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
On our earth, according to Trenbeth’s figures, we lose about 24 watts in convection. The greenhouse plant won’t be losing the convection heat that a non-greenhouse plant loses-it will warm up more during the course of the day. Conversely, it would cool faster each night.
A second factor is evaporation- Both the indoor and outdoor plant will cool thanks to transpiration, but the outdoor plant will cool more thanks to wind, which is blocked by the greenhouse- this factor operates both day and night. The average temp of the greenhouse plant should be higher, but the relative difference should increase up untill around the warmest part of the day, around 2:00 PM or so, then drop off until dawn.
As to the two planets, since they’re in balance with the sun, they should both be receiving and raiating away an average of 324 watts per square meter.
Let us consider the two planets, and just for simplification, suppose that both atmospheres are completely transparent at all wavelengths — except for the CO2 absorption lines. For this scenario, all of the green house effect is due to the CO2. Since both planets are in thermal equilibrium, each must radiate (or reflect back due to albedo) the same total energy out as is incoming. For two identical objects at differing temperatures, the warmer object will radiate more at every wavelength, and will also have a maximum at a slightly shorter wavelength. Of course, these two objects are not identical. One has more CO2 and is at a higher temperature — and yet we know that both radiate the same total energy out. Since we have posited all other, non CO2 gases as transparent, there is only one way to modify the radiation curve for the warmer object so that total area under the curve equals that of the cooler object. The radiation out from the warmer object must have a stronger dip at the wavelengths where CO2 absorbs. After all, the CO2 (in this scenario) is the only thing in the atmosphere which is not completely transparent.
Is this reasonable? Aren’t good absorbers also good emitters? Won’t the CO2 mixed into the upper atmosphere be radiating outward? Why don’t we see a spike in radiation at the CO2 lines? And if we did, then doesn’t that makes an energy imbalance. I know that the green house gas effect is well accepted by almost everyone, so will someone point out the flaw in my reasoning, at least as it pertains to this simplified model?
Assuming the glass is totally transparent (to all EM radiation) then the trapped air in the box and by consequence the plant itself will be at a higher temperature due to no cooling from convection.
Therefore the plant in the box must be hotter and therefore it must emit more blackbody thermal radiation which will be seen by the camera.
I at once suspected this “thought experiment” had everything to do with a mythical beast called “The Greenhouse Effect” and accordingly I therefore opted in both cases to vote where it said “The same”
If I am wrong then I shall, be very happy as no doubt I can then look forward to an explanation for “The Greenhouse Effect”. I mean an explanation that: a) makes sense, and b) Does not prove the complete opposite. (All explanations I have read so far, fall into the ‘No sensible explanation ‘)
I feel certain that if a “Greenhouse effect” natural, forced or otherwise exists then there MUST be somebody out there who can explain it, calmly and rationally.
The usual; “The greenhouse effect is natural, is well understood by scientists bla – bla -bla –” does not cut it and cannot be seen as scientific proof one way or the other. Nor can opinions of how it may be happening, nor do opinions of those who have more opinions than others.
By the way:
Why not make it a project for say 2011’s first couple of months: Write your own essay explaining “The Greenhouse Effect” and poste it on WUWT.
It could become interesting reading!
To all those considering convection, air rising etc etc i remind you of this sentence in the article..
To be clear, the camera integrates to measure the radiative emission temperature of the object.
If the camera is “seeing” the plant, then it is measuring the radiation from that plant even before the air around it has a chance to warm and rise.
Seen as we were not given a time frame, I assume an instant measurement was taken. So both will be the same.
For the planets, the 2xCo2 one is already 1c warmer, meaning it has reached equilibrium already, so radiation out will be same as radiation in. If it was brighter, it would be cooling. If it was dimmer, it would STILL be warming, but it’s not. It has reached equilibrium.
“We take an image of the two plants in the early afternoon on our fancy camera, what do you find in the image?”
Is the image of the plant taken from inside or outside the greenhouse?
Greenhouses work. I dont think there is any question about that…but at equilibrium they dont “manufacture energy” so the net energy leaving the ground where the sunlight hits must be the same in both cases.
In the first experiment, assuming they are in equilibrium to the environment as you stated above the one in the eclosure would show cooler (not in tempereature but emittted radiation, temperatures equal, emissivity different) than the one in the open for it would be have converted some of the co2 and have a lower concentration and since co2 in the atmosphere has greater absorptivity it also has higher emissivity and a lack of this causes the lower emissision though the temps are equal.
In the second experiment the planet with +1°C will show more emission. No brainer. Trouble is you cannot “block some of the emitted radiation”, emissivity disallows this unless you actually CHANGE the emissivity and therefore equally change the absorptivity. The two planets would actually be at the same temperature. Sorry, AGW is a sham.
As a founding member of People Against Acronyms and Abbreviations or PAA&A, I had no idea what IOW meant. My first thought was that IOW meant “I own won”. There was a paranthetical phrase with a missing comma as in “I own won, a cool toy which…” Then I find Id-ian physics. I am beflummoxed, my temper-ature is rising. Maybe it is a typo and should be Ind-ian physics. Well, I did look up IOW and determined a good translation is “In other words”. I did not get anywhere with Id-ian physics. Maybe it’s Idaho-ian physics. Or maybe better yet, Intelligent Design-ian physics.
Willis Eschenbach had a good discussion of CO2 and radiation in People Living in Glass Planets.
Some will find it pretty easy, while I bet others will get all tied in knots over it.
You can say that again. Still working on the first example. Without determining an answer, I think some interesting things can be inferred. Assuming that both plants start surrounded by a normal atmosphere, then at the time of the camera snapshot:
1. The experiment has been running for some time.
2. The greenhouse atmosphere is seriously depleted of CO2.
3. The plant is dead.
These follow from the stated condition of equilibrium. I reason as follows: A live plant is converting some portion of incoming radiation to stored chemical energy through photosynthesis. The emitted radiation from the greenhouse is therefore some function of of incoming radiation less chemical storage. This results in disquilibrium, since photosynthesis is slowing down due to CO2 depletion. Eventually the plant dies, a state of equilibrium is reached, and we can finally take our picture.
I suggest we simplify the experiment by eliminating the plant. Perhaps we can substitute a government bureaucrat, since it is well known that bureaucrats, like other inanimate objects, do not move until they are pushed.
As to the question, if the camera really captures the entire spectrum, and both setups are really in equilibrium, then I would suppose they would be of equal brightness. Except that the live plant is still diverting radiant energy to chemical….
Doesn’t everyone here get it?
I assume most if not all of you are sitting in a room. Objects in that room are all at the same temperature, black or white. All of the objects are in resonance radiantly with the walls and each object affecting each other object much as gravity. Some would propose (AGW proponents) that if your room was evacuated without convection and conduction that the black objects (think co2) would be warmer temperature wise than the white objects (low absorptivity and emissivity) but I for one don’t buy that unless someone can concretely prove it to me, by physics.
Is it any difference with the Earth just because it is only warmed on one side every 12 hours? If there was no sun but a diffuse radiation on every side of equal radiation (TSI) would that not be the same except for the diurnal up and down of the temperatures and seasonal variance? In that case it would be the same as your evacuated room, the same? Any thoughts at all or is this just politics on both sides, to hell with the actual physics and science?
Sorry, sometimes I think NObody is thinking. Just the same dribble.
The planets only have a radiative temperature because free space has an impedance. It’s about 300 ohm. (Any radio ham can tell you that.)
A planet heated from the inside is like a constant current device driving a load through a resistance. The voltage (temperature) drop across the resistor will be proportional to the impedance of that resistor (or thermal insulation). Once equilibrium is reached, all the generated current (heat) will pass into the load (free space) [according to Kirchoff].
The external radiative temperature therefore depends slightly on the insulating layer, but only if its impedance is similar in magnitude to the impedance of free space.
An object with “less escaping thermal radiation” is warmer, not cooler.
The image seen by the camera is cooler in that case because fewer photons are reaching the “film” or retina; but the camera’s film is not the object.
Without knowing the emissivity your camera can not give a temperature reading from the integrated intensity. In both cases, at equilibrium, the radiant energy in will be equal to the radiant energy out. I assume that we are ignoring convection and conductivity, although that may not be a safe assumption with the glass greenhouse. The two cases can be at different temperatures for the same radiant intensity, because they will have different emissivities. This is the essence of the Earth’s Greenhouse effect.
1) Thanks Jeff for posting this, you hit my sore point.
2) My dream for the day would be that by the end of this thread, even if 2000 comments long we would be able to answer the question — Does Kirchhoff’s law (emissivity and absorptivity, directly co2 related for it absorbs more) apply to Earth as other normal matter as view from afar like the moon. That would be an actual accomplishment scientifically here on wuwt. So many, maybe even myself, seem confused.
To me the increased emission equals the equally increased absorption and since in = out there is NO affect from co2. Maybe even getting back into Venus’s atmosphere later for it seems to show why this is so. Think lapse rate.
Both planets will reflect the same amount of energy. The hotter (CO2 planet) will emit more in the longer wavelengths.
Consider an alternative thought experiment. One planet covered with mirrors and the other covered with asphalt. They both reflect the same amount of energy but the mirror planet reflects mostly visible light and the asphalt planet reflects mostly IR radiation.
If tomorrow is clear, I will perform this experiment with my rustbucket Toyota Tercel and report back.
The two planets would have the same total, but if you looked at individual wavelengths, those from the surface would be brighter, and those from the top of the atmosphere would be dimmer, canceling out in the net.
For the first experiment none are correct.
If I assume the camera is outside the box then the camera will most likely see the box and not the plant inside.
Sigh. Jeff tells us that the 2XCO2 planet is 1 degree warmer. That’s a given, and it’s “true” regardless of differences with a glass greenhouse.
So, let’s simplify the question even further. In a world with half of it a given 1 degree warmer than the other half, would the warmer half show a brighter image? Unless you want to put a lot of people at FLIR and other imaging product companies out of business, the answer should be Yes (brighter).
The temperature of the plant is approximately a function of
(a) air temperature and
(b) transpiration.
All things being crudely equal, the plant in the greenhouse will register a higher temperature, since imposing limits on convection will increase both air temperature and humidity, reducing transpiration and the evaporative cooling effect this has on the plant, particularly its leaves.
Increasing wind speed and lowering humidity could reduce the temperature of the plant inside the greenhouse by increasing transpiration, despite a rise in air temperature.
Conversely, increasing the amount of CO2 available to the outdoor plant could increase its average temperature with no apparent increase in air temperature. This is because CO2-enriched plants don’t have to struggle so hard to breathe: stomata close in response to elevated CO2, reducing transpiration, meaning less evaporative cooling and water consumption.
If the first experiment, energy has to leave both planets as thermal EM radiation. One could assume that that the transparent box is high enough on planet 1 so that it is does not increase the atmospheric pressure and therefore planet 1 and planet 2 have exactly the same thermal emission temperature. Energy in = Energy Out.
In the second experiment, the two planets again have the same thermal emission temperature. In planet 1, with the doubled CO2, the layer that thermal emission temperature occurs is 460 metres higher than planet 2 and hence the surface temperature on planet 1 is 1.0C higher. Effectively, the lagtime in which the energy enters and then exits planet 1 is a few minutes longer than planet 2. 44.06 hours in planet 1 compared to 43.44 hours in planet 2.
“William Sears says:
January 1, 2011 at 6:10 pm
Without knowing the emissivity your camera can not give a temperature reading from the integrated intensity. In both cases, at equilibrium, the radiant energy in will be equal to the radiant energy out. I assume that we are ignoring convection and conductivity, although that may not be a safe assumption with the glass greenhouse. The two cases can be at different temperatures for the same radiant intensity, because they will have different emissivities. This is the essence of the Earth’s Greenhouse effect.”
I think you are exactly correct in the first statement but wrong in the second. Is this not why black objects in an IR camera show as bright and white objects (of anything with low absorption) show as dark. How can this be if their temperature is the same? It’s that the bright objects are absorbing more but equally emitting more, Kirchhoff’s law and the opposite for lighter colored objects or i.e. objects of less emissivity. Now apply that to co2 which absorbs more. To temperature it doesn’t matter, it emits more to space therefore preventing any warming, it has to.
In the second statement you are wrong, you said “The two cases can be at different temperatures for the same radiant intensity, because they will have different emissivities.”. If the have different emissivities they also have different absorptivities and the effects equate and the temperatures MUST BE EQUAL in a given radiative field (in the same room).
If speaking of inter-atmosphere effects instead of measurements from afar outside the object it gets more complicated.
At the moment the picture is taken I will use the same camera with midafternoon sun to photo the moon representing the plant out in the open and the earth representing the plant in the greenhouse . My hypothesis is the moon surface will read hotter than the earth surface because is reflects more energy back to space than an enclosed earth.
…now, where did I put the aspirin?
”earth’s land and ocean surface warmed to an average of 14c”…
the graphic seems to indicate that the sun warms the earth’s surface to an average of 14c?
those rescued chilean miners might beg to differ. it was not 90 degrees fahrenheit in that mine not because the sun warmed 390 ppm of CO2 and an inch of soil half a mile above their heads for a few hours a day. it was hot because they were closer to the source of heat.
i’m always amazed that scientists want to completely ignore the overwhelming mass of earth and the incomprehensible amount of btu’s it contains to entertain the thought that a trace gas in the comparatively negligent mass of our thin atmosphere is somehow going to overheat the entire planet with solar radiation that is normal to its surface for, at best, an hour a day.
have we all gone mad?
it’s not a coincidence that the mean annual temperature of air at the surface miraculously matches the temperature of the soil a few inches beneath the surface. if the temperature of our air is increasing, maybe we should first wonder what’s going on under our feet? and if it isn’t land use change, we shouldn’t be so quick to assume that a hair at the end of a dog’s tail is going to wag the whole dog.
maybe these global warming scientists should try warming their tea by heating the air above it? perhaps when they realize that they have to heat the same volume of air above their tea to 800 degrees or so in order to warm their tea by 1 degree, that then they will reconsider AGW?
we deforest, pave, build, convert matter to energy, and redistribute heat and energy all over our earth. we should be pleased that our thermometers tell us we’ve changed our environment by less than a degree, if at all.
michael mann should have been very curious when his tree rings told him things may actually be getting colder – he may have stumbled onto something that we should really be worried about.
Maybe the answers lies in the fact nobody is running around selling home climate systems based on indoor CO2 concentration levels. LOL