses warm
In the second planetary case there is no suppression of convection, just a very slight increase in nighttime temperatures due to a slight reduction in thermal losses.
Ergo, the difference due to greenhouse gases would be lost in the natural oscillation of the atmosphere.
Change my last sentence to. “Does it matter if the energy is coming externally or internally with respect to radiation from TWO EQUALLY WARM BODIES?” To see the paradox that some of you have created.
HankHenry says:
“My question is: what would happen if on these twin earths the normal one had 1 atmosphere but the other had 50 times normal atmosphere?”
Well, scattering would certainly be very different, since any transmission path would be through a much greater density of atmosphere.
Because it is ideal conditions, in=out. Therein lies the problem with ideal conditions.
This should be fun to follow. Personally I agree with John Robertson above. Glass is a physical barrier and it changes things. Think of glass 2 molecules thick versus a half meter thick. The planets, on the other hand, are exposed to space. It doesn’t matter if one is warmer than the other, they will balance as energy in = energy out.
I get A,B,B,A,C,D,D.A,A – I might be wrong though as i have a 31st hang over.
The interior of the greenhouse is warmer not because the glass or whatever traps the radiation, but it is warmer because you do not have convection. In free air, the warm air will rise and be replaced by cooler air from higher altitudes. You can perform this experiment easily with your car. Take your car (when its warm enough, right now it is not so in Boston) and roll up the windows. Leave it in the sun. Then measure the temperature of the interior using one of the remove sensing IR thermometers you can obtain for not a lot of money. You are measuring the quantity of outgoing IR, which is how those operate. Now do the same with the windows of the car open. You will measure a much lower temperature since the convection currents of air out of the car will carry the heat away and replace it with cooler air.
The number of joules of energy retained will reach some constant in all 4 examples.
Those constant’s will not be equal.
Once that constant has been achieved then the inbound/outbound radiation will be identical.
David,
I don’t think the camera would show equilibrium, because the warm greenhouse would still be transfering much of the excess heat by convection along the outside walls, and thermal cameras only record radiation, not conduction and convection.
This might be a flaw in the thought experiment, because such cameras would tell us that Venus is a fairly cool planet since they see cloud tops.
just reviewing some others comments makes me think I’m losing my marbles!
Yes – if the camera was just measuring in and out radiation – in any ‘equal’ system any two identical bodies would register the same on the camera. However, (and I may be thinking too deep here) I presumed the idea was to consider the actual temperatures of the objects in question – and – without doubt, the camera as described, must ‘see’ any differences in actual ‘body’ temperature (i.e. radiative emissions) via any differences in BB radiation.
The effective height from which thermal radiation leaves the higher concentration planet will a bit greater than that of the lower concentration planet, so its radiating surface will be somewhat greater. Therefore, in order to radiate equally, its brightness (radiating temperature) must be slightly less. Extrapolating downward to the earth’s surface from the effective height of radiation, at the adiabatic lapse rate, the surface temperature of the higher concentration planet will be somewhat higher because of the greater height through which adiabatic convection must extend. Presumably both planets will reflect sunlight equally at wavelengths that do not interact with the greenhouse gases.
We could make this hard, by throwing in things like a variation in the emissivity of the greenhouse glass because of accumulating pollen and plant sap.
But that would be cruel.
alcuin says:
January 1, 2011 at 4:27 pm
The effective height from which thermal radiation leaves the higher concentration planet will a bit greater than that of the lower concentration planet….
sorry, but why?
the concentration of CO2 in the atmosphere is described as doubled – but there is no mention of any increase in atmospheric depth (all other conditions are described as identical)?