Guest Post by Ira Glickstein
This series began with a mechanical analogy for the Atmospheric “Greenhouse Effect” and progressed a bit more deeply into Atmospheric Windows and Emission Spectra. In this posting, we consider the interaction between air molecules, including Nitrogen (N2), Oxygen (O2), Water Vapor (H2O) and Carbon Dioxide (CO2), with Photons of various wavelengths. This may help us visualize how energy, in the form of Photons radiated by the Sun and the Surface of the Earth, is absorbed and re-emited by Atmospheric molecules. 
DESCRIPTION OF THE GRAPHIC
The animated graphic has eight frames, as indicated by the counter in the lower right corner. Molecules are symbolized by letter pairs or triplets and Photons by ovals and arrows. The view is of a small portion of the cloud-free Atmosphere. (Thanks to WUWT commenter davidmhoffer for some of the ideas incorporated in this graphic.)
- During the daytime, Solar energy enters the Atmosphere in the form of Photons at wavelengths from about 0.1μ (micron – millionth of a meter) to 4μ, which is called “shortwave” radiation and is represented as ~1/2μ and symbolized as orange ovals. Most of this energy gets a free pass through the cloud-free Atmosphere. It continues down to the Surface of the Earth where some is reflected back by light areas (not shown in the animation) and where most is absorbed and warms the Surface.
- Since Earth’s temperature is well above absolute zero, both day and night, the Surface radiates Photons in all directions with the energy distributed approximately according to a “blackbody” at a given temperature. This energy is in the form of Photons at wavelengths from about 4μ to 50μ, which is called “longwave” radiation and is represented as ~7μ, ~10μ, and ~15μ and symbolized as violet, light blue, and purple ovals, respectively. The primary “greenhouse” gases (GHG) are Water Vapor (H2O) and Carbon Dioxide (CO2). The ~7μ Photon is absorbed by an H2O molecule because Water Vapor has an absorption peak in that region, the ~10μ Photon gets a free pass because neither H2O nor CO2 absorb strongly in that region, and one of the 15μ Photons gets absorbed by an H2O molecule while the other gets absorbed by a CO2 molecule because these gases have absorption peaks in that region.
- The absorbed Photons raise the energy level of their respective molecules (symbolized by red outlines).
- The energized molecules re-emit the Photons in random directions, some upwards, some downwards, and some sideways. Some of the re-emitted Photons make their way out to Space and their energy is lost there, others back down to the Surface where their energy is absorbed, further heating the Earth, and others travel through the Atmosphere for a random distance until they encounter another GHG molecule.
- This frame and the next two illustrate another way Photons are emitted, namely due to collisions between energized GHG molecules and other air molecules. As in frame (2) the Surface radiates Photons in all directions and various wavelengths.
- The Photons cause the GHG molecules to become energized and they speed up and collide with other gas molecules, energizing them. NOTE: In a gas, the molecules are in constant motion, moving in random directions at different speeds, colliding and bouncing off one another, etc. Indeed the “temperature” of a gas is something like the average speed of the molecules. In this animation, the gas molecules are fixed in position because it would be too confusing if they were all shown moving and because the speed of the Photons is so much greater than the speed of the molecules that they hardly move in the time indicated.
- The energized air molecules emit radiation at various wavelengths and in random directions, some upwards, some downwards, and some sideways. Some of the re-emitted Photons make their way out to Space and their energy is lost there, others back down to the Surface where their energy is absorbed, further heating the Earth, and others travel through the Atmosphere for a random distance until they encounter another GHG molecule.
- Having emitted the energy, the molecules cool down.
DISCUSSION
As in the other postings in this series, only radiation effects are considered because they are the key to understanding the Atmospheric “Greenhouse Effect”. I recognize that other effects are as important, and perhaps more so, in the overall heat balance of the Earth. These include clouds which reflect much of the Sun’s radiation back out to Space, and which, due to negative feedback, counteract Global Warming. Other effects include convection (wind, thunderstorms, …), precipitation (rain, snow) and conduction that are responsible for transferring energy from the Surface to the Atmosphere. It is also important to note that the Atmospheric “Greenhouse Effect” and a physical greenhouse are similar in that they both limit the rate of thermal energy flowing out of the system, but the mechanisms by which heat is retained are different. A greenhouse works primarily by preventing absorbed heat from leaving the structure through convection, i.e. sensible heat transport. The greenhouse effect heats the earth because greenhouse gases absorb outgoing radiative energy and re-emit some of it back towards earth.
That said, how does this visualization help us understand the issue of “CO2 sensitivity” which is the additional warming of the Earth Surface due to an increase in atmospheric CO2? Well, given a greater density of CO2 (and H2O) molecules in the air, there is a greater chance that a given photon will get absorbed. Stated differently, a given photon will travel a shorter distance, on average, before being absorbed by a GHG molecule and be re-emitted in a random direction, including downwards towards the Surface. That will result in more energy being recycled back to the Surface, increasing average temperatures a bit.
Arfur Bryant says:
March 29, 2011 at 9:18 am
I think Cassandra King nailed it. To me, this is a problem of scale which, of course, is not represented in the original graphic.
I would use the following analogy to visualise the scale:
Now that is a perfect and descriptive analogy and explanation of the facts, I really hope Ira reads your post and comments on it because it goes right to the heart of my concerns and I am not a scientist nor do I hold any scientific qualifications so your post really appealed to me and was a joy to read and understand and visualise. I say visualise because that is the key to understanding. IMHO you have created a masterpiece that should have a wider airing in order for us mere mortals to gain a better understanding of the issues.
Reply to Thomas: 8.21 Stratospheric drying: http://www.npr.org/templates/story/story.php?storyId=123075836
Solomon thinks it’s fewer thunderstorms: I think it’s more efficient precipitation in thunderstorms, part of the GHG control system that maintains near constant GHG effect in the atmosphere. Miskolczi has found from radiosonde data that the reduction of [H2O] in 61 years’ radiosonde data maintains constant IR optical depth/ 1.87 average IR photon absorptions in passing through the atmosphere [within 0,.1% of the theoretical value]: http://miskolczi.webs.com/
My own work concerns the aerosol optical physics in the climate models. All the two-stream approximations are wrong because they fail to take into account direct backscattering you get from applying Mie theory to the first few scatterings as the light wave enters a cloud. What this means is that what is claimed to be ‘cloud albedo effect’ cooling is either neutral or heating, quite substantial in the palaeo-climate.
Hence you don’t need CO2-AGW: it could well be net zero. The quid pro quo is that none of the models can predict climate.
I am getting a bit depressed by the level of physics being discussed here and in some of the other web sites that we are being directed to. I am beginning to wonder if there is a strategy amongst the warmers to refute the greenhouse effect using bogus physics in order to quote these arguments, say how stupid they are, and thus justify the dismissal of all sceptic arguments as unscientific. Or I might just be paranoid!
There are serious problems with the quantification of the greenhouse effect. There is a serious doubt about feedbacks. There is a serious problem that the “fingerprint” of an enhanced greenhouse effect cannot be identified.
But there is no doubt that the atmosphere absorbs radiation emitted by the earth and eventually radiates it to space at a higher altitude which, being at a lower temperature, means a smaller energy loss. That reduction has to be compensated for by increased radiation losses elsewhere and that means a warmer surface.
This is the overall energy balance scenario and Ira has tried to explain it at a mechanistic level to show what is happening within the climate system. I have problems with some details, as I have already commented, but overall I think it is a good effort.
Some of the objections however are bizarre.
For example some say the sky cannot radiate to the earth because it is warmer yet you can point an infrared detector at the sky at night and measure it! Anyway when a CO2 molecule radiates a photon downwards how does it know the earth is colder and it should not do it!? Of course if the earth is colder the nett energy flow would be upwards (from warm to cold) and that is the only necessary requirement thermodynamically.
Then there are the arguments like CO2 is only a “trace gas” and H2O is a far more powerful absorber. If we a want to make the warmists accept the evidence of measurement we must accept the same rigour. One can measure the absorbtion by CO2 at say 1Km and do the same for H2O. By this altitude both have already trapped all the outgoing radiation at their characteristic absorption frequencies and re-radiated at least once. You can measure the radiation and see the big gaps in the black body spectrum associated with this absorption. You may think that CO2 (or H2O) at such low densities cannot do this but it does and there is already more than enough to do it totally. It is pointless coming up with simplistic arguments to deny these facts when we have data to confirm them.
The greenhouse effect is very badly named but it is a real effect. It would do the sceptic cause a great deal of good for everyone to accept this and then focus on the real issue – what is the effect of further increments of CO2? This question is by no means obvious. When the warmists tackle this question they tend to use the same simplistic arm waving physics that I have been reading above. We need to do a better job not a worse one.
Spartacusisfree, you brought up water vapor with this statement: “And because more latent heat means more efficient precipitation, the upper atmosphere dries and it’s easier for the heat to radiate to space.”
Now you start to talk about the stratosphere which is clearly inconsistent with a statement about precipitation since there is far too little water vapor in the stratosphere to cause precipitation. Anyone brining up Miskolczi obviously have rather shallow understanding of the subject, so I think I pass on further discussion.
Recommend you start here with snow absorption:
http://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGLL_enUS382US382&q=%22black+carbon%22+snow+albedo
The so-called tropical upper troposphere hotspot I understand is the mark of the mythical water vapor amplification that turns 1C per CO2 doubling into as much as 3C per doubling. Supposedly the extra CO2 increases water vapor content of the upper troposphere more than lower troposphere resulting in upper troposphere (which is always far colder than the surface) warming more than the surface. “Hotspot” is an inaccurate name since it’s really differential warming between upper and lower troposphere.
The increasing magnitude of the differential warming, if any, is difficult to measure and controversial. I believe notoriously poor GCM modeling of clouds is why the models are wrong. If the water vapor being delivered to the upper troposphere increases so does the amount of heat carried away from the surface by evaporation and convection and it condenses at an increased rate rather than giving the upper troposphere a higher absolute humidity. Thus evaporation, convection, and cloud formation is a negative feedback that negates the effect of more water vapor due to warmer surface temperature. There are several well developed hypotheses on this effect. The best IMO is
http://www.scribd.com/doc/25071132/The-Saturated-Greenhouse-Effect-Theory-of-Ferenc-Miskolczi
which is basically saying that the atmosphere, with regard to water vapor greenhouse warming, is saturated and we can’t get any more greenhouse warming out of water vapor – we can get additional surface warming only from increases in non-condensing greenhouse gases like CO2 and methane. I am in fundamental agreement with the conclusion simply by considering that the earth has never experienced a runaway greenhouse effect which water vapor amplification must certainly entail. So regardless of whether Miskolczi is right something is putting a ceiling on the maximum surface temperature and this appears to me to explain why. CO2 levels in the remote past have been several doublings greater than present and the global average temperature was several degrees higher fitting perfectly with there being no water vapor amplification.
My own pet theory is that CO2’s primary role is increasing the earth’s average temperature from below freezing to above freezing and this is accomplished by the first 100ppm in the atmosphere. CO2’s LWIR absorption curve is linear for low concentrations of the gas and becomes logarithmic in greater concentration (which has been known for over 150 years since John Tyndall experimentally measured it). It serves as “kindling” to get the water cycle going. Once the water cycle is going it takes over as the primary greenhouse gas and unlike CO2 is self-limiting because it’s a condensing gas. After that CO2’s role is limited to about 1C per doubling. There isn’t enough recoverable fossil fuel in the world to do more than two or three doublings which might possibly mean we could end the ice age and in a matter of thousands of years see the earth get green from pole to pole (which is its usual state – no polar ice caps at all). The downside of having a vastly increased amount of arable land is that ocean levels will return to their normal non-ice age level which is about 100 meters higher. That will take thousands of years to happen. No plants or animals will have any trouble migrating to higher ground. Humans have the bigger problem as we built a lot of immobile crap very close to the ocean under the mistaken impression that ocean level rises and falls only with the tides and is constant otherwise.
Harry Dale Huffman,
I agree with your analysis up to a point. In particular, the temperature ratio of the “effective radiating surface” of Venus should be 1.176 times the temperature of the “effective radiating surface” of earth (ignoring albedo effects). (I have also heard this called the earth’s “photosphere” since that is the regions from which photons are emitted from earth.)
That “effective radiating surface” is a combination of the surface (warm), the clouds (cooler), and the GHGs (cooler yet since they radiate from close to the top of the troposphere). On Venus, almost no radiation comes from the surface, since it is blocked by continuous thick clouds and GHGs. On earth the surface DOES radiate a significant amount.
To a zero-order approximation, you could say all the “effective radiating surface” is around the 500 mBar location on either, so this layer should have similar (within a factor of 1.176) temperatures on either planet.
However, any more accurate estimate would need to include the actual locations of clouds and amount of cloud cover, as well as the location and emission spectrum of GHGs, since these all affect the “effective radiating surface”.
(Once you get below this effective radiating surface, then the lapse rate is the key factor in determining the temperature for lower layers, so Venus with a much deeper atmosphere will have a much higher surface temperature.)
“For example some say the sky cannot radiate to the earth because it is colder yet you can point an infrared detector at the sky at night and measure it! Anyway when a CO2 molecule radiates a photon downwards how does it know the earth is colder and it should not do it!? Of course if the earth is colder the nett energy flow would be upwards (from warm to cold) and that is the only necessary requirement thermodynamically”.
Measure what though. See below that might be relevant. From the slaying blog
And this has been discussed before but the issue is not whether radiation travels downwards. It is whether it has a warming impact if it returns to the surface.
From slayingtheskydragoncom/en/blog/102
Lord: “Back radiation can be simply demonstrated by pointing a simple infrared detector at the underside of a cloud. Try it.”
Chorus: “My IR detector only cost $60! Simple! Agreed! Agreed!”
Slayer: “Clouds do not absorb and re-radiate heat back to Earth. Clouds add THERMAL MASS which takes longer to heat and cool. Warmists ‘support’ this false hypothesis with IR thermometer readings, but the IR readings of a hot Barbie is the same from any distance; ENERGY is not. Your $60 REMOTE thermometer is not measuring the radiant energy you are receiving, it is measuring the resonance of the Barbie.”
Yeah, yeah… that seems to be a very British concern to me. The country where they still have single glasing all over the place, and anyway, doors and windows never seal. I can totally see how they could save 90% heating by simply arriving in the 3rd millenium to that end. After many years, and by sheer luck, I finally ended up in a London house with double glasing for a change. And you can tell the difference – when friends come around for a visit, everybody goes like: ” Hey wow, you got double glasing…”
It is also a national habit here that people simply won’t shut their car engines down in the street and let them run for ever, etc, etc, etc The truth is that the British are wastrels and being a bit more sensible alone would cut their energy consumption in half over night – no policy needed at all.
OK, Spartacusisfree, at what speed does the hot air convect upwards? 1 mile per hour? 10 MPH? 100 MPH? 1000 MPH :^) What?
The downwelling radiation travels at … uh … the speed of light. That is over 600,000,000 MPH. So, what are the chances that convection will yank that heated air up and away before it has a chance to radiate energy in all directions, including back down towards the Surface of the Earth?
@RJ: If the earth radiates 100,000 units of energy and then receives even 1,000 unit through back-radiation, then it’s still warmer than if there were no back-radiation at all. At the same time, the sun is radiating energy to the earth. The net effect is that the earth is warmer thanks to the back-radiation.
I did not say precipitation occurs in the stratosphere, it’s much lower down. And it’s easy to prove the optical physics in the climate models, which predicts the cloud part of ‘global dimming’, is wrong by looking at clouds as droplets coarsen prior to raining: the base gets darker, exactly the opposite of what the models predict.
After NASA discovered there was no experimental proof of this cooling, they published this: http://geo.arc.nasa.gov/sgg/singh/winners4.html
Twomey’s physics, which he warned could not be extrapolated to thicker clouds, is based on Mie theory. There is no ‘surface reflection’ but it apparently fooled climate science, hence AR4 contained the imaginary cooling.
So the high feedback hypothesis was always the artefact of a scientific mistake. Until it’s corrected, no model can predict climate. Correct it and out pops another AGW, ‘cloud albedo effect’ heating that explains palaeo-climate [probably via phytoplankton bio-feedback] far better than CO2-GW, which if not net zero, is much lower than claimed.
I am on record as saying that Natural Cycles and Processes have more of an effect on raising and lowering average Global Temperatures than CO2. In the WUWT series that ended with Global Warming is a Pussy Cat I estimated that only 0.1ºC of the supposed 0.8ºC warming since 1880 is Human-Caused (CO2 and Land Use) . The averaged-results of WUWT commenters on my series put Human-Caused at 0.18ºC, almost twice my original value, but still smaller than Natural Cycles and Data Bias.
Yes, for the past decade and a half, CO2 has continued its merry rise and there has been no net statistically significant Global Warming, and, as you point out, we have lots of charts to prove that. However, there are many factors that contribute to Global Warming and Global Cooling. Some, such as Natural Cycles, sometimes warm and sometimes cool. Others, such as rising CO2, have the effect of warming, but not as much as the official climate Team claims.
The whole point of my Visualizing the “Greenhouse Effect” series is to help WUWT readers understand that the Atmospheric “Greenhouse Effect” (and I put “scare quotes” around it to distinguish it from a physical greenhouse) is a real effect. AGW (Anthropomorphic Global Warming) is real because we are partially responsible for average temperatures being higher than they would be absent human activities. Knowing that AGW is true, but has a minor effect, will (I hope) make you all look well-informed when you dispute the scam behind Catastrophic CAGW.
The argument “backradiation warms the surface of the Earth” is often used when discussing the greenhouse effect. Is it fair to infer from this argument that an object’s temperature will always be higher in the presence of backradiation than it would be in the absence of backradiation? If so, I disagree.
Consider two disjoint blackbody objects (a spherical active object, and an inert object) in the vacuum of cold space. Assume the internal thermal conduction properties of each object are such that the surface temperature is uniform. By blackbody I mean the surface of an object (a) absorbs all electromagnetic energy incident on it, and (b) radiates electromagnetic energy in accordance with Planck’s blackbody radiation law. By “active object” I mean an object with a constant internal source of thermal energy–e.g., radioactive decay. By “inert object” I mean an object that has no internal source of thermal energy.
The active sphere in isolation will experience no back radiation–i.e., all radiation leaving the sphere will escape to space. The active sphere in the presence of the inert object will experience back radiation–i.e., some of the radiation emitted from the active object will be absorbed by the inert object–causing the inert object’s temperature to rise–causing the inert object to radiate energy, some of which will be directed “back” toward the active object. Provided there is no object-to-object thermal conduction or convection, I believe the steady-state temperature (the temperature when the rate of energy leaving an object is equal to the rate of energy entering the object) of the active object will be higher in the presence of the inert object than it will be in isolation. However, if conduction and/or convection between objects is allowed, then depending on the rate of energy transfer via conduction / convection, in the presence of the inert object and hence in the presence of backradiation the steady-state temperature of the active object can be either higher or lower than in the absence of the inert object. If true, then the argument that “backradiation warms (i.e., increases the steady-state temperature of) the active object” is not true.
Ira explicitly stated that his arguments apply only in the absence of conduction and convection; and with that caveat, I agree. However, since conduction and convection are both present in the Earth/Earth atmosphere system, I am not convinced that “greenhouse gas backradiation” produces a net increase in the surface temperature of the Earth. Any attempt to discuss the “greenhouse effect” in the Earth’s atmosphere without including a discussion of conduction/convection is in some sense like trying to get your car to the top of a steep hill by pushing–you’ll waste a lot of time; and when you’re done pushing, your call will likely be farther down the hill than when you started.
Ira this from Jennifer Marohasy blog. The calculation for the emissivity of CO2 is found in this thread.
Comment from: Nasif Nahle March 30th, 2011 at 1:01 am
” The algorithm was developed (derived) from experiments by Hottel, Leckner, Lapp, etc., etc., etc.”
Additionally, you can see the insignificant total emissivities of the carbon dioxide on the tables that you can find in any book on heat transfer by radiation. I’m not alone on this THEORY and LAWS. Mine is a humble calculation based on experimentation and observations made by many physicists.
…” The total emissivity of the atmospheric carbon dioxide is 0.0017, whether you like it or not. …”
:)Hottel found also that the total emissivity of the carbon dioxide in a saturated state was very low (Ɛcd = 0.23 at 1.524 atm-m and Tcd = 1,116 °C). [6]
As Hottel diminished the partial pressure of the carbon dioxide, its total emissivity also decreased in such form that, below a partial pressure of 0.006096 atm-m and a temperature of 33 °C, the total emissivity of the carbon dioxide was not quantifiable because it was almost zero. [6] [7] [8]
If has shown in ( http://jennifermarohasy.com/blog/2011/03/total-emissivity-of-the-earth-and-atmospheric-carbon-dioxide/#more-7739) that at STP the emissivity off CO2 is almost zero then no back radiation can take place.
Matt
All this might do is slow the rate of cooling. It will not warm the planet. And as radiation travels at the speed of light\(and the quantity of CO2 is small) the slower cooling if it occurs would be very minimal.
But there is still the problem of a cooler body heating a warmer one. If radiation leaves earth’s surface at say 10 degrees and collides with CO2. Won’t this lower the potential energy in the radiation. So when it returns (if it does) its vibrational energy will be lower than the surface of earth. So can not warm the planet unless the planet has in the mean time cooled by more than the radiations potential energy.
RJ says:
in reply to March 29, 2011 at 10:31 am
“For example some say the sky cannot radiate to the earth because it is colder yet you can point an infrared detector at the sky at night and measure it! Anyway when a CO2 molecule radiates a photon downwards how does it know the earth is colder and it should not do it!? Of course if the earth is warmer the nett energy flow would be upwards (from warm to cold) and that is the only necessary requirement thermodynamically”.
Measure what though. See below that might be relevant. From the slaying blog
And this has been discussed before but the issue is not whether radiation travels downwards. It is whether it has a warming impact if it returns to the surface.
—————————–
If it returns to the surface and does not warm it must be totally reflected. If it is totally reflected then the surface cannot emit at that wavelength either (basic law of thermodynamics). Therefore it is not possible for radiation to be emitted from the surface, absorbed by the atmosphere and re-radiated downwards without increasing the temperature of the surface.
Ira
I always had a problem with the logic of the GWG theory. And thankfully all the queries I had were answered and explained in the brilliant slayers book.
I now debate with warmist in various ways. One is to dispute the GWG theory as per the slayers book. Another to dispute the extent of the warming assuming the GHG theory is correct. Another to point out the pointlessness of CO2 taxes and cap and trade etc.
Assuming the flawed GHG theory is almost a fact (as you are doing) without understand how weak this theory is, and its flaws, was a hindrance not a help to me.
Dave Springer (march 29@2:11pm)
“So-called back radiation, which is what you’re describing, does not warm the surface except in limited and exceptional circumstances where a mass of air transported convectively (read winds) is warmer than the surface over which it blows and even then it is limited because the air has so little heat capacity compared to the surface – imagine trying to heat a cup of coffee by stirring it with a hot feather.” So let me get this straight you think backradiation is caused by a warm wind blowing across the surface, and that this is a limited and exceptional circumstance? You’re kidding, right?
“What actually happens is the back radiation slows down the rate of surface cooling.”
Does it now, and where are the experiments that quantify this? Objects above absolute temperature emit electromagnetic radiation sure, but HOW MUCH? How much IR does a block of iron at freezing emit. Is it sensible. Backradiation is a myth, go measure it and tell me how many btu’s or degrees kelvin it’s good for. I’d really like to know as it’s 3 weeks into spring, it’s freezing, and we have been below normal temps by 8-10 degs. How’s this, you or anyone else can’t measure or quantify it. To see what you’re up against see Dr. Roys Spencer’s amusing but failed “cavity experiment”.
” But the one thermodynamic fact is always operative in the real world – the farther out of equilibrium the system is the harder it tries to move back to the theoretical equlibrium temperate.” Does it now. You already invalidated any equilibrium by calling it a moving target. Any evidence that it “tries harder”.
The bottom line is that there is no greenhouse effect. It is an atmosphere effect due soley to water vapor. Both Feedbacks and forcing are nothing but plausible fantasies.
Ira, I really enjoy your contributions and the discussions they create. With so may ideas, one really has to think and ponder what is the most realistic explanation.
After reading through the various contributions to this thread, it seems to me that the dualistic nature of electromagnetic radiation is not taken into account sufficiently. We talk of wavelength (the wave nature of electromagnetic radiation) and photons (the particle nature). Your diagrams are of molecules being hit by photons. So it is like a billiard ball situation.
Someone said it was more like a tsunami wave. While I agree with that somewhat, radiation is not a wave front. It is more like a constant field. The billiard ball type diagram makes it seem as though the chance of a CO2 molecule being hit is small. And for one ray of radiation it is. But, with a constant field of radiation, the CO2 molecule is always being hit by some ray of radiation.
Nevertheless, it is hard to understand how this could cause enough increase in the water vapor in the air for the water vapor to produce the increase in atmospheric temperature that the models show.
Reed Coray says:
March 29, 2011 at 11:20 am
The argument “backradiation warms the surface of the Earth” is often used when discussing the greenhouse effect. Is it fair to infer from this argument that an object’s temperature will always be higher in the presence of backradiation than it would be in the absence of backradiation? If so, I disagree.
Consider two disjoint blackbody objects (a spherical active object, and an inert object) in the vacuum of cold space. Assume the internal thermal conduction properties of each object are such that the surface temperature is uniform. By blackbody I mean the surface of an object (a) absorbs all electromagnetic energy incident on it, and (b) radiates electromagnetic energy in accordance with Planck’s blackbody radiation law. By “active object” I mean an object with a constant internal source of thermal energy–e.g., radioactive decay. By “inert object” I mean an object that has no internal source of thermal energy.
The active sphere in isolation will experience no back radiation–i.e., all radiation leaving the sphere will escape to space. The active sphere in the presence of the inert object will experience back radiation–i.e., some of the radiation emitted from the active object will be absorbed by the inert object–causing the inert object’s temperature to rise–causing the inert object to radiate energy, some of which will be directed “back” toward the active object. Provided there is no object-to-object thermal conduction or convection, I believe the steady-state temperature (the temperature when the rate of energy leaving an object is equal to the rate of energy entering the object) of the active object will be higher in the presence of the inert object than it will be in isolation. However, if conduction and/or convection between objects is allowed, then depending on the rate of energy transfer via conduction / convection, in the presence of the inert object and hence in the presence of backradiation the steady-state temperature of the active object can be either higher or lower than in the absence of the inert object. If true, then the argument that “backradiation warms (i.e., increases the steady-state temperature of) the active object” is not true.
If I follow the logic here you are suggesting that the loss of heat via convection and conduction to the inert body can be greater than the heat gained by back radiation. That means that the inert body is inside the atmosphere of the radiating body (or vice versa). In either case the inert body is in dynamic equilibrium with this atmosphere. In which case it would be the properties of the inert body relative to the atmosphere it replaces which would determine whether the surface warmed or not. If the inert body was thermally more conductive the surface would be cooler and if it were a better insulator the surface would be warmer. However I do not see how this is relevant to the situation we have where, in order to simulate the actual atmosphere, your “inert body” would have to have exactly the same composition and properties as the atmosphere itself.
You were doing fine until you said no back radiation takes place. CO2 has no line emission in a cold dense gas. It has an absorption line. After absorption it loses the energy kinetically via collisions primarily with oxygen and nitrogen. This thermalizes the gas mixture i.e. sensibly warmer. The air is ALWAYS emitting a continuous blackbody spectrum! Everything with a temperature above absolute zero emits a continuous blackbody spectrum with a peak frequency that rises with rising temperature. The higher the peak frequency the more energy in the emission. This is where the downwelling radiation comes from and its existence is without doubt.
Brian W says:
March 29, 2011 at 12:00 pm
Dave Springer (march 29@2:11pm)
“So-called back radiation, which is what you’re describing, does not warm the surface except in limited and exceptional circumstances where a mass of air transported convectively (read winds) is warmer than the surface over which it blows and even then it is limited because the air has so little heat capacity compared to the surface – imagine trying to heat a cup of coffee by stirring it with a hot feather.” So let me get this straight you think backradiation is caused by a warm wind blowing across the surface, and that this is a limited and exceptional circumstance? You’re kidding, right?
“What actually happens is the back radiation slows down the rate of surface cooling.”
“Heat: A Mode of Motion” by John Tyndall, 1859. This is old stuff. If you don’t know this you don’t know basic experimental physics.
Dave Springer
” This is old stuff.” OH IS IT NOW! That’s funny NOWHERE in the 512 pages of Tyndall’s great experimental work does he EVER mention backradiation, nor does he mention the term greenhouse EVEN ONCE. You are blowing a lot of hot MOIST air and I’m calling you on it. Bring out some of that old stuff and let’s find out!
Argone gone to lunch :^)
As I mentioned, this simplified depiction is of a small (acually teeny tiny to use the technical term) section of the Atmosphere and, by luck, there was no Ar in that section.
Dave Springer says: Everything with a temperature above absolute zero emits a continuous blackbody spectrum with a peak frequency that rises with rising temperature. The higher the peak frequency the more energy in the
It’s not a peak frequency. It’s a median energy. Blackbody spectrum is a spectrum with a central energy and energys higher and lower thus forming the BB spectrum.