Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Support

Bluesky Facebook LinkedIn Mastodon MeWe

Twitter YouTube RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

New? Register here
Forgot your password?

Latest Posts


Joseph Postma and the greenhouse effect

What the science says...

Joseph Postma published an article criticizing a very simple model that nonetheless produces useful results.  He made several very simple errors along the way, none of which are very technical in nature.  In no way does Postma undermine the existence or necessity of the greenhouse effect.

Climate Myth...

Postma disproved the greenhouse effect

"Skeptics hope that Postma’s alternative thermal model will lead to the birth of a new climatology, one that actually follows the laws of physics and properly physical modeling techniques...Postma deftly shows how the systemically tautologous conjecture that is “back-radiative heating” just doesn't add up. We see how climatologists fudged the numbers to make it appear as if Earth actually raises its own temperature by having its own radiation fall back upon it - a conjecture contrary to fundamental physics." (John O'Sullivan)

Some recent attention has recently been going around the web concerning a new “paper” done by Joseph E. Postma (PDF here) which claims to “…physically negate the requirement for a postulation of a radiative atmospheric greenhouse effect.”  

The claims are of course extraordinary, along the lines of Gerlich and Tseuchner’s alleged falsification of the atmospheric greenhouse effectAs is often the case with these types of “skeptics,” the more extravagant the claim, the more obscure the publishing venue; in this case the host is Principia Scientific International, which according to the website “…was conceived after 22 international climate experts and authors joined forces to write the climate science bestseller, ‘Slaying the Sky Dragon: Death of the Greenhouse Gas Theory.’” Most rational people would stop here, but this is the Americanized age where we need to glorify everyone’s opinion and must provide rebuttals for everything, so here it goes:

I ask that the reader have the paper open in a new window so they can follow along with this article.

The Foundations

Most of Postma’s first 6 pages are actually correct.  He describes the greenhouse effect through the so-called layer model, which is a simple way to break up the planet into a “surface” and an “atmosphere,” with outer space overlying the top layer.  This model is described in many climate books such as Dennis Hartmann’s Global Physical Climatology, David Archer’s Understanding the Forecast, Marshall and Plumb’s Atmosphere, Ocean and Climate Dynamics, and radiation books like Grant Petty’s First Course in Atmospheric Radiation.  I will say that I do not particularly like this model as a suitable introduction to the greenhouse effect.  It is useful in many regards, but it fails to capture the physics of the greenhouse effect on account of making a good algebra lesson, and opens itself up to criticism on a number of grounds; that said, if you are going to criticize it, you need to do it right, but also be able to distinguish the difference between understood physics and simple educational tools.

The atmosphere in Postma’s paper is just a single slab, so he has two layers (atmosphere+surface), but in general you can have many atmospheric layers.  He goes on to solve for the energy balance of each layer (see equations 11-14). RealClimate derived the same result in less than a page here.

 Figure 1: Layer model is Postma's paper.  Click to Enlarge


Postma actually doesn’t get the atmospheric radiative flux right.  The emission is not σTa4, it is fσTa4, where f is the atmospheric emissivity/absorptivity (following his notation) and Ta is the atmospheric temperature.  The emissivity is a unitless factor between 0 and 1 descrbing how good of an absorber/emitter the object is relative to an ideal body.  f = 1 describes a blackbody.  By Kirchoff's law, the absorptivity of a layer must be equal to the emissivity (at the same wavelength),  Both right hand sides of equations 11 and 12 are thus wrong, but it turns out that those errors cancel each other out and he gets equation 14 right.  The factor of 2 in Equation 12 comes about because the atmosphere emits both up and down, although Postma clearly doesn't know how to derive this result formally, based on later statements he makes about this.  Toward the end of page 14 he says this is invalid since the atmosphere radiates in 3-D, not just up and down.  In fact, the quantity σT4 refers not only to the total power output of an object (the rate of energy emission), but it also refers to isotropic (equally intense in all directions) radiation.  The result σT4 is obtained if one assumes that a plane radiates uniformly over a hemisphere (for example, the domed "half sphere" field of vision that a human can see  when you stand outside, with the base of that half-sphere being the surface you sre standing on; the other hemisphere is invisible (see this image).

So far, it is simple textbook stuff with not much promise.

Geometry of the Global Energy Budget

Postma then goes on to describe fictitious “boundary conditions.”  In particular, he seems to have serious objections to the averaging of the solar radiative flux over the Earth.  In essence, he would prefer we had one sun delivering 1370 W/m2 of energy to the planet, with a day side and a night side, noon and twilight, etc. instead of the simple model where we average 1370/4=342.5 W/m2 over the planet (so that the whole Earth is receiving the appropriate "average" solar radiation).  The number becomes ~240 W/m2 when you account for the planetary albedo (or reflectivity). 

The factor of 4 is the ratio of the surface area to the cross section of the planet, and is the shadow cast by a spherical Earth.  It is therefore a geometrical re-distribution factor; it remains “4” if all the starlight is distributed evenly over the sphere; it is “2” if the light is uniformly distributed over the starlit hemisphere alone; with no re-distribution, the denominator would be 1/cosine(zenith angle) for the local solar flux.

In simple textbook models, we like to prefer explanations that get a point across, and then build in complexity from there (see Smith 2008 for descriptions on a rotating Earth).  Of course, students who use this model are probably educated to the point where they know that day and night exist, and certainly GCMs have a diurnal cycle.  The radiative calculations are done explicitly by accounting for the temperature distribution and absorber amount that is encountered at each grid box.  Postma is simply tackling a non-issue, just as how people criticize the term “greenhouse effect” for not working like a glass greenhouse. Postma objects to teaching this simple model because it is not real.   All that is done, however, is to use a brilliant and sophisticated technique, taught only to the geniuses among us, called averaging! And of course, simple models are used in any is how we learn.

But, in actuality, the globally averaged solar re-distribution approximation is not bad when we use it to describe the temperature for planets like Earth or Venus.  These planets have an atmosphere or ocean that transport heat effectively, especially Venus with virtually no day-to-night or pole-to-equator temperature gradient.  The atmosphere and/or ocean help smooth the diurnal temperature difference very well.  Therefore, when coming up with a temperature estimate, it is a great first approximation.  If you want the local equilibrium temperature for an airless body like Mercury or the Moon (that does not transport heat), then you want to use the no-redistribution or hemisphere only solar factor.  This is well-known (see e.g., Selsis et al 2007).  On Mercury, there is no heat distribution and very little thermal inertia; before the sunrise the temperature on the surface is somewhere near 100 K (-173 °C) and by noon the temperature on the surface of Mercury rises to about 700 K (427 °C).  This may also be relevant for tide-locked planets (very slow rotation since one side is always facing the host star, the other in perpetual darkness).  Earth does not experience any such changes of the sort.  On Venus, the variability is even less, and most of the planet is at around 735 K.

Summary so far...

To summarize so far, Joseph E. Postma did not like a simple model of Earth’s radiative balance where we approximate the Earth as a sphere with uniform solar absorption.  Of course, this is never done in climate modeling or in more detailed analyses appropriate for scholarly literature, so it is more an exercise in complaining about undergraduate education than an attempt to correct what he calls a “paradigm” in climatology.  Nonetheless, the 0-D energy balance model is a useful approximation on Earth when coming up with an average emission temperature (~255 K), since air circulations and oceans tend to even out the diurnal temperature gradient on Earth, in addition to the thermal inertia provided by the system.

Venus is More Optically Thick Than a One Layer Model Can Give You

Postma starts by using Venus as a template for where the greenhouse model he is using breaks down.  And indeed, he is right.  His argument is that f (the emissivity) cannot possibly be greater than 1 (which is correct), and yet it must be in order to produce the Venus surface temperature in his Equation 29)  Based on this, he then states that the standard greenhouse model does not work in general.  The problem is that his Equation 29 assumes a one-layer atmosphere, which is an absurd assumption when you approach the extremely high optical thickness of Venus. Venus has a 90 bar atmosphere that has well over 90% carbon dioxide, some water vapor, and a greenhouse effect generated by suluric acid droplets and SO2.  The radiative transfer on Venus works much differently than on Earth, owing in part to intense collisional broadening of CO2 molecules.  A photon has an extremely difficult time escaping Venus, unable to do so until it reaches the very outer parts of its atmosphere.

Using the layer model, you would need many atmospheric layers to produce something close to Venus; with enough layers you would find that you could produce the surface temperature of Venus without violating conservation of energy.  With just one perfect absorbing atmospheric layer, the surface temperature cannot exceed 21/4 times the emission temperature (Te=~230 K on Venus).  But with two perfectly absorbing atmospheric layers, it can rise to 31/4Te.  With three layers, the maximum temperature is 41/4Te, and so on.  The reason the surface temperature is capped in this way is because the atmosphere itself must be emitting radiation and heats up when it absorbs photons from the surface, which in turn increases emission.  If the atmospheric layer were instead a good infrared reflector (i.e., it has a high thermal albedo), then you could delay heat loss to space that way and increase the surface temperature well beyond this value.  This could happen with CO2 clouds instead of H2O clouds, the latter are much more effective IR absorbers than IR scatterers, whereas the former could raise the IR albedo.

In essence, Postma stretches a simplified model to areas that it was never designed to go to, and then declares that its failure to work means the whole paradigm of the greenhouse effect is wrong.  The incompetence is overwhelming.  Postma is not done though, and decides to dig in further.  His next argument is amusing, but perhaps a bit strange to follow, so I will try to explain.

Lapse Rate Confusion

He claims that observations of the atmospheric lapse rate (the rate at which temperature declines with height) disallow the greenhouse effect.  His reasoning is that the atmosphere is at a fixed height.  When greenhouse gases warm the surface, and cool the upper atmosphere, that height still remains fixed, but obviously the temperature difference between the bottom and top of the atmosphere must increase.  Postma then claims that this necessarily implies that the lapse rate must have a greater slope than the theoretical value that he derived of about -10 K per kilometer (which is about right for a dry air parcel ascending).  That is, if the atmospheric height remains fixed, and the temperature difference between bottom and top is increased, then the rate at which air cools with height must increase.  Since this is not observed, then we have a problem, right?

In actuality, the atmospheric height is a distraction.  The adiabatic lapse rate does not extend beyond the point where convection breaks down, which is the tropopause.  The whole point of the greenhouse effect is that increasing atmospheric greenhouse gases does increase the “average” height at which emission to space takes place (and the tropopause increases in height too), so one IS allowed to extrapolate further down the adiabat to reach a higher surface temperature.  On Venus, the optical thickness forces the tropopause to some 60 km altitude. Additionally, it is worth pointing out that greenhouse gases warm the upper troposphere, not cool it, but they do cool the stratosphere.


Figure 1: Qualitative schematic of the old (blue) and new (e.g., after CO2 increase) temperature with height in a dry atmosphere.  Moisture tends to enhance the tropical upper atmosphere warming relative to surface.  Temperature increases to the right. 

TOA vs. Surface

Perhaps just as crucial to all of this, Postma cannot get around the surface energy budget fallacy, which says that increased CO2 causes surface warming by just increasing the downward infrared flux to the surface.  This problem is described in standard treatments of the greenhouse effect, which he does not seem to know exist, such as in Ray Pierrehumbert’s recent textbook. The primacy of the top of the atmosphere budget, rather than the surface energy budget, has been known at least since the work of Manabe in the 1960s (see also Miller 2012)

In reality, the top of the atmosphere budget controls the surface temperature even more than the surface forcing, because the atmosphere itself is adjusting its outgoing radiation to space (and much of the radiation to space is originating in the upper atmosphere, owing to its IR opacity). Where the atmosphere is well-stirred by convection, the adjustment in temperature at this layer is communicated to the surface.  I described this in more detail here. 

Postma runs into this mistake again when he claims that the low water vapor in hot deserts is a problem for greenhouse theory, but this is largely due to the lack of evaporation cooling, which is just one component of the surface energy budget, and nearly absent in a desert.  This is one scenario where a detailed consideration of the surface budget is critical, as well as in other weakly coupled regimes.

The way CO2-induced warming really works in a well mixed atmosphere is by reducing the rate of infrared radiation loss to space.  Virtually all of the surface fluxes, not just the radiative ones, should change in a warming climate, and act to keep the surface and overlying air temperature relatively similar.  The back-radiation will indeed increase in part because of more CO2 and water vapor, but also simply because the atmosphere is now at a higher temperature. But if the lower atmosphere was already filled with water vapor or clouds to the point where it emitted like a blackbody (at its temperature), increasing CO2 would not directly increase downward emission before temperature adjustment, but would nonetheless warm the planet by throwing the TOA energy budget out of whack.


In summary, Joseph Postma published an article criticizing a very simple model that nonetheless produces useful results.  He made several very simple errors along the way, none of which are very technical in nature.  More sophisticated models are obviously designed to handle the uneven distribution of solar heating (which is why we have weather!); nonetheless, the educational tools are useful for their purpose, and in no way does Postma undermine the existence or necessity of the greenhouse effect.  Without a greenhouse effect, multiple studies have shown that the Earth collapses into a frozen iceball (Pierrehumbert et al., 2007; Voigt and Marotzke 2010, Lacis et al 2010) and indeed, after an ice-albedo feedback, plummets below the modern effective temperature of 255 K.  This work makes extraordinary claims and yet no effort was made to put it in a real climate science journal, since it was never intended to educate climate scientists or improve the field; it is a sham, intended only to confuse casual readers and provide a citation on blogs.  The author should be ashamed.

Last updated on 1 January 2015 by Chris Colose. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.


Prev  1  2  3  4  Next

Comments 51 to 75 out of 99:

  1. Dikran, I like your explanation in #50, and with your indulgence I would like to ask one "alternate physics" question. If the mirror sends the same frequencies of IR that the blackbody emits back to the blackbody, is there any physical mechanism by which the blackbody could "reject" those frequencies?
  2. Eric (skeptic) Radiative physics isn't really my area, but the definition of a blackbody is that it absorbs all radiation that falls on it, so if it rejects some particular frequency then by definition it isn't a blackbody.
  3. (-Snip-)

    [DB] Again, you chose to "hang your hat" on the Postma paper you linked to.  You were then challenged to defend a particularly egregious distortion of physics Postma makes, here.  You cannot through dereliction run away from your defense of this paper, as it is your chosen field of play. 

    A failure to follow through on your self-assumed duty will have consequences.

    Off-topic snipped.

  4. Eric (skeptic) @51, if the emitting body does not absorb in the same wavelengths of light as well as it emits, then it is in violation of Kirchoff's Law of Thermal Radiation. As Kirchoff's Law is a special case of the 2nd law of thermodynamics, that would also mean it is a violation of the second law of thermodynamics. There are some nuances here. If emissivity is greater than zero at a given wavelength (ie, the body radiates in that wavelength, then it must absorb some of the reflected radiation, but will not absorb all of it unless the emissivity and hence absorptivity is 1, ie, the body is "black" at that wavelength. Because some radiation is absorbed, the body will warm up if it is otherwise in thermal equilibrium. If the mirror is so arranged that all light emitted or reflected from a section of the body is reflected back to the same section of the body, then radiation intensity reflected from the mirror will quickly build up until radiation absorbed by the appropriate section of the body equals radiation emitted by that section. At that time, the intensity of radiation at that wavelength reflected from the mirror (and either emitted or reflected from the section of the body) would be the same as if the body had an emissivity of 1 at that wavelength.
  5. Thanks Tom. I read that absorptivity and emissivity were equal for an object in thermal equilibrium and figured that covered it. But the arguments I read (pages and pages) talk (vaguely) about physical processes at the surface of the object with individual photons rather than the net amounts of emissivity, absorptivity and energy.
  6. I think the link in post 53 should be:

    [DB] Fixed, thanks!

  7. Postma`s mirror... OK forget the mirror atm and imagine the experiment to be in the vacuum of space, and there are two identical black bodies being warmed by the same light bulb at the same distance to the same state of equilibrium. When the two black bodies are in close proximity but not touching each other, is there a reciprocal warming in both bodies ?
  8. YOGI Forgetting the mirror avoids discussing the error in Postmas' paper. However, in the case of two black-bodies in close proximity and at the same temperature, there will be a constant exchange of photons between them, but this exchange will be balanced (assuming symmetry). However this exchange flux will mean that both bodies would be at a higher equilibrium temperature than they would be at if the other body were not there. This is because the outbound photons that would have been absorbed by the other body will now simply be radiated out into space and won't be balanced by the reciprocal photons from the other body.
  9. That`s what I thought, which implies that there is a problem with the mirror example, as equal and mutual exchange amounts to the same as reflection. A mirror is hardly an analogy of the real atmosphere as re-emission after absorption is in all directions, and there are gaps between absorption bands. So I would not throw the baby away with the bathwater, most of the paper is very good.
  10. YOGI Please can you state unequivocally whether the mirror example in Postmas paper is incorrect or not. Do you agree that with the mirror in place the blackbody will have a higher equilibrium temperature. Also please state unequivocally whether you agree that in the example you provided that the presence of another blackbody increases the equilibrium temperature of both bodies. In a scientific debate it is essential that such things are stated absolutely unequivocally, rather than "which implies there is a problem", which is vague enough for there to be a question of whether it is actually right or wrong.
  11. YOGI - "...most of the paper is very good." As stated in the OP, and in multiple comments, this is simply not the case - Postma's paper is riddled with basic errors. An object is in thermal equilibrium when it is radiating/conducting/convecting away as much energy as it is receiving from it's surroundings. A nearby mirror (as part of those surroundings) increases the amount of energy coming in over and above the light source (relative to a cold background), and the only way for that object to radiate away that increased energy is to become warmer. That holds whether the mirror is perfect, or a 50% mirror with wavelength dependencies (more akin to the atmosphere). Increased energy from the surroundings, or rather decreased energy absorbed by the surroundings, will drive the temperature of the object up until it is radiating as much energy as it absorbs. Postma's statement that the object would not heat up, as Dikran noted here, is completely wrong, violating the first law of thermodynamics, conservation of energy. There's nothing wrong with the mirror example, just with Postma's incorrect description of what would happen. Leaving behind inexact language (such as "implies") - do you, or do you not, agree with Postma?
  12. While I too consider the Postma paper under discussion a travesty, YOGI's statement that 'most of it is very good' could almost be true in a way... if this is an indication that YOGI is accepting that the 'greenhouse effect violates the 2nd law of thermodynamics' argument (i.e. the central premise) of the paper is incorrect then there actually is quite a bit of other information in the paper which is accurate and detailed enough to be useful to someone who hasn't been exposed to the more nuanced intricacies of the greenhouse effect previously. However, as Dikran notes, it isn't entirely clear whether YOGI is accepting that or not... and in any case there are many other references which can provide a similarly detailed background without the occasional shockingly wrong argument sprinkled in.
  13. Back-rad works for sure, you can feel the warmth of clouds on a cold night. So as long as it warmer than the surface, I have no objection.
  14. Clouds warmer than the surface? It's called temperature inversion and does not lead to the formation of clouds.
  15. YOGI you have been asked to clarify your position and give an explicit answer on two questions. Instead you have given only the vaguest response possible. This is an indication that you are trolling. Please demonstrate that you are not by giving direct, unambiguous answers to the questions you have been asked.
  16. YOGI, the presence of clouds at night will keep the earth's surface warmer (i.e. will inhibit radiational cooling), all other things being equal. The clouds can be any temperature from relatively warm low clouds to very cold high clouds but will mostly be cooler than the surface. Yet they cause the surface to be warmer than it otherwise would be.
  17. Re YOGI@63 Hmmm. You seem to feel that cloud must be warmer than you (or the surface) to "feel the warmth". How about these conditions?: a) clear sky, warm night, surface at 30 degrees C, so upward-emitted IR is about 480 W/m^2. Back radiation from clear sky about 340 W/m^2, which translates to an apparent sky temperature (for the purposes of IR radiation) of about 5 C. Net IR at the surface is -140 W/m^2 (i.e. a loss of 140 W/m^2). Feels cool. [Although I've selected these temperatures arbitrarily, they are realistic.] b) Identical to a), except that cloud has moved in at an altitude of about a kilometer, and the base of cloud is at about 25 C (i.e. still colder than surface). Back radiation from sky is now from cloud, not clear sky, and is about 450 W/m^2 (based on 25 C). Net IR at the surface is now only -30 W/m^2 (loss of 30), so the loss of energy from the surface is reduced from 140 to 30 W/m^2 - or only about 21% of what it was. [Again, arbitrary, but realistic, numbers.] To me, the second case (substantially less net IR loss from surface) will "feel warmer", even though the cloud is cooler than the surface (or me). Do you disagree with this scenario? Are you trying to say that the surface (or you) will feel no extra warmth unless the overlying cloud is actually warmer than the surface?
  18. Eric (skeptic) So what's up with nighttime clouds? Clouds emit infrared radiation to the efficiently absorbing ground, keeping the ground (and thus the overlying air) warmer.:
  19. YOGI, before you go any futher, please answer the questions I asked here. You have not yet clarified your position on the errors in the Postma paper that you chose to hang your hat on.
  20. Bob Loblaw "a) clear sky, warm night, surface at 30 degrees C.." What all night long ? "Net IR at the surface is -140 W/m^2 (i.e. a loss of 140 W/m^2). Feels cool." Yes once it is much less than 30°C it will feel cool. "Are you trying to say that the surface (or you) will feel no extra warmth unless the overlying cloud is actually warmer than the surface?" Correct. In b) the presence of clouds will slow the rate of surface cooling, but cannot increase surface temp` unless they are at a higher temp` than the surface.
  21. YOGI, you are incorrect. A simple extension of Postma's mirror example will demonstrate why you are incorrect. Your continued avoidance of answering the questions that would clarify your position on Postma's paper is beginning to look like evasion.
  22. "Dikran Marsupial" YOGI Please can you state unequivocally whether the mirror example in Postmas paper is incorrect or not. Do you agree that with the mirror in place the blackbody will have a higher equilibrium temperature. I think it incorrect as I implied in my analogy of two black bodies, that got snipped earlier.
  23. YOGI, thankyou for answering one of my questions, the other was: "Also please state unequivocally whether you agree that in the example you provided that the presence of another blackbody increases the equilibrium temperature of both bodies."
  24. YOGI "clear sky, warm night, surface at 30 degrees C.." What all night long ?" I don't know where you live that causes you to be surprised at this, but it is quite realistic. Where I live the temperature at 2.00am was 30.7C a couple of nights ago. And it wasn't much less at 6.00am.
  25. YOGI@70 Wow. Basically three short sentences (not counting the single word "Correct" that starts the third paragraph) and we have both a strawman and a goalpost shift. Plus a fundamental misunderstanding of physics.... 1) Who said "all night long"? I'm comparing two scenarios at a single point in time, and how something/someone "feels" at that instant. No need to consider what might be going on hours away. [Strawman - I never said the conditions had to be maintained for a period of time.] 2) At a particular point in time, 30 C with rapid heat loss will "feel cooler" than 30C with a slow heat loss. No need to wait. [Goalpost shift - now "feels warm" or "feels cool" is replaced by "needs to be substantially cooler than skin temperature, or previous temperature". I can't tell which YOGI means.] 3) Whether something warms or cools is dependent on it total energy balance, not just the net IR. Even if net IR is negative (a loss), reducing that loss (making it less negative) can lead to warming if all other heat fluxes remain constant. [Fundamental misunderstanding of physics.] It may be that you're just expressing yourself poorly, but for me, it's "three strikes, you're out".

Prev  1  2  3  4  Next

Post a Comment

Political, off-topic or ad hominem comments will be deleted. Comments Policy...

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

Link to this page

The Consensus Project Website


(free to republish)

© Copyright 2024 John Cook
Home | Translations | About Us | Privacy | Contact Us