RealClimate logo

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. 151
    Frank Giger says:

    One of the largest problems with keeping concern up for AGW within the public is the policy that goes with it, not the science itself.

    The solution domestically seems to consist of A) raising taxes (such as on gasoline and electricity) and B) inventing new taxes (carbon taxes).

    The solution globally seems to pretty much consist of handing over large sums of money to “developing” countries, with the USA tagged to pay most of it.

    It looks to the average Joe to just be another excuse for a money grab out of his pocket. I don’t know if y’all are keeping up with the economic news, but folks aren’t real excited about having to pay more for things or have the government just hand over huge amounts of cash to some of the most corrupt nations on the planet under the guise of “saving the planet.”

  2. 152
    Iskandar says:

    Dear Gavin,

    I am not getting confused, you are. In a “vacuum” a “hot ” molecule can travel a long way, losing kinetic energy due to increasing gravitional energy. It will collide with an other molecule, exchanging energy, thus cooling down. Both can and will emit IR radiation when in in excited state, going down in temperature in the process. It is exactly this process which has made sure that Earth still has an atmosphere.

  3. 153
    Zach says:

    That is far too complicated for consumption by the general public. Right up front you define photons as electromagnetic radiation going above most folks’ heads. Conduction? Is that like convection? Etc.

    I’ve thought about this a lot, and I think the best initial strategy is to simply lay out how unprecedented the current situation is:
    1. Temperature as high as it’s been since the discovery of fire.
    2. CO2 as high as it’s been since the continents separated.
    3. This has all happened in the years since we began extracting and burning fossil fuels.
    4. Natural change cannot account for how fast the planet is changing.
    5. If we do nothing in the next few years, it will either be impossible or much, much more expensive to clean up the atmosphere in the future.
    6. Reversing global warming is possible, but it will require real sacrifice from you and everyone else.

    These are more likely than not to be correct statements, but probably aren’t accurate with a ton of confidence. That doesn’t matter. It’s important to be accurate, but any statement will be assailed regardless of its accuracy.

  4. 154
    Iskandar says:


    Mohaaaa! You calculated the 800 mm sea level decline in stead of the 1.5 mm, not I. It seems my objections are within the error range of your calculations, so why do you dismiss them as being ridiculous?

    [Response: Perfect I am not, and I apologise for the error. There is still no latent heat loss to space (though the margin by which I demonstrated this is substantially less than I first suggested). And note that for this to be as important as the radiative forcing associated with anthropogenic CO2, you’d still need to multiply it by 20. If you think sea level has been falling at between 15 cm a century or 3 meters a century because we are losing water to space, then point to some actual evidence. – gavin]

  5. 155
    Chris Dudley says:

    Further to my #125,

    Gavin especially, do you remember when the sense of the Arctic Oscillation was wrong in this article and I provided the correction? The markup is still present in the discussion of fig. 6 and I believe the corrections propagated into subsequent versions. I am quite sure there is a similar sort of mistake in this article as described in my #125. You need to fix it or else the article remains misleading and confusing.

    I know you get a lot of posts but do please respond to this one.

  6. 156
    Chris Colose says:


    Gavin has corrected you numerous times and judging by your responses it seems only to be an exercise in futility. Your comment #152 is even more confused. The vertical presence of our atmosphere is allowed by a rough balance between gravity and a vertical pressure gradient force, the so-called hydrostatic balance. Here’s one last attempt.

    The Earth is surrounded by the vacuum of space. For a compressible fluid like air, there is no well-defined top such as the ocean surface and so it is possible to find trace molecules even half way out to the moon. But this does not at all matter for the argument, and in fact once you get above the stratosphere or so the air is so thin as to have a negligible impact on planetary energy balance. There is a whole science to uppermost atmosphere physics, where a lot of stuff typically breaks down (like the ideal gas law and Local Thermodynamic Equilibrium which climatologists take for granted) but it’s almost a different field all together with little, if any, influence on surface temperature and energy budget discussions.

    All of the other energy transfer terms (sensible and latent heat fluxes, advection, conduction, etc) require some medium and are not operative as space-to-planet energy transfer terms. If the planet was held up by giant tortoises or in contact with some other object (or in the case of early Earth, when there are significant extraterrestrial impact events) then this would not be the case. Gas giants like Jupiter also have significant internal energy sources although this is negligible on rocky planets with a surface which reduce the interior heat flow to several orders of magnitude less than the solar flux. There is also very small mass flux to the Earth associated with particles from the solar wind, but again, it has no measurable impact on Earth’s surface temperature. It is quite safe to say that ~100% of the energy Earth receives is radiation from the sun. Similarly, radiation is the only way you can get rid of energy going out to space by similar rationale.

    This is not the case with surface-to-air heat exchange(which involves evapo-transpiration, sensible heat flows, and radiation) or even within the troposphere where impacts of latent heating on atmospheric circulations are realized on scales ranging from hundreds of meters to thousands of kilometers. The distinction between the top of atmosphere energy budget and the surface or troposphere energy budget is crucial, and are explicitly considered separate in many texts on global climate, such as in Dennis Hartmann’s “Global Physical Climatology” or in Ray Pierrehumbert’s upcoming text. In fact, it is the TOA energy balance which takes primacy in many climate change discussions (there are some exceptions, such as if you could introduce a moist surface in the Sahara desert, the evaporative cooling would likely be more important regionally than CO2 increase). The general argument however is being discussed by rasmus in the context of planetary energy balance: the impact of additional CO2 is to reduce the outgoing longwave radiation term and force the system to accumulate excess energy; the imbalance is currently on the order of 1.45*(10^22) Joules/year over the globe, and the temperature must rise allowing the outgoing radiation term to increase until it once again matches the absorbed incoming stellar flux.

    In equilibrium, both the top of atmosphere and bottom of atmosphere energy budget must be satisfied of course, but there’s no non-radiative heat flux to space. There is non-radiative heat flux in the atmosphere though and energy can be transported above the level where the greenhouse effect is dominant but eventually must be lost by thermal radiation. This is accounted for going back to simple radiative-convective models half a century ago.

    So no more “Earth loses latent heat to outer space” please? Otherwise I’d challenge you to find a data set that is measuring this latent heat loss to space if you’re so convinced it exists.

  7. 157

    Matthew–how much rain is falling on the periphery of Australia as opposed to the center? Has that changed?

  8. 158

    Snapple 140,

    You raise a salient point. I think the emphasis on the US right wing is due to the fact that most editors and posters on RC are Americans. Different countries have different interest groups.

  9. 159

    FG 151: The solution globally seems to pretty much consist of handing over large sums of money to “developing” countries, with the USA tagged to pay most of it.

    BPL: Where in the world did you get that idea?

  10. 160
    Patrick 027 says:

    Re 150 Robert Means – That could be clarified. If there is any place where more energy goes in than comes out or vice-versa, then energy is accumulating or being depleted, or else there is a conversion to some other form of energy (which must then accumulate or be depleted or go to or come from somewhere else, etc.). For energy in the form of heat, such an imbalance tends to cause a change in temperature. Changes in temperature cause changes in emission of radiation, so that as the temperature changes in response to an energy flow imbalance, the imbalance tends to decay toward zero as equilibrium is approached. Within the troposphere and between that and the surface, convection (and at the surface, conduction and molecular mass diffusion) are also important – these also respond to changes in temperature so that an imbalance causes a temperature change that causes the imbalance to decay.

    Of course, the planet as a whole is losing geothermal (and tidal) energy, but that is a very very small flux that is insignificant to the climates (surface of crust, ocean, atmosphere) of the inner planets. The layers that make up the climate system can come to equilibrium with both the solar heating and the geothermal flux from below by adjusting temperature (and circulation, water vapor and clouds, etc.) until the LW radiation to space is equal to the sum of solar and geothermal (and tidal) heating and the LW and convective/conductive heat fluxes within the system are such that the energy (as well as momentum and mass) fluxes in and out of each part are balanced (for a time period over which a climatic state can be characterized; obviously there will be redistributions of energy, mass, and momentum over the course of a day, a year, or on the time periods of weather patterns and some internal variability).

    (PS the production and consumption of kinetic energy is relatively small compared to heat fluxes, although it is very important to the circulation of the atmosphere (and ocean); kinetic energy is converted from heat by motions that act like heat engines; it can be converted back to heat by the opposite kind of motion, and also by viscosity. …

    (It can also be converted from gravitational potential energy that originates from density variations that are not due to thermal expansion – this is not very important in the atmosphere (clouds, water vapor) but it is important in the ocean (fresher vs saltier). Ultimately, though, such potential energy in the ocean or atmosphere is produced by (differential) heat(ing).)

    … Thus, for the purposes of accounting for heat gains or losses, the propagation of kinetic energy ultimately accomplishes a transport of heat analogous to convection. As with sensible and latent heat, transport of kinetic energy to space is insignificant.)


    Re 153 Zach – some minor corrections:

    2. CO2 as high as it’s been since the continents separated.

    No. But it is considerably higher than it’s ever been before in at least the last several hundred thousand years. (?) Maybe a few million (?). Which is plenty.

    3. This has all happened in the years since we began extracting and burning fossil fuels.

    Production of cement and deforestation also have made contributions, and the later extends farther back in time (And then there’s CH4 from agriculture), though it is true that the changes were small before fossil fuels became a significant source.

  11. 161
    Patrick 027 says:

    Re 151 Frank Giger

    To those, I say:

    Do we not respect property rights, etc.?
    Should we not generally have responsibility for the consequences of our actions?
    Should our moral principles not extend to how we deal with other countries?
    Is change always unfair?

  12. 162

    Titus #77: if you think more powerful computers revealing gaps in knowledge is a problem, you’ll probably want to close down the whole field of biology. The problem there is far more extreme than in climate science, where the basic physics is well known and widely tested. In biology, we don’t have a plausible theory of the overall system yet. Every time we can drill down to more detail and do better computer models, it’s back to the drawing board. I see no sign of that in climate science, rather the basic physics has been confirmed repeatedly.

    Back to the article:

    In other word, why would a negative feedback act for GHGs but not for solar forcing?

    As someone else noted, this should be “other words” – but otherwise this is a great point, one I’ve made several times before. This goes back to the other point I raised earlier: we are talking here about a general theory of planetary climate, not a theory of AGW.

  13. 163
    Ray Ladbury says:

    Frank Giger,
    What would be your proposal to developing countries for foregoing the use of fossil fuels in their development?

    If they agree to limit CO2 emissions to avoid catastrophic warming, should they not expect some assistance in development of a replacement energy infrastructure at the very least?

  14. 164
    Patrick 027 says:

    Re 103 Gilles
    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 ?

    It is very obviously that humans will go extinct beyond some threshold(s), but that isn’t really what we’re dealing with here. We’re dealing with loss of comfort, wealth, well-being, health, and some life, plus the ethics of the unfair distribution (in space and time) (given the distribution of the responsibility of the cause), plus the natural human reactions (war, etc.) – though on that point, I of course recognize that people like the Janjaweed (sp?) must be held responsible for their actions even though a drought was involved.

    “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.

    Implying that the people only lost a year (or less) of their lives from that cause? I agree that premature death at 70 is not the same as premature death at 1 (when keeping in mind that death eventually gets everyone), but it’s still not desirable.

  15. 165
    Patrick 027 says:

    Re 147 Geoff Wexler
    Question: Is pressure broadening really secondary?
    (and temperature broadenning, compositional dependence of pressure broadenning, and temperature-dependence of line strengh)

    We must of course distinguish between the relative importance to a condition in total verse the relative importance to relatively small changes. (ie all these things are important, but would be minor contributors (so far as I know) to climate feedbacks, relative to the Planck response, water vapor, snow, ice, lapse rate, biological stuff, etc.)

  16. 166
    Chris Dudley says:

    Further to my #155,

    You all are posting my comments but without response. One more try, I think this is simple. From the article:

    ‘Hence, a reduced 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 the the estimates from the planetary energy balance model (Figure 3).’

    This paragraph simply does not make sense as written. If the word ‘reduced’ is changed to ‘increased’ then it starts to. Look at it schematically: a ‘reduced’ optical depth situations is proposed as opposite to (on the other hand) a thin atmosphere. A thin atmosphere is also optically thin, it is the ‘reduced optical depth’ situation, not the former.

    The paragraph contradicts itself.

    [Response: Thanks, You”re right. The optical path is indeed defined as: τ = ln(I0) – ln(I). I got my signs mixed an apologize for that. -rasmus]

  17. 167
    Radge Havers says:

    Jim Harrison @ 141
    “Scientists disastrously overestimate their ability to communicate their findings to non-scientists.”

    Or perhaps paraphrasing another commenter, scientists don’t always adequately define their target audience. I’m not saying that applies to this article. On the other hand, if you’re trying to cast the broadest possible net, I would say it’s of utmost importance to communicate what excites you about the subject (and NOT just that you happen to be one of those rare breed who are obsessed with solving problems that no one else can).

    Climate scientists are probably at a disadvantage in this regard. Topics like space (think Sagan) and dinosaurs are automatically interesting. Even string theory, which apparently nobody understands, is captivating (‘Whoa, dude! Like reality is so weird!’). Climate science, not so much. Just a bunch of soupy chemistry that doesn’t explode when you light it and heaping gobs of multi-layered physics that can’t even make your car go faster. It’s an uphill slog for you guys in that regard, and you have my deepest sympathies for that and your tireless patience with the public.

    Sad to say though, this discussion does remind me a little bit of the comments I saw on an article somewhere else a while back. The article was about the problems of getting Harry Potter translated in to Scottish Gaelic and what effect that had on promoting the language. One commenter demanded that Harry Potter be damned and all that was needed to promote the spread of Gaelic was for the best writers to write more poetry. This was rightly pounced on. Think about it, would being able to read the obscure scribblings, in their original form, of a handful of professional academics make you want to go out and learn Gaelic to use as a living language?

    I mean really?

  18. 168

    Re #151 (Frank Giger)–

    I have to agree with Barton’s questioning stance about this post: I haven’t noticed anything resembling an actual climate change policy in the US yet.

    I’m eagerly awaiting same, and I’ve heard some proposals. But so far, I don’t think we’re all that much past the vaporware stage.

    As to the policy proposals, I don’t think they resemble Frank’s caricatures all that closely.

  19. 169
    Phil Scadden says:

    “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.” True – for now – but what the science shows is validation for a model that makes more dire predictions for the future and with increasing costs associated with delays to action. 3mm/year of sea level is fine, but over 10mm/yr? The risks to human lives in my opinion is in starvation from food production disruption, and in risk of war from migrations triggered by delta losses and drought. The rich in the west being the least vulnerable to both but whose emissions are the ultimate cause.

  20. 170
    Garrett says:

    Radge Havers @ 167

    “Just a bunch of soupy chemistry that doesn’t explode when you light it …”

    Maybe methane-fuel-air explosions will get everyone’s attention!

  21. 171
    Patrick 027 says:

    Re 166 Chris Dudley, quoting main post ‘Hence, a reduced 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 the the estimates from the planetary energy balance model (Figure 3).’

    The best way I can interpret that (using what I already know) is that, as climate reaches equilibrium after any forcing changes, the outgoing longwave radiation (OLR) has to balance the solar (SW) heating (setting aside typically tiny geothermal and tidal heat fluxes). IF the atmosphere is optically thick, then most of the OLR is not coming from the surface, and in fact is coming from the upper atmosphere – in a region whose optical thickness is smaller than the total atmosphere (hence, ‘reduced’).

    If the temperature declines with height through much of the OLR source, then adding more LW optical thickness will still reduce the OLR. If the temperature doesn’t decline with height in that layer (perhaps because of solar heating), it is still the case that increasing the LW optical thickness will, by concentrating the source of OLR into a yet thinner layer at the top of the atmosphere, remove some of the cooling of the lower part of the original OLR source (by adding additional downward LW flux from above, replacing the darkness of space), thus tending to cause warming there. In general, so long as there is some solar heating beneath some level, there must be a net LW + convective heat flux upward at that level to balance it in equilibrium; convection tends to require some nonzero temperature decline with height, and a net upward LW flux requires either that the temperature declines with height on the scale of photon paths (from emission to absorption), or else requires at least a partial ‘veiw’ of space, which can be blocked by increasing optical thickness above that level. But there will always be some layer of air at the top of the atmosphere that has a partial to nearly complete ‘veiw’ of space.

    (PS a skin temperature can be lower than the brightness temperature of the OLR because a very thin layer at the top of the atmosphere will absorb a tiny fraction of OLR, thus barely affecting OLR, but must in equilibrium emit that same amount of energy both upwards and downwards; if it were as warm as the brightness temperature of the OLR then it would emit twice what it absorbs and thus cool. But OLR and the necessary thinness of such a layer can vary over wavelength and there’s also solar heating.)

    If the optical thickness varies over wavelength, then it is possible to saturate OLR at some wavelengths – but the greenhouse effect generally can’t be saturated at all wavelengths, because there has to be some net upward LW flux (above the level where convection becomes insiginificant) that balances solar heating (beneath that level) to maintain equilibrium, and there will generally always be ‘room’ to reduce that net upward LW flux at the wavelengths where it occurs. (CO2 is saturated at the tropopause level in the central portion of it’s dominant (for Earth) band (centered near 15 microns), but the optical thickness per unit CO2 declines away from the center in such a way that the width of the band exceeding a given optical thickness tends to widen by some amount with each doubling; it is this shifting of the ‘edges’ of the band where adding more CO2 has an effect.)

  22. 172
    Patrick 027 says:

    Note that it is possible, hypothetically, to introduce so much optical thickness that the tropopause level or any level besides the very top of the atmosphere becomes saturated (zero net LW flux); however, the resulting climate response will tend to ‘unsaturate’ the effect at some level(s) – for example, by shifting the position of the tropopause so that convection balances solar heating where the net LW flux is zero and convection carries the heat to where a net LW flux can balance the convective heat delivery.

    PS a question came up earlier about absorption lines of CO2:

    Generally, absorption lines in the absence of any line-broadenning, would absorb over an infinitesimal portion of the spectrum, and saturate (ie reduce the photon travel distances to a scale where there is very little temperature variation) relatively more quickly. Line broadenning takes the optical thickness and spreads it out a bit. For a very thin layer or very small amount of substance, the fraction of radiation absorbed wouldn’t change (so far as I understand), but the saturation at the line centers is delayed while significant absorption can occur over some finite interval of the spectrum, so that absorption can increase to some significant amount as the layer is thickennned or more material is added.

    The absorption bands of gases are generally not the result of the broadenning of a single line; they are actually groupings of lines; the variation in optical thickness over the band would be related to variations in line strength (and line spacing, I’d assume). The line broadenning takes some optical thickness from the line centers and puts it into gaps between closely-spaced lines, so that there can be significant absorption over a contiuous band of wavelengths.

    PS when molecular collisions are frequent relative to photon emissions and absorptions (as is generally the case in most of the mass of the atmosphere), the radiant heat absorbed by any population of molecules is transfered to the heat of the whole population within some volume, and molecules that emit photons can then gain energy from other molecules. Then the temperature of the whole population of molecules in some volume is approximately the same temperature as the molecules that are responsible for emitting and absorbing photons. This is approximately LTE.

    If collisions are more rare, then molecules might absorb photons, become exited, and then emit photons. This can still produces a greenhouse effect (analogous to a scattering greenhouse), but the mathematics is different. (You could even have a greenhouse effect based on LW-phosphorescence (maybe with nanoparticles suspended in the air) – but that’s a bit exotic.)

  23. 173
    Patrick 027 says:

    Re my second to last comment, about a skin layer: “but must in equilibrium emit that same amount of energy both upwards and downwards” – I mean in total – it would emit half that energy upward and half downwards.

  24. 174
    Chris Dudley says:

    Just to note that BPL in #109 agrees with my assessment (#92 and forward) that there is a problem with using the word ‘reduced’ in the article. Sorry I did not see that sooner Barton.

  25. 175
    Jim Redden says:

    Fine article. However, before for a litmus test, try asking someone to explain the basic mechanics of the relationship of the Sun and the Earth with respect to annular seasonal change. If they can’t pass that test, then it is very unlikely an understanding of an enhanced greenhouse effect is in the range of possibilities. However, these same folks feel confident holding an opinion about anthropogenic climate change (or the lack thereof).

    As others point out, a basic scientific literacy is pre-requisite and necessary to understand to most basic of climate concepts. Regrettably, a scientific literacy is not necessary to hold public office, or to guide the reins of a global corporation: cleverness, cunning, greed, and a lust for power are more potent indicators.

  26. 176
    Harmen says:

    “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. ”

    I liked how Scott Denning explained this in a way that even economist understand.

  27. 177
    McGahill says:

    Excellent to have this review of the science of atmospheric warming for us who have not been fully blessed with a scientific education. One thing that I have never had explained and that I would very much like to understand is this: I believe carbon dioxide and methane are greenhouse gasses effectively because they retain heat more or longer than the Oxygen and Nitrogen that make up the bulk of the atmosphere. But why are these carbon gases more retentive of heat? I assume it is something to do with the electronic structure of the molecules and the quantum energy levels in the molecules? Can someone please givean explanation sufficiently simple for a non scientist to grasp.

  28. 178
    Edward Greisch says:

    [removed – please calm down]

  29. 179
    EL says:

    If I was targeting the general population for my audience, I would not bother to mention a formula of any kind because the general population has very little to no understanding of algebra; instead, I might put a technical note in some places with formulas for those who are comfortable enough with algebra to use it, but my entire article would not depend on any formula of any kind, and I would make an effort to keep them separated. The general population is probably around a 7-8th grade level. So any story targeting these people must be geared for that level. (Most people seem to have trouble grasping fractions.) Also, technical words should be avoided completely. And I wouldn’t use any kind of graphing.

    I would try to show people the greenhouse effect instead of telling them.

  30. 180

    Chris Dudley 166,

    the optical depth is reduced at higher altitudes, which means it never saturates. The upper reaches of an atmosphere will always be at lower pressure–less mass in the same gravitational field. With five guys lying on top of each other (perhaps on Fire Island?) the one at the top is the one experiencing the least pressure.

  31. 181
    Chris S says:

    I’m normally a lurker but I feel compelled to post because several people have said that this is inappropriate for the non-scientist. Wrong, oh so wrong. I am not a scientist (I’m a teacher of IT) and I found it comprehensible, well explained and very useful. Like the man said – you can’t please all the people all of the time, so don’t try.

  32. 182
    Matthew L says:

    #157 BPL
    It is difficult to tell the long term trends as the Australian Govt. don’t appear to have done maps or graphs showing variance, only absolute figures. They do have some long term maps at the Australian Bureau of Meteorology showing 10, 20 and 30 year rainfall averages. Examples:
    30 years 1911-1940

    30 years 1976-2005

    Practically no rain falls in the Australian interior. From a quick visual comparison there appears to be very little change in the coastal areas. However it would seem that rainfall is penetrating further into the interior from the north coast towards Alice Springs and from the South West corner inland away from Perth. However, these areas are so sparsely populated, I wonder how accurate rainfall measurements were in 1911-1940?

    They last did a full climate assessment in 2006, and another one is due in 2011.

  33. 183
    Mike Ellis says:

    This is an excellent summary of the principles of the GHE and it is well aimed and highly needed for professional scientists who are in one way or another engaged in climate change research. “Climate change” has an enormous scope, and there are probably more non-physicists/chemists engaged in the science than not, and this article is excellent for that community. (Our community should do the same for you guys!)

    So thanks very much.

    A question: as I understand it, the radiation aborption characteristics of GHG molecules is dependent on the fraction of time they spend in particular dipole configurations. Is this fraction dependent on temperature or on the partial pressure of other gas species in the atmosphere?

  34. 184
    Confused says:

    In your notation to Figure 4 you say “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.” But surely the planet’s main heat loss takes place on the side of the globe facing away from the sun. You seem to have assumed permanent sunlight.

  35. 185
    Gordon says:

    I applaud your interest in communicating with the broad spectrum of humanity, but I believe you are neglecting a significant aspect of human psychology. Ernest Becker wrote a book, several decades ago, called The Denial of Death. In it he investigates the ideas put forth by Otto Rank, a contemporary of Freud, who believed that fear of death (or, more properly, anxiety about mortality) is the primary motivator for the vast majority of humans. Rank supposed that, in order to assuage this anxiety, persons attach themselves to a hero, or to a heroic notion (such as “western culture,” or “science,” or a religion), convincing themselves that so long as their hero persists, they, too, will “live on,” becomming “immortal,” like their heroes. To give up the hero is to give up “immortality.”

    Climate change, and its anthropogenic source, directly challenges the “heroism” of western culture. To it up will be to give up the dream of immortality for billions of people. All the careful explanation in the world will not help.

  36. 186
    Didactylos says:


    A good graph can explain a lot. This is the reason (probably the only genuine reason) why the hockey stick graph has been attacked so brutally. The adage “a picture is worth a thousand words” remains true to this day.

    Not all graphs are created equal, of course. Try to cram in too much information, distort it, or choose to express concepts that can’t be easily grasped, and your graph is useless to most people. Do it well, and your graph will project an idea that can live beyond simply reading the accompanying prose.

    Graphs can also be deployed in a “lie-to-children” sense, expressing a simpler version of a more complicated truth. The Al Gore ice core graph is a good example – the basic fact, that CO2 and temperature are linked, is expressed perfectly. The more complicated details of feedbacks are glossed over.

  37. 187
    Steve R says:

    Philip @120 describes the receptive audience best: scientists in other fields who can understand the numbers, graphs & equations in this piece, who need reassurance that the science is sound, and having read and understood the piece, will be able to pass along their explanations to other people.

    Make no mistake, this article is far too complex for John Q. Public*: not because John Q. couldn’t possibly understand it (he could, with some assistance from the helpful and infinitely patient people posting here), but because very few people work that hard to understand something unpleasant, unless it’s a direct and immediate threat, and even then… End of story.

    *I say this as a representative John Q: high test scores, master’s degree in literature, an engineer father, and an aptitude for reading technical material outside my field. This piece tested my capability, but I persevered & I think I get it. The concept of “optical depth” was useful, and for some reason, this is the first time I’ve encountered it in the 100’s of explanations of AGW I’ve read.

  38. 188
    Ray Ladbury says:

    Confused–you are indeed living up to your moniker. Outgoing IR increases with temperature. Since the daytime side is on average warmer than the night-time side, then all other things (e.g. cloud cover, etc.) more IR will escape on the day side. Also, day or night side, the planet is warmer than the inky blackness of space, so both sides radiate IR.

  39. 189
    Ray Ladbury says:

    Edward Greisch@178,
    Fossil fuels are cheap and demand no advanced technology for exploitation. Alternative energy technologies are more expensive and have greater technology requirements. With respect, why should a developing country with a growing population and on the verge of economic takeoff not tell you to pound sand when you tell them not to take the cheapest and most rapid route to economic prosperity?

    And with regard to bombing countries using coal, I’m afraid I will not follow you down that slippery slope to genocide. You’re on your own there, pal.

  40. 190
    Brian Dodge says:

    The lead story at right now is
    “‘Climategate’ review clears scientists of dishonesty”
    Unfortunately, their other stories at are about the politics, not the science.

  41. 191
    Anonymous Coward says:

    Ed’s #178 is an amazingly hate-filled comment. Hopefully these off-topic fantasies about US taxpayers financing “their” population growth can be put to rest with a reality check: has figures for US economic aid by major recipient countries (table 1263). I added up everything except Iraq and Afghanistan for the latest year available (2007): 13.2 billion dollars, over 10% of which went to Russia. By comparison, the USA’s trade deficit was 711.6 billions dollars in 2007.
    Going to, here are latest numbers for the holdings of US treasury securities: UK +167.4, Japan +109.6, China & HK +107.6 and so on (from April 2009 to April 2010, billions of dollars).

    [Response: The comment has been removed. No further OT commentary please. – gavin]

  42. 192
    Chris Dudley says:

    Barton (#180),

    I tried to read it that way but it does not work. Consider also where the term ‘reduced optical depth’ is introduced:

    ‘The term known as the ‘enhanced greenhouse effect’ describes a situation where the atmosphere’s becomes less transparent to infra-red light (reduced optical depth)’

    As you also pointed out: less transparent=increased optical depth.

  43. 193
    ghost says:

    I like the article. I also wonder if the reception to the quite excellent piece would be warmer if the presentation media were changed from more static text/graphics to more animation, at least for online presentations. In that, I’m thinking of the way that The Mechanical Universe programs illustrated physics principles, and especially the math manipulations. (I might be biased, because I loved that series, viewable here: If it is a useful goal to give the interested viewer a comfortable understanding that the physical model is logical, sensible, and correct/reliable, rather than to convey a firm everyday grasp of the precise details to most, then it seems to me that animating the mathematics could help toward that end. The atmospheric math certainly is more complex than Newton’s First Law, but maybe any progress toward simplifying the communication would be worthwhile. We probably are happy when non-specia-lists comprehend the material, but it probably is more important to know how and why the non-comprehenders react to the material. I also wonder how audiences with demographics different from RC’s audience would react to the presentation, and to various presentation formats. Maybe younger audiences would comprehend better with a more animated format. I don’t know what sites would have younger audiences–maybe Kate from ClimateSight could chime in on that question.

    RE: Gordon # 184, I see merit to your thesis. Especially as, the more synthetic our daily existence becomes, the more difficult it is for the average person to comprehend the vulnerability of our habitat and the notion that humans aren’t the only species in that habitat. Maybe I disagree a little with a possible implication that western culture can maintain only if AGW does also. It seems to me that the grail is whatever type of energy is required to run that doomed amusement park ride. In that vein, the hero worship doesn’t care whether it is sustainable energy rather than the rapidly depleting fossil sources, as long as the music continues. A person might think that more readily/easily available energy simply will turbocharge population growth and consumption, but that’s a different train wreck discussion.

  44. 194
    Hank Roberts says:

    McGahill says: 7 July 2010 at 2:02:
    “… I believe carbon dioxide and methane are greenhouse gasses effectively because they retain heat more or longer ….”

    That’s wrong. Don’t get confused by the guy who often posts confusing energy and temperature. The explanation won’t make any sense as long as you read it while assuming that mistaken idea is correct.

    Start from scratch with the explanation.

    Authors, it might help to address commonly held misunderstandings and areas of confusion — assumptions people may be holding as they read your piece that undermine comprehension.

  45. 195
    Nick Gotts says:

    [edit – sorry, the original offensive comment was removed, and so are any replies].

  46. 196

    Fred Moolten’s comments made me go back and re-check my understanding of the stratosphere and GHG cooling. Please correct me where needed.

    The stratosphere is found between the tropopause to about 51 km or so. The tropopause heights varies between 8 km and 18 km due to latitude and season. Cooler latitudes and seasons result in a lower tropopause height.

    The greatest ozone density is around 20-25 km because the ozone destruction processes are slower there. Maximum incoming solar heating occurs there but the greatest temperatures are near 50km where the low density air requires very little energy to raise its temperature (think KE=1/2 mv^2 and fewer collisions with fewer air molecules yield greater v). Therefore, there is a temperature inversion throughout the stratosphere above the ozone maximum. This inversion severely inhibits convection (mixing) so radiative and conductive processes dominate. The inversion also means that air from the troposphere has difficulty entering the stratosphere so heat is not easily transported between the two by convection or conduction. (Note: Srongly thunderstorms can transport gases such as water vapor up across the tropopause and there are breaks in the jet stream westerlies allowing interchange of stratospheric and tropospheric air. Also, gravity waves can help mix the air in the stratosphere.) One can assume that radiative transfer must dominate between these two spheres.

    So what cools the stratosphere?

    1) Increased GHGs and the tropospheric enhanced GHE
    2) Ozone loss
    3) Decreased solar radiation

    The simplest explanation is that the troposphere is warming primarily through enhanced GHE and not because incoming solar radiation is increasing (in fact it is decreasing recently). There should be less heat available to the spheres above which means they will all cool. Energy cannot be created, right? If there is more energy in one sphere there must be less for the others assuming an energy balance with the sun.

    As mentioned here, as the troposphere becomes more optically thick, the height at which IR is released to space is higher and cooler which means there is less upwelling IR to the regions above the troposphere. So is this causing the stratospheric cooling? It appears that the answer is “yes, to some degree” but there are other factors that make it difficult to nail down this thing.

    Before 1995, it appears that the most significant cause of the lower stratospheric cooling was ozone depletion which means less absorption of solar radiation. Because ozone rapidly decreases with height (very little ozone above 35 km), ozone loss is estimated to have caused only half of the cooling at the higher levels of the stratosphere. Increased CO2 in the stratosphere at higher levels increases upwelling radiation to space which appears to have been greater than absorption from below resulting in cooling at higher levels.

    Since 1995, ozone is increasing in the lower stratosphere and ozone absorbs incoming sunlight to cause warming. Since 1995, there has been no cooling of the lower stratosphere but no real warming which one would expect due to the rebound of ozone. This may suggest that the decreased upwelling IR from the troposphere is offsetting the warming caused by increasing ozone.

    It also appears that increased CO2 is cooling the mesosphere and thermosphere and there is very little ozone in that region. A complicating issue for all spheres is that the sun has been “very weak” recently which has resulted in some cooling of these spheres, especially the thermosphere.

    Some good references:,0/2__Ozone/-_Cooling_nd.html

    Scott A. Mandia, Professor of Physical Sciences
    Selden, NY
    Global Warming: Man or Myth?
    My Global Warming Blog
    Twitter: AGW_Prof
    “Global Warming Fact of the Day” Facebook Group

  47. 197
    Jim Eager says:

    McGahill @177, I’m just a lay reader here, but I’ll take a shot at your question. I’m sure I’ll be corrected shortly if I get it wrong.

    CO2, H2O (water vapour), CH4 (methane), N2O (nitrous oxide), O3 (ozone), etc. are greenhouse gases because their molecular structure allows them to absorb packets of infrared light energy (photons) of specific wavelengths and become vibrationally excited. (Yes, it is a quantum effect.)

    But they can’t retain this extra energy for very long. Almost immediately (nanoseconds) they relax from their excited state by either 1) emitting that energy as a new photon, some of which will continue up towards space, some of which will go back downward to be reabsorbed, thus keeping the energy in the atmosphere longer, or 2) by colliding with another gas molecule, most likely an O2 (oxygen) or N2 (nitrogen) molecule since they make up over 98% of the atmosphere, thereby converting the extra vibrational energy into kinetic energy by transferring it to the other gas molecule, which will then collide with other molecules, and so on, making the air warmer.

    In other words, greenhouse gases don’t retain heat themselves, but rather redirect outgoing energy radiated by the surface and lower in the atmosphere, thereby delaying it’s ultimate escape to space. You might say energy is retained, but it is the vastly more numerous O2 and N2 molecules that are warmed and retain the heat, even though they can not directly absorb outgoing infrared energy themselves.

    Another way to think about it to is to compare outbound IR photons to the child’s game of snakes and ladders. Progress around the game board is hindered when your game piece (outgoing photon) lands on a slide (greenhouse gas molecule) that sends you backwards (photon emitted back downward), but eventually your game piece will make it to the finish line (space), it’ll just take longer.

  48. 198
    t_p_hamilton says:

    Mike Ellis asks:”A question: as I understand it, the radiation aborption characteristics of GHG molecules is dependent on the fraction of time they spend in particular dipole configurations. Is this fraction dependent on temperature or on the partial pressure of other gas species in the atmosphere?”

    As a first approximation, no. The molecules, even in the vibrational ground state are vibrating back and forth about the equilibrium value (in CO2 the primary importance is bending away from linear, back and forth). Vibration excited states primarily increase the amplitude, but not so much the rate of change in dipole moment as a function of angle, which is what IR intensity depends upon.

  49. 199
    Edward Greisch says:

    [edit – please, no more. This is a thread about the GHE!]

  50. 200

    Yes, the GHE basics constantly need to be retaught to each new generation, and to those who at long last are tuning in.

    It’s probably still too complicated, except for those with some time on their hands.

    I was fortunate as a teen in the early 60s either to learn about the natural GHE in school science class or in my outside readings in science. When AGW came to public consciousness in the late 80s, I was well prepped to understand it.

    A more simply scheme is the idea of a real greenhouse, or a car in summer with windows up. Even tho this may not be totally scientifically correct, it is a good way to get the point across. A simple picture — like — is a very good start.

    When people understand the GHE from this greenhouse or car analogy and from such simple images, then it becomes very easy to understand that adding more GHGs to the atmosphere will increase the global warming. For a sincere person, this is all they need to know to understand it — plus learning something about how the warming will affect the earth and life thereupon — for them to be motivated to mitigate AGW. For the heartless, fearful denialists, no amount of science and no amount of making it easy to understand will ever help them to accept AGW and start mitigating.