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A simple recipe for GHE

Filed under: — rasmus @ 5 July 2010

According to some recent reports (e.g. PlanetArk; The Guardian), the public concern about global warming may be declining. It’s not clear whether this is actually true: a poll conducted by researchers at Stanford suggests otherwise. In any case, the science behind climate change has not changed (also see America’s Climate Choices), but there certainly remains a problem in communicating the science to the public.

This makes me think that perhaps a new simple mental picture of the situation is needed. We can look at climate models, and they tell us what we can expect, but it is also useful to have an idea of why increased greenhouse gas concentrations result in higher surface temperatures. The saying “Everything should be made as simple as possible, but not simpler” has been attributed to Albert Einstein, which also makes me wonder if we – the scientists – need to reiterate the story of climate change in a different way.

Gavin has already discussed this (also see here and here), but it may be necessary to tell story over again, with a slightly different slant. So how can we explain how the greenhouse effect (GHE) work in both simple terms and with a new angle? I also want to explain why the middle atmosphere cools with increasing greenhouse gas concentrations associated with an increased GHE. Here I will try to present a conceptual and comprehensive picture of GHE, explaining both the warming in the lower part of the atmosphere as well as the cooling aloft, and where only the most central features are included. Also, it is important to provide a good background, and we need to start with some very fundamental facts.

Four main physical aspects
Several factors are involved, and hence it may be useful to write a simple recipe for the GHE. This recipe then involves four main ingredients: (i) the relationship between temperature and light, (ii) the planetary energy balance, (iii) the distance light travels before being absorbed, and (iv) the relationship between temperature and altitude.

(i) Temperature and light
Energy can be transmitted in many different ways, involving photons (light or electromangetic radiation), conduction, and motion. Most of these require a medium, such as a gas, fluid, or a solid, but space is basically a void through which photons represent virtually the only form for energy transfer. Hence, planets tend to gain or lose energy to space in the form of photons, and we often refer to the energy loss as ‘radiative heat loss’.

A fundamental law of physics, known as the Planck’s law, says that radiative heat loss from any object depends on its temperature. Planck’s law also explains the colour of the light, or its wavelength, and hence explains why iron gets red hot when heated sufficiently.

Figure 1. Illustration of Planck's law, where the different curves represent objects with different temperature. The y-axis is marks the intensity and the x-axis the wave length (colour) of the light emitted by bodies with a given temperature (PDF-version and R-script generating the figure.)

Planck’s law predicts that the light from an object with a temperature of 6000K – such as the solar surface – produces light that is visible, whereas objects with a temperature of 288K produce light with a wavelength that our eyes are not able to see (infra red). This is illustrated in Figure 1 showing how the light intensity (y-axis; also referred to as ‘flux density‘) and the colour of the light (wave length) vary for objects with different temperatures (here represented by different curves). The yellow curve in the figure represents the solar surface and the light blue curve the earth.

(ii) The planetary energy balance
The planetary energy balance says that our planet loses heat at the same rate as it receives energy from the sun (otherwise it would heat or cool over time). This is because energy cannot just be created or destroyed (unless it involves nuclear reactions or takes place on quantum physics scales).

The planets’ distance from the sun and the brightness of its surface dictates how much energy it receives from the sun, as the light gets dimmer when it spreads out in space, as described by Gauss’ theorem.

Figure 2. A schematic of the solar system, where the energy received by the earth is the sunlight intercepted by its cross-section, and where the heat loss on average is due to thermal emission from the whole surface area of the planet. As the sunlight travels away from the sun, it spreads out over larger space and gets dimmer.

The energy flowing from the sun is intercepted by the earth with energy density described by the ‘solar constant‘ (S0=1366W/m2), and the amount of energy intercepted is the product between this flux density and the earth’s disc (minus the reflected light due to the planet’s albedo: A ~0.3). The average heat loss is given by the product of earth’s surface and its black body radiation:

S0/4 (1-A) = σT4,

where σ=5.67 x 10-8W/(m2 K4) is the Stefan-Boltzman constant. This gives a value of 255K, known as the emission temperature.

Figure 3 shows a comparison between observed surface temperature and calculated emission temperature for the planets in the solar system, based on the balance between energy from the sun and heat loss due to black body emission. In these simple calculations, the greenhouse effect is neglected, and the black body radiation can be derived from Planck’s law. The calculations agree quite well with the observations for most of the objects in our solar system, except for Venus which is known to harbour a strong GHE and has a hotter surface than Mercury despite being about twice as far away from the sun.

Figure 3. Comparison between calculated emission temperature and the observed surface temperatures for planets and moons in our solar system. The calculations estimate the reduction in the energy flux density with distance away from the sun (Gauss' theorem) and the black body radiation describing the rate of planetary heat loss. Here, the greenhouse effect has been neglected in the calculations, but the GHE does affect the observed surface temperatures. Venus is a bright planet (high albedo) with a thick atmosphere mostly made up of CO2, which explains higher surface temperature than inferred from a pure energy balance (PDF-version and R-script generating the figure).

(iii) Light absorption
It is also clear that our planet is largely heated at the surface because the light from the sun – which is visible for our eyes – penetrates the atmosphere without much absorption (hence we can see the sun from the ground). However, the atmosphere is a medium of gas and particles that can absorb and scatter light, depending on their wavelength (hence explain why the sky is blue and sunsets orange).

The distance light travels before being absorbed – optical depth – can vary with the light’s wavelength and the medium through which is travels. The optical depth in our atmosphere is different for visible and infra-red light.

Infra-red light is absorbed by molecules, which in turn get more energetic, and the excited molecules will eventually re-emit more infra-red light in any random direction or transfer excess energy to other molecules through collisions. In a optically thick (opaque) atmosphere, there will be a cascade of absorption and re-emission.

Hence, whereas the planet is heated at the surface, it’s main heat loss takes place from a height about 5.5 km above the ground, where most of the radiation is free to escape out to space. The optical depth dictates how deep into the planet’s atmosphere the origin is for most of the planet’s infra-red light (the main planetary heat loss) that can be seen from space. Furthermore, it is the temperature at this level that dictates the magnitude of the heat loss (Planck’s law), and the vertical temperature change (lapse rate) is of course necessary for a GHE. The temperature at this level is the emission temperature, not to be confused by the surface temperature.

We know that the optical depth is affected by CO2 – in fact, this fact is the basis for measuring CO2 concentrations with infra-red gas analysers. Molecules composed of three or more atoms tend to act as greenhouse gases because they can possess energy in terms of rotation and vibrations which can be associated with the energy of photons at the infra-red range. This can be explained by theory and be demonstrated in lab experiments. Other effects are present too, such as pressure and Doppler broadening, however, these are secondary effects in this story.

(iv) The relationship between temperature and altitude
There is a well-known relationship between temperature and height in the troposphere, known as the ‘lapse rate‘ (the temperature decreases with height at a rate -6K/km). The relationship between temperature and altitude can also be seen in the standard atmosphere. The lapse rate can be derived from theory for any atmosphere that is the hydrostatically stable condition with maximum vertical temperature gradient, but it is also well-known within meteorology. Thus, given the height and value of the emission temperature, we can get a simple estimate for the surface temperature: 255K + 5.5km * 6K/km = 288K (=15oC; close to the global mean estimated from observations given by NCDC of ~14oC).

Enhanced greenhouse effect
The term known as the ‘enhanced greenhouse effect’ describes a situation where the atmosphere’s becomes less transparent to infra-red light (reducedincreased optical depth), and that the heat loss must take place at higher levels. Moreover, an observer in space cannot see the infra-red light from as deep levels as before because the atmosphere has become more opaque.

Figure 4. A simple schematic showing how the planet is heated at the surface, how the temperature (blue) decreases with height according to the lapse rate, and how infra-red light (wiggly arrows) is absorbed and re-emitted at various stages on its way up through the atmosphere. Energy is also transferred through vertical motion (convection), evaporation, and condensation too (latent heat), but that doesn't affect this picture, as they all act to restore the vertical structure toward the hydrostatically stable lapse rate in the long run. At the top of the atmosphere, the infra-red light escapes freely out to space, and this is where the planet's main heat loss takes place. This level is determined by the optical depth, and the heat loss depends on the temperature here. (click on figure for animation)

The effect of heightened level of heat loss on the surface temperature is illustrated in Figure 4, where the emission temperature and lapse rate are given if we assume an energy balance and a hydrostatically stable atmosphere on average (a generally hydrostatically unstable atmosphere would be bad news).

Hence, a reducedincreased optical depth explains why atmospheres are not easily ‘saturated‘ and why planets such as Venus have surface temperatures that are substantially higher than the emission temperature. Planets with a thin atmosphere and insignificant greenhouse effect, on the other hand, have a surface temperature that is close theto the estimates from the planetary energy balance model (Figure 3).

Feedback processes
The way the atmosphere reacts to changes in the optical depth is more complicated, due to a number of different feedback mechanisms taking place. But to get a simple overview, it is useful to keep in mind that the optical depth is sensitive to how much water vapour (humidity) there is in the air, and that the lapse rate is sensitive to the composition of the atmosphere (i.e. humidity). Furthermore, the albedo A is affected by clouds, snow, ice, and vegetation, all of which affect the way the earth receives energy from the sun.

In our simple picture, feedback processes affect changes in the height of the level where most heat loss takes place, the slope of the lapse rate, and heating at the surface (and hence the emission temperature).

So why is the upper atmosphere cooled then?
The upper atmosphere, comprising the stratosphere and mesosphere, is expected to cool during an AGW, as shown by the GCMs. So what is happening there? This is when the picture becomes more complicated.

Since CO2 mostly absorbs/re-emits infra-red light at around 14 microns, an increased concentration in the troposphere will reduce the upward infra-red radiation at this band. The total energy is roughly constant, but it is made up from increased emissions at other bands because it’s warmer. There is less absorption by CO2 of upwelling infra-red light above the troposphere, but increased emission as a function of increased concentrations. Thus there is a cooling.

Can this picture be falsified, e.g. if other factors were to play a role too? For instance, can this situation be altered by changes in the sun?

Changes in the sun can of course affect the amount of energy received by the earth through changes in its output, variations in the intensity of UV-light, or perhaps even clouds through galactic cosmic rays. But it’s hard to see any systematic long-term trend in the level of solar activity over the last 50 years, and it is difficult to see how solar activity may have an effect while other factors, such as GHGs, don’t. Gavin and I recently published a study on the response to both solar activity and GHGs, and found similar magnitude for both forcings in both observations and the GISS GCM.

There have been claims of negative feedbacks, such as the “iris effect“. One would expect negative feedbacks in general to dampen the response to most forcings, unless they involve a particular process that is active for a particular forcing. In other word, why would a negative feedback act for GHGs but not for solar forcing? Many feedbacks, such as changes in atmospheric moisture, cloudiness, and atmospheric circulation should be similar for most forcings.

Another question is why we do see a global warming trend if the negative feedbacks were most important (Figure 5). Negative feedbacks usually imply quiet conditions in a complex system, whereas positive feedbacks tend to lead to instabilities, often manifested as internal and spontaneous oscillations (see Figure 5). It is reasonable to expect the feedback processes to affect natural variations as well as forced changes such as an enhanced GHE, orbital changes, volcanoes, or changes in the sun.

Figure 5. Estimates of the global and annual mean temperature based on a number of different data sets, including both traditional analyses as well as re-analyses (also see the last 15 years).

The point about negative feedback also brings up another interesting issue: Negative feedbacks usually act to restore a system to a particular zero-level state. What would the zero-state be for our climate? No greenhouse effect or some preferred level of greenhouse warming? There is already a natural GHE that, together with other atmospheric effects, can account for about 32oC higher global mean surface temperature. What makes this state so special, and can we explain the present natural GHE in the presence of negative feedbacks (consider starting from a state with no GHE)?

Hence, claims of negative feedback is controversial because all these tough questions then need to be addressed. We can write down a simple recipe for the GHE, but it is indeed challenging to reconcile a presence of a negative feedback with our observations, or explain the current observed global warming in any other terms.

446 Responses to “A simple recipe for GHE”

  1. 101
    ccpo says:

    Re: #2 or #3

    This is actually a big project. Teachers know not to produce something that tries to be all things to all people. Decide on content. Lay it out in the clearest way you can at your level, then take that and translate it for age/target. Have four or five of the same presentation, all at different levels.

    Likely best to recruit people to do the various levels who have the “knack” for that level. I realize that is sort of what you are doing, but it’s a bit haphazard. Define the audience for *this* iteration, design from there.

    Just a thought. Probably already thought.


  2. 102
    Martin Vermeer says:

    And there is no vacuum, there is only a lower concentration of matter. That is why the void of space has a temperature of 2 to 4 K.

    Where I come from, we study the subject first before shooting off… the vacuum of space “has” a temperature of 2.7 degrees because it is filled with black-body radiation at that temperature. Any matter present may equilibrate with that.

  3. 103
    Gilles says:

    “The records study that I was a part of demonstrates, statistically, that deadly heat waves during the summer are becoming increasingly more likely.”

    I think that the main issue for the public is : WTH does this have to do with the extinction of human race that is supposed to happen above some threshold ? “deadly heat waves” (in France at least, in 2003) has only abridged the life of weak and old people by some months – it has been almost exactly compensated by a decrease of mortality the year after. To my knowledge, the overall impact on the French population after some years has been statistically zero. And of course there have been many heat waves in the past.

    So the main issue for me is that all “serious” studies show only “statistical trends” having some effects on some measurable quantities , (slight increase of average temperature , slight increase of sea level , slight decrease of northern , but not southern , sea ice, ..), but actually none of this would have been noticed by average people in their all day life, if these studies wouldn’t have been done. If we wouldn’t we have modern satellites , network of thermometers, and so on, and we had asked people how their life has changed for 30 years due to climate, the general answer would probably have been :” climate? eeeeh? what are you talking about ?”

    The divorce between scientists and population starts here : scientist congratulate themselves for having found a statistically significant difference in 30 years data, but they have almost no influence on the all day life of most people.

  4. 104
    John Monro says:

    Gavin, thanks for the effort. You have distilled a good deal of difficult to understand information into a coherent whole, and as someone already pondered, this moderately intelligent physician has no major difficulty grasping most of it, but it does take quite a bit of mental effort to do so, and to follow your arguments. But here I must observe a contrary difficulty – one of the most vociferous, extreme and dismissive to the point of rudeness global warming deniers that I have ever met is a medial colleague of mine here in Wellington, he’s obviously not as intelligent as I am ;-), but he is a qualified doctor, and a very good one too. So our attitudes to global warming, GHE or whatever you call it, are not predicated by intelligence of the capacity to understand, but our capacity to refuse to do so and to rationalise this refusal. It’s difficult to know what motivates this man, a mature man, much of my age, married with a family, to so manifestly deny the patently obvious, which he does exactly in the same way as all the other contrarians, using the same spurious arguments, the same fallacies and the same plainly wrong information as all the other contrarians use ad nauseam -the sort of arguments that he would see through in an instance if advanced by homeopathy practitioners or iridologists.

    I think you are dealing in GHE with something that is immune to logic or intelligence or reason. Try telling a German before the Second World War for instance that following Adolf Hitler would lead to ruin, how many would have believed you? Or take your own country before your dreadful Civil War, how could so many in the southern states be so destructively stupid? The only thing that would change any of these people’s minds is their personal experience of the need to do so which, of course, is rather too late.

    I think it is the same with global warming. I don’t think it matters how “simply” or intelligently you present the science or the reasoning, or even the ethics, until the majority of people are inconvenienced or damaged by what is happening, they will not change their life-styles for any non-immediate threat, especially when they can rationalise their actions on the basis of the arguments around global warming, or in the case of those that do have a capacity to understand, their choice not to do so.

    As someone very concerned indeed about global warming, all I can do is fervently hope that there isn’t quite enough oil, or gas, or coal, or methane available to mankind to burn in sufficient quantities to make this planet entirely uninhabitable; that the high emissions scenario of the IPCC is itself fundamentally flawed because there aren’t enough fossil fuels available to power it, or that getting them productive would be so prohibitively expensive, or not producing sufficient energy out for energy invested, to make it worth while. Or that the oil leak in the Gulf of Mexico gets a lot worse over the next ten years, and so pollutes a vast area of the American continent that this change of mind is forced by other realities. Another worthwhile hope would be that continuing our dependence on the diminishing fossil fuels resources will produce such a profound and long lasting economic depression, the problem won’t arise.

    I’m sorry to be such a party-pooper, but I have on my side of the argument about 10,000 years of so-called civilised human history to back me up.

  5. 105
    jyyh says:

    Thank you, good to see the same explanation that was in my studies in university (on Environmental Protection Studies 1st year), but of course this course didn’t go to the details of Stefan-Boltzmann, optical depth measurements or Plancks law, these were explained in more detail in the spectroscopy courses, I took for the chemistry studies.

    One might also try to explain GHE thus: One builds a house, with an unheated greenhouse all around it, this doesn’t cover the roof of the house (this is for eliminating the direct heat transfer by gas movements, so the greenhouse mainly gets radiative heat transfer only). What happens to the temperature difference between the greenhouse and the house if one changes simple glazing to double glazing in the house? This situation is somewhat analogous to Stratosphere-Troposphere relation to me, the glasses have some optical depth (as does any material by spectroscopic principles, though for helium this is somewhat high).

  6. 106
    Pierre Allemand says:

    Your formula should be more exact if you took emissivity in account. This correction is of the same order of magnitude than GHG effect (a few percent).

    Also, why not to simply say that : Unlike major gases of the atmosphere, GHG (mainly water vapor) are heated by IR radiations and cool by IR emission and/or by convection. (This is never said in simple terms, physicians talk always about rotation and vibration of molecules and photons emission which is not very clear for most people).
    Convection acts so that temperature of surrounding gases and temperature of GHG become equal, while IR emission tends to decrease temperature of the whole system. But convection is less and less efficient when pressure decreases, while cooling by radiation is more and more efficient. So the amount of energy lost to space by radiation is proportionally more and more important when pressure decreases (at higher altitude). That explains why temperature is lower and lower with altitude.
    That is the (always simple) view of a thermic engineer who prefers radiation to photon emission and temperature to vibration and rotation of molecules.

  7. 107
    The Ville says:

    Spencer@63 wrote:

    For a super simple explanation to a general audience, I quote what John Tyndall wrote back in 1862:
    “As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial [infrared] rays, produces a local heightening of the temperature at the Earth’s surface.”

    This is essentially a 100% correct analogy.

    You highlight the problem by stating it is an analogy.
    Analogies, although useful, don’t explain the science. They are also open to ridicule by those opposing the science.

    I think it is important not to be afraid of explaining science as best as we can in all it’s glorious detail. However I do think scientists need help with good quality media producers and creators. Sadly I think sometimes media people shy away from the detail and do not know how to tell a story about the science (even though that is supposed to be their speciality!).
    I think sometimes this is due to their own misunderstandings of the science.

    Every so often though, the right scientist(s) and the right artist(s) or the right interpreter(s) of science manage to get together and do a good job.

  8. 108

    A great overview of the background of the GHE workings; thank you!

  9. 109

    “Optical depth” is greater with more GHGs. The definition is:

    tau = k rho ds

    where k is the extinction coefficient, rho the density of the absorber or scatterer, ds the distance. Clearly greater rho of the GHG means GREATER optical depth.

    If you mean “reduced distance at which tau = 1,” you’ll need a new term.

  10. 110

    I think I have to side with the folks that say this post doesn’t really work–and I hate to say that about ANY RealClimate post. But it is too complicated for anyone who isn’t interested in learning about science in the first place. The target audience should be those who

    A) do NOT have any particular interest in learning more science, but

    B) want a clear explanation of how the greenhouse effect works. Just enough and no more.

    I’d go with something like this:

    * * *

    Radiation physics is very well understood. We know, from how far Earth is from the sun, and how much light it reflects, that it should be a lot colder than it is. The French physicist Jean-Baptiste Fourier first noticed this in 1824.

    The greenhouse effect keeps Earth warm enough to be habitable. Some gases in Earth’s atmosphere–mostly water vapor and carbon dioxide–let sunlight pass through mostly unhindered, but absorb infrared light from the ground. That heats up the atmosphere, and the warmer atmosphere warms the Earth, as well. And the more of these “greenhouse gases” or “GHGs” in the atmosphere, the warmer Earth’s surface gets.

    With a very few exceptions, every material object above absolute zero radiates! Really hot things, like the sun, radiate visible light. Things at room temperature, like the Earth’s surface, and you and me (!), radiate less powerful infrared light. And gases are material objects! When GHGs absorb infrared light from the Earth, they warm up–and that means they radiate more. And some of that radiation goes right back down to the Earth. So Earth has both sunshine and “atmosphere shine” warming it. The more GHGs, the more “atmosphere shine.”

    The energy isn’t coming out of nowhere. It all comes from the sun, but it stays in the system longer, and gets distributed differently, the more GHGs you have in the atmosphere. We’ve been adding GHGs by burning fossil fuels, and as a result, the Earth has gotten warmer. And that could be a big problem. Our agriculture and industry are exquisitely adapted to the relatively stable temperatures we’ve had for the past few thousand years. Raise the Earth’s average temperature just a degree or two, and you can move agricultural growing belts hundreds of miles! And it moves the rain. We could wind up completely crashing our agriculture. No food.

    Sea-level rise is a long-term danger of global warming. But drought is one we’re already having trouble with–ask the Australians! Without hurting our standard of living significantly, we can get all the power we need from other sources–solar, wind, geothermal, biomass. But if we just let things go on as they are, burning more and more oil and coal each year, we’re going to be in serious trouble.

  11. 111

    I’ve revised the explanation I posted to make it simpler still, without making it stupid. It’s now posted at my web site:

  12. 112
    idlex says:


    I’ve been using the solar flux and planck’s law and ground heat conductivity and capacitance to build a (very simple) ‘climate’ simulation model that showed the energy exchanges over a day. I began with no atmosphere, and now I want to add one.

    I started off thinking that maybe I could treat the atmosphere like I was treating the surface of the earth, as a black body which had an emissivity and temperature and heat capacity. I looked around for some equations to tell me how much solar energy got absorbed by the atmosphere, and how much of the IR emitted by the surface of the earth got absorbed in the atmosphere. I’ve not managed to find these equations yet.

    But I have a new problem. From what I’m reading, CO2 absorbs photons of a particular energy, and no other. And then it re-emits photons of the same energy in some random direction. While a CO2 molecule is ‘holding’ a photon, it remains in an excited state (variously described as electron orbit shifts or inter-atomic bond flexings). It’s not at all clear to me that this excited state corresponds to an increase in temperature. It seems much more like photons are captured by CO2 and re-emitted (much like footballs are captured by soccer players and then kicked away to be captured by some other players).

    So which is it? Do photons from the surface of the earth heat up the CO2 molecules that absorb them (where heating up would mean making them move faster), and transmit this heat to other air molecules by collision. Or is it that photons are captured by CO2 molecules, and the captured energy stored internally in the molecule (like energy is stored in a spring), until released as a photon of the same energy?

  13. 113
    Edward Greisch says:

    72 Bob Doppelt: You added a “)” to a URL so it won’t open. Thanks for doing research on communication for GW. It is sorely needed.

  14. 114
    jyyh says:

    BPL, yes I was thinking of that, strike over the last sentence concerning Helium) in my prev post and change accordingly (that’s for not tackling optics in the university).

  15. 115

    Some nice formulations in the comments here–Jim Harrison back at #16, Chris Colose on feedbacks (#98, I think), Pierre Allemand (#105) and Barton just now at #109, as well as others. Thanks to all–very useful!–and of course to rasmus, too, whose essay succeeds IMO, though not at the simplest possible level. (The trouble, I think, is that “no simpler than necessary” necessarily invokes subjective value judgments, some of which are illuminated by the various alternate formulations such as those that I listed.)

  16. 116


    I don’t see why RC shouldn’t also cater to those who do have a particular interest in learning more science.

    It will indeed be more complicated to cater to different audiences with different posts, as some audiences will like one post and not another. The alternative, to only cater to the not-so-interested-in-learning-science, would be a missed opportunity.

    One might even say that those who are interested in the science are a very important target audience, as they could be very important in shaping other people’s opinions in their personal network. As Michael Tobis has noted ( ), currently a large fraction of this group (scientifically literature layfolk) are taken in by the likes of McIntyre. We should perhaps do more, not less, effort to educate them with the basics so that they’re not as easily swayed by the scientifically-sounding-but-in-the-end-not-so-scientifically-sound “skeptics”.

    I wrote down some thoughts about this group of skeptics (“citizen scientists”) on my blog here:

  17. 117

    That should be “scientifically *literate* layfolk” of course…

  18. 118
    The Ville says:

    Can I ask a really basic question about the vibration bending mode of CO2 and it’s energy levels?

    Am I right in saying that CO2 has 4 energy levels that correspond to IR radiation absorption?

    Does this mean it can absorb up to 4 IR frequency photons (hence causing vibration), after which it effectively becomes IR transparent?

    eg. under intense IR radiation CO2 will effective ‘fill up’ and become saturated, unable to absorb any more until it has emitted some IR photons?

    Answers from commentators or blogging scientists would be apprciated.

  19. 119
    Mike Donald says:

    Eli is too modest to mention his own helpful contribution. Here’s his simplest explanation FYI.

  20. 120

    For those complaining that the article is too mathematical etc.: remember, there is a large group of scientists out there who need to be reassured that the basic science is correct. Many of these people are in other fields, retired, etc., and may not have the time, resources or inclination to read a text book or do a detailed literature survey. Material like this is great for that group, as well as for science journalists who know enough to read a few equations.

    Others whose skill is more in communication to the nonscientist can take it from here, but I found this a useful summary (PhD computer science, working with biologists, involved in Green politics – not illiterate but by no means an expert). Thanks very much for the time and effort that went into this.

    One point I really like to emphasise when talking about the science is that we are not talking about a theory of AGW, but a theory of climate, which predicts AGW. This is important because it emphasises that AGW is something that is not a peculiar discovery that can be knocked down by attacking a few trivia.

  21. 121

    Gilles #102: many scientists do influence everyday life of ordinary people, including biologists, who work to a much lower standard of evidence than you advocate.

  22. 122
    bestquest says:

    I think you have missed the mark as far as understanding why John Q. Public does not worry about global warming. The reason he doesn’t need to worry is explained here:

  23. 123
    Ray Ladbury says:

    idlex, one thing you are missing is that most CO2 molecules relax not by emitting a photon by by colliding with, say a Nitrogen, molecule and imparting the extra energy to that molecule. You can look at this in terms of equipartition. The IR flux from the warmer surface excites much of the CO2–much more than would be excited at thermal equilibrium at the temperature of the atmospheric layer where the photon is absorbed. To move toward equilibrium, the CO2 then has to impart energy to the surrounding gas. Make sense?

  24. 124
    Veidicar Decarian says:

    A nice, simplified explanation suitable for a high school graduate, or perhaps a grade or two less.

    Of course, the willfully ignorant stopped reading after the first sentence.

    Do you expect to improve things when you appeal to a minute minority between those who are smart enough to understand the science and who are already convinced, and those who will remain willfully ignorant to the grave?

    Your chosen method might make some progress over the next 30 to 60 years, as it has with smoking.

    I think progress is needed at a slightly faster rate.

    Don’t you?

    Where is the AGU or the APU review of the scientific merrit of the Trash coming from Inhofe’s office of disoonfirmation?

    Rapidity requires breaking some heads.

    Polite explanations are just more of the same failure that has been tried before.

  25. 125
    Chris Dudley says:

    Further to my #92:

    In the section ‘Enhanced greenhouse effect’ The first sentence is incorrect. ‘reduced optical depth’ should read ‘increased optical depth.’

    The sentence starting ‘The effect of heightened level of heat loss’ is awkward since it is not a matter of increased heat loss but rather a changed altitude where heat loss mainly occurs. ‘The effect of heat loss from a higher altitude’ might be better.

    The sentence starting ‘Hence reduced optical depth explains’ is both incorrect and somewhat incongruent with the next link. Something like ‘Hence increased optical depth, particularly from a well mixed gas like carbon dioxide which affects the altitude of heat loss, explains’ might get the idea across better.

    In the section: ‘So why is the upper atmosphere cooled then?’ I think either mention ozone destruction as a current cause or say the subject is complicated and link here:

    Hope that helps.

  26. 126
    Didactylos says:


    Nice troll. Sorry, nobody is falling for it. Bye bye.

  27. 127
    MarkLPolska says:

    Great post! As someone who has a long unused undergraduate degree in chemistry, the level of math and physics in this post was near perfect for me. I can understand the complaints by some that the post was too complex, but I appreciate your efforts to provide information at different levels. Keep up the good work.

  28. 128
    Rod B says:

    Martin Vermeer, radiation has what has been defined by convention of convenience as a “characteristic” temperature. It is not a real temperature in the common physics accepted meaning. Ray’s answer was more apt.

  29. 129
    idlex says:

    @Ray Ladbury

    I understand what you mean, but you’re the first person I’ve encountered who’s said that. Got a link?

  30. 130
    t_p_hamilton says:

    idlex asked:”Am I right in saying that CO2 has 4 energy levels that correspond to IR radiation absorption?

    Does this mean it can absorb up to 4 IR frequency photons (hence causing vibration), after which it effectively becomes IR transparent?”

    Ray’s answer was about why multiphoton absorption is not important, however, I will answer your particular question about your understanding. Multiple photon absorption is certainly possible for each vibration. In IR spectroscopy there are “hot bands” (where the first excitation is presumed via thermal) for quantum number v=1 to v=2. This frequency is almost the same as the fundamental absorption (v=0 to v=1), so the hot band peak may be difficult to see within the fundamental band.

    Electronic absorption has a tendency to “bleach out” like you describe.

  31. 131
    mircea says:

    Thank you for the answers! I think I understand, but this article is sign that I need to go back relearn the basics.

  32. 132
    Gilles says:

    Ray : “The IR flux from the warmer surface excites much of the CO2–much more than would be excited at thermal equilibrium at the temperature of the atmospheric layer where the photon is absorbed.”

    actually if the medium is optically thick , the IR flux has a characteristic radiation temperature from the location where it was emitted, that is around one mean free path away. It may be not “much more” if the optical depth is high enough (the order of magnitude is ∆T = l.grad T = grad T .h/tau ) ; it can be a small difference for high tau , but it insures the gradual transfer of heat from one layer to another one. That’s the essence of diffusion approximation.
    The fact that the energy of a photon is most often transferred by collisions to other molecules does not really matter, since it means that collisions can also excite molecules that will sometime emit a photon – both process cancel exactly in LTE. In thermal equilibrium, there is no net “heating” in the sense that the atmosphere would gain temperature, the temperature is steady on average everywhere. This is really a transport process, heat flows throughout the atmosphere but without temperature variation. Locally, absorption and emission do cancel exactly : only the DIRECTION of photons is slightly anisotropic : a little bit more photons come from the lower, hotter layers and a little bit less from the upper, colder ones. They are reemitted isotropically, so the budget is slightly positive outwards and negative inwards , but vanishes when integrated on all directions. The net result is a transport outwards.

  33. 133
    Matthew L says:

    #110 BPL
    Excellent clear post and pitched just right for the general public such as me (although I would politely suggest fewer exclamation marks! They come over a touch patronising).

    I am with you all the way until you get to this bit:
    But drought is one we’re already having trouble with–ask the Australians!

    Actually, over the last 110 years average annual rainfall totals have (apparently) been on an upward trend. Variability over the last 40 years has been more pronounced (though that may have more to do with more accurate and extensive measurement).

    I can easily find this graph:
    and there are others on the net showing the same thing, although the Australian Met Office rather unhelpfully does not have one.

    The problem for Australia is less to do with climate change driven drought and more to do with water shortage. Easily confused but very different cause.

    They have a big problem of chronic over-use of scarce water resources, both for farming and rapidly expanding cities. This means that when of their perfectly normal occasional droughts hits they are much more vulnerable to its effects.

    We must be very careful not to repeat easily falsifiable climate change myths like this as that simply plays into the hands of the likes of WUWT and their ilk.

    Keep up the good work, your site are an invaluable source of good data.

  34. 134
    Ray Ladbury says:

    Gilles says “…both process cancel exactly in LTE”

    Yup, but by definition as we add greenhouse gasses, we depart from equilibrium, so the processes do not cancel and there is a net flow of energy from radiative to kinetic.

  35. 135
    Ray Ladbury says:

    idlex, the gist of the argument is here.

    Basically, it comes down to the long lifetime of CO2 in its excited state–much longer than the collision time.

  36. 136
    Matthew L says:

    Found one from the Aussie Govt clearly showing the upward rainfall trend.

    If that link does not work try this tinyurl:

  37. 137
    Martin Vermeer says:

    Rod B #128, don’t try to confuse the issue. You know damn well what I meant. And so, I hope, do the other readers.

  38. 138
    john aislabie says:

    A “scientific fact” thrown around is that at current levels CO2 is already at 97% of its absorption limit and therefore further CO2 makes little more difference. The analogy of putting on a second ski hat is offered.
    I do not understand (and maybe they don’t!)where this plays in the science and whether or not it has any substance as an argument. Apologies if this is too basic to merit comment.

    [Response: Not facile at all. Indeed, that you suspected that the ‘97%’ (or sometime ‘98%’) number is bogus is to your credit. The fact is that you can keep increasing CO2 forever and it will not saturate in this sense (see Venus for an example). CO2’s effectiveness per ppm does decrease as you go to higher concentrations (which is why we discuss the sensitivity to 2xCO2 rather than per 100 ppm for instance), but this is very well understood and has been incorporated into the models from the beginning. – gavin]

  39. 139
    A. Ros. says:

    Mr. Rasmus,

    I found this entry on the following blog. A study ( ) Published in the Proceedings of the National Academy of Sciences confirms what people who have been paying attention already know:
    97–98% of the climate researchers most actively publishing in the field support the tenets of ACC outlined by the Intergovernmental Panel on Climate Change, and (ii) the relative climate expertise and scientific prominence of the researchers unconvinced of ACC are substantially below that of the convinced researchers.

    Furthermore, researchers with fewer than 20 climate publications comprise ≈80% the UE group, as opposed to less than 10% of the CE group. This indicates that the bulk of UE researchers on the most prominent multisignatory statements about climate change have not published extensively in the peer-reviewed climate literature.

    If these claims are true…does that not suggest that restraint & caution are indeed needed in light of the Fact that reformating the economies of both the developed and developing world, are not without their own set of dire & destabilizing consequences?

  40. 140
    Snapple says:

    I can’t understand climate science. I never took Physics, and I am a senior citizen. Still, I try to keep up on new developments in science.
    I’ve voted Republican for many years, but that may change.

    I know that scientists are a lot smarter than I am and that they aren’t communists plotting against me. So-called “conservatives” who are saying this do not speak for me.

    The Guardian seems to be pretty sure that much of the blame for this propaganda is coming from the U.S. Maybe we have met the enemy and he is (the)US.

    Still, I never hear you all discuss the Russian oil/gas politics. Do you really think the Russian monopolies or the Russian government won’t mount the sort of political operations that the American corporations seem capable of? I read the American scientists complain about the “right wing,” in the Guardian, but they don’t seem to realize that the “right wing” is saying exactly the same thing as the Russian media. They even cite some of the same scientific “experts.”

    At first, the Russian media bragged about the Russian hackers. They stopped bragging when the FSB (domestic security) denied it was involved. But maybe the SVR (foreign intelligence service) mounted this from the US.

    It is no accident that Pravda and Russia Today really played up Climategate in both their Russian and English-language media. People from the CATO Institute appeared on Russia Today in English. Pravda is quoting FOX. Media in Russia is often owned by the oil/gas industry and the political line is controlled by Russia’s ruling Unity Party. Gazprom is half owned by the Russian government and pays the government’s bills. President Medvedev is the former Chairman of the Board of Gazprom. They have a lot of power. So maybe talk about Gazprom when you talk about Exxon.

    Why don’t you discuss the scientific fallacies in some Pravda articles or Russia Today programs?

    Historically, some anti-scientist campaigns have come from Russia, tho’ America has its share among religious fundamentalists and conspiracists.

    The Guardian says the hacking happened from the East Coast of N. America, but that doesn’t mean an American individual or entity did this. There are tens of thousands of computer experts in America who are not Americans. Eleven of them seem to have been Russian agents. So far, the FBI only told the court enough to arrest them for probable cause. That’s right in the complaint.

    Russian operatives (usually the military intelligence) steal secrets, but the KGB and its successors also run influence operations.

    The very shrill tone of this Climategate hysteria sounds exactly like the short-lived Stalinist KGB Doctors’ Plot (Jewish doctors were supposedly plotting to exterminate the Soviet leaders) or the KGB lie that crafty Pentagon scientists cooked up AIDS as an instrument of genocide.

    In both those cases, the KGB finally admitted there was no substance to the conspiracy theories and threw their collaborators under the bus. Even the Russians have to face reality and solve problems.

    Just my personal view.

  41. 141
    Jim Harrison says:

    Scientists disastrously overestimate their ability to communicate their findings to non-scientists. Their idea of what a simple but adequate explanation of something would be is far beyond what most adults can understand. I recommend the following experiment: assemble a group of well-educated non-scientists and give them a half hour lecture on any real scientific topic you choose. Write down an estimate of how much of your message was communicated and then share this prediction with your audience. The results will obviously vary, but the typically outcome will be that the scientist will be afraid that he or she was guilty of patronizing the listeners by oversimplifying too much while the lay audience will express utter bewilderment. So much of what technical people take as obvious is news to the general public.

    In the (very good) explanation of the effect of greenhouse gases that begun this thread, we read that “Molecules composed of three or more atoms tend to act as greenhouse gases because they can possess energy in terms of rotation and vibrations which can be associated with the energy of photons at the infra-red range.” Unfortunately, the concept that particular molecules emit radiation at wavelengths dependent on specific modes of vibration and rotation is news to people who don’t know how electromagnetic waves are produced (ask ’em!). Of course you can explain about modes of molecular vibration and rotation, indeed you can explain everything eventually, but it not only takes time and many sentences to unpack these concepts, it takes time to comprehend them, years, in fact.

  42. 142
    Snapple says:

    I should add that Russian scientists publically dissociated the Soviet Academy of Sciences from the AIDS propaganda in 1987, long before the KGB’s 1992 admission that it had spread this lie.

    The Russian scientists stood up to the KGB, so this is really your fight.

    I support you and believe you because I can see enough of the other side’s TRICKS, but you can’t really expect ordinary people to see through all the sophistry.

    I think scientists are doing pretty well, but keep it up.

    If it’s the Russians, they will probably come clean long before Monckton or Morano do. After all, they have to run their country and they can’t look like total idiots to their own scientists.

  43. 143
    Ray Ladbury says:

    @139 Huh? How does you suggestion of proceeding cautiously follow from the findings you quote. The findings indicate that the more an expert understands the evidence, the more he/she finds it convincing that we are warming the climate and that this is an issue. Maybe you want to revisit that post or just chalk up an “own goal”.

  44. 144
    RichardC says:

    Gavin said, “Not everything needs to be written for everyone to understand.”

    But this is an attempt to reach those who don’t know or care what the Stephen-Bosnian whatever constant is. At best, math stopped with Algebra2, and that was a decade ago.

    One of the best lines in the post was, “Energy is also transferred through vertical motion (convection), evaporation, and condensation too (latent heat), but that doesn’t affect this picture, as they all act to restore the vertical structure toward the hydrostatically stable lapse rate in the long run.” Add a few sentences of explanation and you’ve got a grand answer to a core skeptic argument. Add answers for each of the Skeptics claims and you’ll end up with a stellar first primer. Provide a link to this post for those who care about Stefan-Boltzman and everyone’s happy.

  45. 145
    Chris Colose says:

    I have written a brief response to the comments here focusing on communication style and level of technicality of Rasmus’ effort.

    Rasmus and all are welcome to offer opinions…

  46. 146
    Jen Schneider says:


    I suppose I agree with the comment above that your explanation here is too complex for many segments of the population, though if you were in fact going for a particular segment (the educated? Doctors? Amateur scientists?) maybe it’s okay.

    The problem I see (as a communications scholar and a social scientist) is two-fold. One, you start out pretty slippery in terms of who you are aiming at as your audience, and why you’re aiming at them in particular. Having a clear sense of Audience and Purpose are key elements of basic communication.

    But I think the much bigger problem is that there is no “basic communication” when it comes to climate science. I don’t think we can assume that if people/the public/laypersons (whatever audience you are defining) have the same information as us (the “educated,” the scientists) they will agree with us. There’s loads of social science out there indicating that things like values are more important predictors of viewpoint and policy attitudes than scientific knowledge.

    I’m not saying science education isn’t important, of course. But having a more informed public shouldn’t have to be the prerequisite for democratic debate or science policymaking. If scientists wait for the public to be as well-informed as this post supposes in order to move forward with the science policy scientists want, they’ll be waiting for a long time.

    I guess I would argue for improved forms of public dialogue and participation as much for improved science literacy (see for an example). But engagement is a lot more work intensive than just saying the public needs to understand better.

    I sound overly harsh, and don’t mean to. I think you do great work. I hope I’m being constructive and not overly critical.


  47. 147
    Geoff Wexler says:

    Question: Is pressure broadening really secondary?

    In other words if you ran a model at zero pressure (as far as the spectra are concerned) would it really make little difference?
    Are we really being invited to suggest improvements? It would be an interesting example of collective teaching. I’m not surprised at the number of critical comments given the potential size of the team.

    I support the person who wanted this deleted:

    (unless it involves nuclear reactions or takes place on quantum physics scales).

    Why attract criticism?

    At first glance I thought of the following ignorable suggestions:

    It might perhaps be easier for beginners if it was preceded by a qualitative discussion/summary of the energy balance of the whole globe rather than jumping straight into the distinct issues of tropospheric warming and upper atmospheric cooling.

    E.g. “it all boils down to the fact that cool bodies emit less heat than hot ones and as we have seen (shall see) the radiation escaping from a greenhouse planet tends to come from higher in the atmosphere where it is cooler”; if less energy escapes the globe as whole will warm (on average)until…..

    The next bit might come as a shock to those used to seeing the over-simplified discussions. It might be worth softening this by some additional comments such as:

    (a) the vertical distribution of the enhanced effect is important to understand because observing it can be used as a finger-print.
    (b) The greenhouse effect involves two mechanisms, absorption (normally emphasised) and emission (occasionally omitted from the discussion although vital to the physics) .
    (c) A possible mechanism for cooling which can be understood easily is when, for some reason, the first of these is unimportant for the energy balance of a small volume of gas. For example if the temperature is being determined by a short wave heater from above rather than a long wave heater from below. In that case we might as well start by ignoring the latter altogether. The cooling follows.

    I know that you can’t cover everything, but most of the discussions you see in the blogs rapidly move on to feedbacks, so I suggest that this section might be extended and deepened a bit. Otherwise you may be confronted early on by Lindzen’s disciples. After all the positive feedbackfrom water vapour was in the theory from its inception and is now more secure than before.

  48. 148
    Patrick 027 says:

    Re 97Ron DeWitt Do I understand correctly that in the way engineers and such use the term, the feedback can always be expected to be negative, and people’s fears that a catastrophic response to CO2 will destroy life on the planet are misplaced?

    1. The feedback can become zero – or to avoid confusion regarding what is and is not a feedback – the equilibrium climate sensitivity can become infinite (or negative) in some conditions. That is refered to (in climate terms) as a runaway feedback. This isn’t expected to occur for present Earth-like conditions or a range of conditions encompassing that.

    Also, when runaway occurs, it occurs over some range until it reaches a limit and then stops. For example, if the Earth got cold enough, the encroachment of snow and ice toward low latitudes (where they have more sunlight to reflect per unit area), depending on the meridional temperature gradient, could become a runaway feedback – any little forcing that causes some cooling will cause an expansion of snow and ice toward lower latitudes sufficient to cause so much cooling that the process never reaches a new equilibrium – until the snow and ice reach the equator from both sides, at which point there is no more area for snow and ice to expand into. (There are equilibrium climates between the points where the runaway starts and where it ends, but they are unstable equilibria, and the equilibrium coverage of snow/ice increases with forcing that would cause warming.) Once the ice reaches the equator, the equilibrium climate is significantly colder than what would initiate melting at the equator, but if CO2 from geologic emissions build up (they would, but very slowly – geochemical processes provide a negative feedback by changing atmospheric CO2 in response to climate changes, but this is generally very slow, and thus cannot prevent faster changes from faster external forcings) enough, it can initiate melting – what happens then is a runaway in the opposite direction (until the ice is completely gone – the extreme warmth and CO2 amount at that point, combined with left-over glacial debris available for chemical weathering, will draw CO2 out of the atmosphere, possibly allowing some ice to return). The combination of such freezing and thawing is an example of hysteresis, where the equilibrium climate for a given ‘external’ forcing is different depending on prior history.

    2. It is possible to have runaway effects over shorter ranges. On the very small scale, one could have a runaway between whether or not a weather pattern has a thunderstorm at a specific time and place or whether it is dry and sunny at that specific time and place – but that’s not the same as a change in climate (see internal variability, chaos, butterfly effect). It’s possible to imagine a scenario where, once we hit x degrees of warming, we hit a runaway process that jolts us some additional y degrees of warming, and then stops. I’m not saying that’s expected though.

    3. The approx. 3 K expected warming for a doubling of CO2 or equivalent forcing doesn’t include some longer-term processes and also doesn’t include feedback via CO2 or CH4 themselves. The sensitivity of the climate system to human activity (ie how much fossil C we emit, etc.) can be a bit different – especially in the more distant future trajectory. Charney sensitivity refers to the climate sensitivity when fast-reacting feedbacks (Planck response is a given – also, water vapor, clouds, … I think sea ice, seasonal snow) occur but with other things (land-based ice sheets, … vegetation(?)) held constant. Charney sensitivity can be expressed for such forcings as CO2 changes; longer term processes involve CO2 as a feedback (ice ages).

    4. Depends on what you mean by ‘destroy’. From either human activity or likely natural forcings out to some hundreds of millions of years, extinction of all life is quite unlikely (so far as I know) – needless to say anhilation of the Planet Vogon-or-Death-Star style is even less likely still. A mass extinction, however, is possible, though I’m not sure if it’s expected for BAU AGW. A smaller extinction event might be more likely(?). (Of course, other anthropogenic pressures besides climate change (and ocean acification) are/will be contributing to extinctions.) However, it needn’t take a significant extinction to make things unpleasant. If a wild species survives but in low numbers, that might be a loss of food or some other ecosystem service for us humans. Plus, there’s the direct effect of climate change (and associated sea level rise) on agriculture and infrastructure and living. So destruction of wealth (conventional and otherwise) and health is a very real concern.

  49. 149
    Iskandar says:

    It does seem that many readers are not aware of the molecular processes that play around in a “vacuum”. As Gavin showed, even when assuming a ridiculously low effect, it still boils down to a yearly 800 mm of ocean evaporation. What Gavin, the joker, did not mention was that most of the evaporated water came back to earth by precipitation. In the mean time, it carried enough energy into the upper troposphere where it could be emitted into space. As for molecular cooling, when a hot gas comes into contact with a very dilute, very cold gas, energy is exchanged with every collision, cooling the hot gas down to slightly above the temperature of the surrounding diluted gas, thus tranferring lots of energy to the upper layers of the atmosphere, as Gavin calculated. From these layers it will be emitted to the background temperature of space. Concluding: not only radiative process contribute to the transfer of energy from the surface to outer space.

    [Response: The numbers above aren’t quite right – I’ve corrected them though. The answer is still that there is no climatically important contribution of latent heat flux to the flux to outer space. You are getting confused between fluxes that are important at the surface and fluxes that are important at the Top of the Atmopshere (TOA). There are multiple non-radiative energy fluxes at the surface (latent and sensible heat fluxes predominantly) which obviously affect the atmospheric temperature profiles, but when it comes to outer paces, that flux is purely radiative. – gavin]

  50. 150
    Robert Means says:

    Section ii states: “The planetary energy balance says that our planet loses heat at the same rate as it receives energy from the sun (otherwise it would heat or cool over time). This is because energy cannot just be created or destroyed (unless it involves nuclear reactions or takes place on quantum physics scales).” This appears to state that the energy content of the entire Earth system remains constant, so that energy gains for the troposphere/land surface/ocean are precisely offset by energy loss for the stratosphere. This obviously is true at equilibrium, but the quotation appears to state that it is true continuously, that is, there are no transient imbalances. Do I correctly understand the point?