Guest post by Ira Glickstein
A real greenhouse has windows. So does the Atmospheric “greenhouse effect”. They are similar in that they allow Sunlight in and restrict the outward flow of thermal energy. However, they differ in the mechanism. A real greenhouse primarily restricts heat escape by preventing convection while the “greenhouse effect” heats the Earth because “greenhouse gases” (GHG) absorb outgoing radiative energy and re-emit some of it back towards Earth.
The base graphic is from Wikipedia, with my annotations. There are two main “windows” in the Atmospheric “greenhouse effect”. The first, the Visible Light Window, on the left side of the graphic, allows visible and near-visible light from the Sun to pass through with small losses, and the second, the Longwave Window, on the right, allows the central portion of the longwave radiation band from the Earth to pass through with small losses, while absorbing and re-emitting the left and right portions.
The Visible Light Window
To understand how these Atmospheric windows work, we need to review some basics of so-called “blackbody” radiation. As indicated by the red curve in the graphic, the surface of the Sun is, in effect, at a temperature of 5525ºK (about 9500ºF), and therefore emits radiation with a wavelenth centered around 1/2μ (half a micron which is half a millionth of a meter). Solar light ranges from about 0.1μ to 3μ, covering the ultraviolet (UV), the visible, and the near-infrared (near-IR) bands. Most Sunlight is in the visible band from 0.38μ (which we see as violet) to 0.76μ (which we see as red), which is why our eyes evolved to be sensitive in that range. Sunlight is called “shortwave” radiation because it ranges from fractional microns to a few microns.
As the graphic indicates with the solid red area, about 70 to 75% of the downgoing Solar radiation gets through the Atmosphere, because much of the UV, and some of the visible and near-IR are blocked. (The graphic does not account for the portion of Sunlight that gets through the Atmosphere, and is then reflected back to Space by clouds and other high-albedo surfaces such as ice and white roofs. I will discuss and account for that later in this posting.)
My annotations represent the light that passes through the Visible Light Window as an orange ball with the designation 1/2μ, but please interpret that to include all the visible and near-visible light in the shortwave band.
The Longwave Window
As indicated by the pink, blue, and black curves in the graphic, the Earth is, in effect, at a temperature that ranges between a high of about 310ºK (about 98ºF) and a low of about 210ºK (about -82ºF). The reason for the range is that the temperature varies by season, by day or night, and by latitude. The portion of the Earth at about 310ºK radiates energy towards the Atmosphere at slightly shorter wavelengths than that at about 210ºK, but nearly all Earth-emitted radiation is between 5μ to 30μ, and is centered at about 10μ.
As the graphic indicates with the solid blue area, only 15% to 30% of the upgoing thermal radiation is transmitted through the Atmosphere, because nearly all the radiation in the left portion of the longwave band (from about 5μ to 8μ) and the right portion (from about 13μ to 30μ) is totally absorbed and scattered by GHG, primarily H2O (water vapor) and CO2 (carbon dioxide). Only the radiation near the center (from about 8μ to 13μ) gets a nearly free pass through the Atmosphere.
My annotations represent the thermal radiation from the Earth as a pink pentagon with the designation 7μ for the left-hand portion, a blue diamond 10μ for the center portion, and a dark blue hexagon 15μ for the right-hand portion, but please interpret these symbols to include all the radiation in their respective portions of the longwave band.
Sunlight Energy In = Thermal Energy Out
The graphic is an animated depiction of the Atmospheric “greenhouse effect” process.
On the left side:
(1) Sunlight streams through the Atmosphere towards the surface of the Earth.
(2) A portion of the Sunlight is reflected by clouds and other high-albedo surfaces and heads back through the Atmosphere towards Space. The remainder is absorbed by the Surface of the Earth, warming it.
(3) The reflected portion is lost to Space.
On the right side:
(1) The warmed Earth emits longwave radiation towards the Atmosphere. According to the first graphic, above, this consists of thermal energy in all bands ~7μ, ~10μ, and ~15μ.
(2) The ~10μ portion passes through the Atmosphere with litttle loss. The ~7μ portion gets absorbed, primarily by H2O, and the 15μ portion gets absorbed, primarily by CO2 and H2O. The absorbed radiation heats the H2O and CO2 molecules and, at their higher energy states, they collide with the other molecules that make up the air, mostly nitrogen (N2), oxygen (O2), ozone (O3), and argon (A) and heat them by something like conduction. The molecules in the heated air emit radiation in random directions at all bands (~7μ, ~10μ, and ~15μ). The ~10μ photons pass, nearly unimpeded, in whatever direction they happen to be emitted, some going towards Space and some towards Earth. The ~7μ and ~15μ photons go off in all directions until they run into an H2O or CO2 molecule, and repeat the absorption and re-emittance process, or until they emerge from the Atmosphere or hit the surface of the Earth.
(3) The ~10μ photons that got a free-pass from the Earth through the Atmosphere emerge and their energy is lost to Space. The ~10μ photons generated by the heating of the air emerge from the top of the Atmosphere and their energy is lost to Space, or they impact the surface of the Earth and are re-absorbed. The ~7μ and ~15μ generated by the heating of the air also emerge from the top or bottom of the Atmosphere, but there are fewer of them because they keep getting absorbed and re-emitted, each time with some transfered to the central ~10μ portion of the longwave band.
The symbols 1/2μ, 7μ, 10μ, and 15μ represent quanties of photon energy, averaged over the day and night and the seasons. Of course, Sunlight is available for only half the day and less of it falls on each square meter of surface near the poles than near the equator. Thermal radiation emitted by the Earth also varies by day and night, season, local cloud cover that blocks Sunlight, local albedo, and other factors. The graphic is designed to provide some insight into the Atmospheric “greenhouse effect”.
Conclusions
Even though estimates of climate sensitivity to doubling of CO2 are most likely way over-estimated by the official climate Team, it is a scientific truth that GHGs, mainly H2O but also CO2 and others, play an important role in warming the Earth via the Atmospheric “greenhouse effect”.
This and my previous posting in this series address ONLY the radiative exchange of energy. Other aspects that control the temperature range at the surface of the Earth are at least as important and they include convection (winds, storms, etc.) and precipitation that transfer a great deal of energy from the surface to the higher levels of the Atmosphere.
I plan to do a subsequent posting that looks into the violet and blue boxes in the above graphic and provides insight into the process the photons and molecules go through.
I am sure WUWT readers will find issues with my Atmospheric Windows description and graphics. I encourage each of you to make comments, all of which I will read, and some to which I will respond, most likely learning a great deal from you in the process. However, please consider that the main point of this posting, like the previous one in this series, is to give insight to those WUWT readers, who, like Einstein (and me :^) need a graphic visual before they understand and really accept any mathematical abstraction.
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Relative humidity in the upper atmosphere (300mb level) has fallen from around 55% in 1950 to about 45% now.
Should this not have a material impact on the greenhouse effect?
It is hard to get concerned when such substantial changes don’t even warrant discussion, much less explanation. Or is there some obvious offset that makes this moot?
I realize the first graphic is attributed to Wikipedia, but I am used to wavelength graphs with the longer wavelengths (lower frequency) on the left and shorter wavelengths (higher frequency) on the right. Also confusing are the colors which are backwards from the norm.
Unless 2/3 of a beer has completely ruined me mentally (I’m willing to negotiate that point)…
Of course, the warmists have manipulated every other convention of science and engineering, so what’s new?
Gerry
Ira,
Are there any experiments which detect the incoming re-emitted radiation from the atmosphere? Did a quick search and only came up with models. It seems like we should be able to detect the radiation coming back at us from the atmosphere if this model is valid.
Does it heat the earth or does it slow the rate of cooling?
I do not think you should say its a scientific truth etc. It’s a theory that some experts do not agree with.
I’m not an expert but I have a problem with a cold body (a small amount of CO2 and water vapour) heating a much larger warmer earth. Logically it does not seem right. If for example a human was put into a container filed with CO2 would they cook or warm up. I do not think so.
And does energy leave the earth mainly by convection or radiation.
Slaying the sky dragon addressed these issues. Maybe our understanding of this issue is still not that advanced and the majority viewpoint is flawed.
Can you please give me the references for CO2 IR re-emittance
Ira, I think this explanation is clearly written and understandable and is helpful to people who are new to the topic or who are less technically inclined.
Most serious skeptics have little disagreement with the basic physics of the green house effect. The big issue (as you point out) is sensitivity. Many of us believe that sensitivity is a function of a large number of interrelated variables and as such lends itself to stochastic modeling rather than deterministic modeling. I’d be interested in your comments on this regard.
Many on the True Believer side argue that the skeptics don’t ‘understand simple physics’ when the discussion turns to the green house effect. In fact, what the skeptics argue is that the green house effect is anything but simple and that the simplistic modeling approaches so in vogue with the AGW crowd are naive. Having said that, I do like Pielke’s approach of measuring ocean heat content anomaly as the best way to estimate sensitivity.
Finally someone has addressed the “atmospheric window” effect. There is a lot more to be said, but for now, “it’s about time”.
Most of the above is well understood by many (but not all) on both sides of the argument. The AGWs think that the solar input is constant, but the output is affected by CO2 (hence hockey stick).
Sceptics think that the solar input is variable so the output has to change accordingly.
There is also possibility of a half way house:
Solar input is constant, but the energy storage – release relationship is not a linear function, but kind of thermal hysteresis, where the oceans absorb and store large chunks of energy well above the equilibrium, followed by more than the accumulated excess release. The effect of this is a sequence of the warm periods (roman, medieval & current one) separated by the cooler ones.
From the looks of that first chart, ~100% of 7μ and 15μ radiation gets absorbed by the atmosphere on the way back to space, mainly by H20 and CO2. Maybe a tiny trickle bounces its way from molecule to molecule and makes it to space.
If that’s so, then doubling or even tripling the values of H2O in our atmosphere should have no effect on absorbtion. You can’t absorb more than 100% of the radiation, right?
Your graphs are completely misleading.
They show that the intensity of the Solar Radiation as being equal to the Earth surface IR upward radiation.
Did you really intend to show this?
Further the colder atmosphere cannot heat or “warm” the warmer Earth Surface.
All it can do is to slow to some extent the heat loss from the surface.
RJ,
I would guess that the vast majority of energy leaves earth by radiation. It has recently been discovered that our atmosphere is being continuously shed by the solar wind, and replenished by volcanism. This would be the only vehicle for convection that I can think of, and no idea if it is significant.
Gerry,
I must be that 2/3 beer. You should have downed the whole draft. Most physics and chemisry texts go from shorter to longer, left to right.
“Other aspects that control the temperature range at the surface of the Earth are at least as important and they include convection (winds, storms, etc.) and precipitation that transfer a great deal of energy from the surface to the higher levels of the Atmosphere.”
I would suggest CO2 in the atmosphere is much like the “correct” expanation of the Crookes radiometer (the black and white rotating thing in a glass vacuum bulb).
According to one theory, the black absorbs light so it rotates away from the light, or is it that the black emits IR? Or is it that the black surface is slightly hotter therefore the few air particles exhibit a higher pressure. Or to put it correctly: “Over the years, there have been many attempts to explain how a Crookes radiometer works:” http://en.wikipedia.org/wiki/Crookes_radiometer.
Just like the Crookes radiometer, carbon dioxide doesn’t just absorb IR, it also emits it. It doesn’t just interact with heat from the surface, it also interacts with the cold of space.
The so called “science” of global warming can be completely turned on its head: increasing CO2 blocks heat being emitted by the earth –> increased CO2 emits more radiation to the cold sink of space.
Just like the Crooke radiometer, the “obvious” explanation is neither obvious when you look at the deatils nor (in the case of the “greenhouse” effect which is a convective blocking effect.) is it right.
Ira, first, I think I will mostly agree with your entire post and that’s a great animated visual, but a question; where in the heck did 5525K for the sun’s temperature come from? Is that a hypothetical effective temperature as if all albedo and reflection has already been removed and falling on a flat Earth? (sounds like more Wikipedia’s hidden ‘clarity’)
Inspection of the infrared absorption windows for CO2 and H2O shows the much greater greenhouse effect of H2O. Yet I’ve often heard that the H2O only appears in the computer models as being forced by CO2, and therefore it doesn’t matter that H2O is a far more effective greenhouse gas than CO2. (People who want to regulate CO2 generation say this) My opinion is that the sensitivity to CO2 is weak, and it is an artifact of the written models that show CO2 effects predominating over H2O. Furthermore, because H2O changes from gas to liquid and back again, the H2O may even have a negative, regulating influence on the CO2 contribution to the greenhouse effect. Certainly the density driven convection caused by cloud formation and evaporation has a strong stirring effect that carries heat aloft. (A key difference between Earth and Venus is Venus’ lack of winds and vertical convection that carry heat upward)
I would like to see a discussion of these issues, as I think this is key to the AGW enthusiast/ skeptic differences of opinion.
Thanks,
Dan
Thanks Ira. Interesting post. A couple of questions, which will undoubtedly demonstrate my lack of physics knowledge. I thought that atoms emitted photons based on quanta specific to that atom. As such the H2O and CO2 molecules will absorb and emit only specific wavelengths. That would also be true of the O2 and N2 molecules. But here you are talking about “blackbody radiation” doesn’t that imply that the wavelength emitted will not be specific to the molecule but will, instead, be a function of the temperature. Is there a conflict here or is it a misunderstanding on my part?
Assuming that my understanding will be resolved, then a further understanding on my part was that the 7u and 15u wavelengths were absorbed very quickly (in terms of distance traveled through the atmosphere. That would it seems to me to effectively “trap” the energy in a fairly narrow region with the vast amount of the energy kinda just hanging around near where it was absorbed neither going up or down very far due to radiation. That then would heat that region resulting in expansion and the physical raising of the molecules. This convective upward flow of the energy would be dominant over the downward portion of the radiative flow. Thus the energy would be lifted toward space. As it got higher more and more would escape outward and the downward radiative portion would serve mainly to keep the convection going with then very little energy actually being returned to the surface.
Thus you mentioned convection as not being treated here but isn’t it possible that convection is a dominant not secondary driver of energy transmission out.
Thanks for any enlightenment you can give me.
etudiant says:
February 28, 2011 at 2:18 pm
Relative humidity in the upper atmosphere (300mb level) has fallen from around 55% in 1950 to about 45% now.
Should this not have a material impact on the greenhouse effect?
It is hard to get concerned when such substantial changes don’t even warrant discussion, much less explanation. Or is there some obvious offset that makes this moot?
——–
Sadly Dr. Miskolczi’s paper and it’s implications are rarely mentioned here, for that aspect of which you speak is probably why increased co2 has had little effect in reality (or it least it hasn’t had any measureable effect in the last 60 years). The slight warming came from some other reason and that is the real mystery, what other reason.
Well some people like a wavelength Horizontal scale, and some people like a wave number scale.
The AGW types, like the frequency scale, because that puts the CO2 frequencies near the peak of the curve; which creates the illusion that CO2 is far more important than it is. CO2 in the 606 to 741 wave number range looks much more impressive than the water bands beyond 1250 cm^-1, where the thermal emissions are only half the flux values as at the CO2 peak.
Well the width of the water bands in wave numbers is a lot more than double what it is for CO2, so water still wins, although it is not as apparent on their frequency graphs. It may be true that frequency, which relates more directly to photon energy, is the right way to do it; but then Wien’s displacement law is always stated as a product of wavelength and Temperature, and the Planck function is often plotted as a function of the single variable lambda.T.
I don’t like Ira’s wiki graph, because it creates the incorrect impression that the earth thermal emission spectrum is the same amplitude, as the incoming solar spectrum; which it is not.
The area under the solar spectrum graph is four times the area under the thermal radiation spectrum, and the spectral width of the thermal spectrum, is 20 times that of the solar spectrum, so the actual amplitude of the thermal spectrum should be about 80 times smaller than it is.
The solar spectrum goes from 0.25 to 4.0 microns for 98% of the energy, while the thermal spectrum goes from about 5.0 to 80 microns for the same 98% of the energy. Even if you equate those two energies, the thermal spectrum amplitude should be 20 times lower than plotted.
jknapp; it is a misundertanding on your part. The wavelength specific nature of say CO2 or H2O is a function of the absorption modes of those molecules, which relate to specific mechanical resonances of the molecules themselves.
BUT most of those excited molecules DO NOT get a chance to re-radiate, that photon, and return to the unexcited state. The time interval between molecular colliwsions is a few nanoseconds; but the lifetimes of the excited states, can be microseconds to milliseconds; so the energy is lost in collisions, before the molecule gets a chance to re-radiate. That thermalization heats the whole atmosphere; the N2, the O2, the Ar, and anything else; and it is that mixture of gases, at somer average temperature in the 250-300 K range, that is the source of the atmospehric emitted thermal continuum spectrum, that is not related to specific molecular energy levels, but only to the Temperature of the gases; whcih determines the dynamics of the collisions, and the resulting chaotic accelerations of the colliding molecules, and the electric charges they contain.
So the atmosphere does radiate a black body like thermal spectrum, and the presence of the gHG molecules simply means that there will be bands of that continuum emission, that are also captured by the GHG molecules, as well as the emissions from the surface.
The only function of the GHG molecules is to heat the atmospheric gases; along with all the other mechanisms that are heating it. After that, the GHG molecules serve no function in the climate process whatsoever.
A nit pick; the ‘curves’ (from wiki) in that first figure all indicate to have the same peak amplitude (in reality they do not, of course). This is in the same category of drawing sine waves using half-circles from a drawing template which yield a ‘slope’ that is seemingly infinite as (delta y / delta x) -> infinity as x -> 0. (Never mind that the same that amplitude scale seem applicable to both solar and earth intensities … one may click my name and see the 2nd image down to see something drawn to scale relating sun and earth intensity. )
Using this depiction does not convey graphically the idea that ever-greater amounts of energy ‘escape’ through the atmospheric window (out to space or to the ‘great beyond’) as the temperature rises.
Please, correct me if I am wrong …
.
In your animated diagram what happens to the downward re-emitted 7μ radiation when it reaches the earth surface? I believe that what happens is that it may emitted again as 7μ IR radiation, or at a longer wavelength such as a photon in the 10μ band. In the former case it will have a high probability that it will be absorbed and downward emitted again, and in the latter case it will be lost to space.
Thus the 7μ band sees not an equilibrium of up and down radiation flux, but a continuous stream of loss to 10μ. This would cause more energy to be lost than in your model.
I’ll wait to see how this is adjudged.
(IR Spectroscopy explaining spectral response regarding gas molecule vibrational modes would seem to indicate otherwise.)
.
This graph, about the atmospheric absorption, is from the 1970’s. Could anyone provide a recent one? Just to compare things.
Thanks in advance.
jknapp says: February 28, 2011 at 3:01 pm
Thanks Ira. Interesting post. A couple of questions, which will undoubtedly demonstrate my lack of physics knowledge. I thought that atoms emitted photons based on quanta specific to that atom. As such the H2O and CO2 molecules will absorb and emit only specific wavelengths.
————————————————–
This is true at low pressures, such as inside a fluorescent light bulb or a gas laser. At higher pressures, such as atmospheric, the molecules interact with each other and smear the lines into bands. Think of it as trying to make a perfect baseball pitch with 3 people bumping into you.
etudiant said:
Can you provide a reference for that?