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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. 51
    The Ville says:

    Re 29- Ike Solem:

    If that’s so, why was Steven Hawking’s “A Brief History of Time” so popular, selling over 9 million copies? People appreciate clear scientific explanations, generally speaking – which is why popular science books continue to be top sellers.

    Hardly anyone read “A Brief History of Time”.
    I have a copy and gave up half way through. 22 years later it remains on the bookshelf, still only half read.

    People bought it because it was cool to have a copy. When it became a best seller, even more people bought it.

  2. 52
    wili says:

    One small typo just above the “feedback” section:

    You typed: “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).”

    Presumably you wanted “close TO the estimates…” rather than “the the.”

  3. 53
    wab says:

    29 asks: “Why was Steven Hawking’s “A Brief History of Time” so popular, selling over 9 million copies?”

    Answer: Because he is in a wheelchair and needs a voice synthesizer to speak (although when the book first came out he didn’t have that and could only be understood by a few people), and this captured the imagination of the world. How many people who bought the book actually read it past the first few pages?

    My two cents: I gave up on the current article not even half way in. I would not have minded more equations, but that was not the issue. It was just not a clear exposition to me, although no doubt it will be for some people.

  4. 54
    David B. Benson says:

    Heat due to random molecular motion is not lost to space. Heat due to radiation can be lost to space.

  5. 55
    wili says:

    Sorry, one more quible: in the second paragraph above the last graph, you start the third sentence “In other word” while the usual phrase is “In other words.”

    In general, I think it is a good article, mostly well written. Reading the (substantive) responses then going back over the article certainly helped.

    It reminded me of old “Scientific American” articles from the ’70’s that I would struggle to understand as a highschooler and college student. Often I didn’t get past the first page or so, but if I really worked at it and talked about it with my nerdy, more-scientifically-literate friends, I could usually figure out most of it and felt better for the mental exercise.

  6. 56
    Didactylos says:

    “If that’s so, why was Steven Hawking’s “A Brief History of Time” so popular”

    The book contained a grand total of one equation. Look up the related anecdote….

    And maybe you should remember that 9 million copies equates to one copy for every 744 people on earth – or one for every 34 Americans. Compare that to the hundreds of millions of book sales for more popular fiction.

    I’m quite happy to conclude that the 9 million readers of “A Brief History of Time” are not a random sampling of Joe Public.

    This leads me to the subject of making information accessible to everyone. Obviously, this isn’t easy. I’m fairly sure Edward Greisch was indulging in irony when he spoke of using Bugs Bunny as a narrator, but he made some good points. Just because our audience are mostly adults doesn’t mean that the level they can best grasp isn’t sometimes the same level that is taught in primary school.

    Terry Pratchett and his scientific collaborators coined the term “a lie-to-children”. This is one of those scientific “facts” you learn in school that simply isn’t true. But it’s close enough to the truth for you to deal with it, and understand everything you need to. The lie is then discarded trivially as you learn more detail.

    This should apply to climate science just as easily. Does our model for how the greenhouse effect causes surface cooling need to include such nuances as stratospheric cooling? Heck no! How many people even know what the stratosphere is? Sure, people probably have a general idea, but the technical details? Not a chance!

    Rasmus used the word “simple” 10 times in his article. Gavin can try and backtrack if he wants, but Rasmus even wrote it in the title! Yes, the article fills a niche. But I’m not sure it was the niche you had in mind.

  7. 57
    The Ville says:

    I think I have my head sorted regarding cooling in the upper atmosphere.
    Probably worth noting a previous RealClimate post:

    Which links to this very useful page that has a nice clear explanation of the cooling effect CO2 has in the upper atmosphere:

    My summary would be:
    Upper atmosphere receives less IR energy from below, yet the greenhouse gases in the upper atmosphere remain a potent emission source of IR, so there is a net loss to space in the upper atmosphere. eg. Input from below is less than that being emitted.

  8. 58
    pointer says:

    What a confused post. I don’t fault the science at all – I’m not a scientist! What I do fault is that you never pin down who your intended audience is. Are you writing for science nerds? For the general public? You start off talking about “communicating science to the public” but then throw in charts! Equations! Numbers! Would Carl Sagan have done that? Does David Attenborough?

    As a piece of science communication for the masses, this is a great section for a science textbook.

  9. 59
    pointer says:

    You know what? My main gripe here is that rasmus said he was selling me an orange and then gave me a persimmon. Where’s my orange?!

  10. 60
    pointer says:

    Oh, this is priceless. From wili (#55):

    It reminded me of old “Scientific American” articles from the ’70’s that I would struggle to understand as a highschooler and college student. Often I didn’t get past the first page or so, but if I really worked at it and talked about it with my nerdy, more-scientifically-literate friends, I could usually figure out most of it and felt better for the mental exercise.

    That’s right. This blog post on explaining the GHE in “simple” terms — an exercise in “communicating science to the public” — is only a success if every member of the public has access to “nerdy, more-scientifically-literate friends”. Because that’s so likely.

  11. 61
    Rich Thompson says:

    If any of the “general public” that this post ostensibly targets actually reads it, it will only reinforce the idea that scientists are arrogant and so out of touch that they can be safely ignored. This could have been entirely avoided by not referring to this explanation as “as simple as possible” for the purpose of “communicating science to the public”. Treating people like they’re stupid doesn’t communicate science to them, it pisses them off.

  12. 62
    Brian Dodge says:

    “why is the measured temp of our moon significantly colder than the expected T?”

    because the Apollo astronauts secretly painted the back side of the moon with Dupont Lucite Acrylic White Lacquer, which increased its visible albedo, but maintained its thermal emissivity near 0.9

  13. 63
    Spencer says:

    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. I may go on to explain that you can look at a rising water level as equivalent to the rise in temperature necessary to “push” energy out to maintain energy out = energy in (less the amount embedded in the warming).

    For the upper-atmosphere cooling, I simply remark that infrared coming up from below is blocked more, as more greenhouse gases are added, so of course it’s cooler above the blocking. Maybe not 100% accurate but good enough for a one-sentence explanation.

  14. 64
    John Atkeison says:

    I am not a scientist.

    As a communcator & activist I feel it is my job to apply the Duck Test– if it looks like a duck, and walks like a duck and quacks like a duck, it is a damn duck! And if weird weather matches my understanding of what we expect to see as expressing the climate changes produced by global warming, I think it is my duty to say so, and I hope more climate scientists find ways to say so as well. (Yes, I know it really is “trickier” for y’all.)

    This is the kind of post I need to keep my level of understanding at a decent level, so I can play my role.

  15. 65
    Michael says:

    The post starts off wondering if there may have been a recent decline in public concern over global warming and then considers if a simpler method of communicating the science would help. In other words they would be concerned if only they understood. Have you considered that if people begin to question the motives of those espousing the science then it doesn’t matter how good or robust the science is; how well it is communicated; or even how perilous the situation appears to ‘those in the know’, the average person will filter that communication out at the start and you won’t get a look in? Climategate has done an enormous amount of damage to the credibility of climate science, rightly or wrongly, and that is what I believe to be driving this recent (real) decline in concern.

  16. 66
    Patrick 027 says:

    Re 39 Patrik – water vapor absorbs both LW (terrestrial) and SW (solar – UV,vis,solar IR) radiation. But not to the same extent. Solar heating at all levels beneath the tropopause, whether at the surface or aloft, still must be approximately balanced by the net upward LW flux at the tropopause in an equilibrium climate. At the point where there is so much H2O vapor in the atmosphere that there is very little solar heating of the surface (very very far from happenning), there will also tend to be almost no net LW cooling at the surface, so a tropospheric-type lapse rate could still tend to extend down to the surface (as long as the net LW cooling is smaller than the SW heating, there will be some non-radiative flux from the surface for equilibrium conditions).

    Re 40 simon abingdon – there is very little mass loss to space (can be significant for evolution of conditions over geologic time or in more extreme conditions, but not for Earth like conditions over the timescales over which climatic equilibrium is determined), and latent and sensible heat are transported by conduction and convection and mass diffusion, which can’t significantly extend outside the atmosphere.

    Non-radiative heat fluxes drop to approximately zero (at least for the global time average) going above the tropopause (there is a little leakage of convection through the stratosphere and mesosphere via upward propagation of kinetic energy and the Brewer-Dobson (does that term include the mesospheric part?) circulation that it drives, but even that can’t really go directly to space. Maybe some of the kinetic energy in the ionosphere produces radio waves that take energy into space ? (radiation, but not of the sort emitted as a function of temperature) – I’m quite sure that’s small enough to ignore in this context.

    (PS I only know that those non-radiative fluxes are small – I would very much like to know numerically what they are (the upward kinetic energy flux and the heat flux of the thermally-indirect overturning).)

    Re 32 Titus – the amount of mass of the atmosphere above the mesosphere is an extremely small amount compared to the total atmosphere. The thermosphere experiences huge temperature swings over the diurnal cycle and due to solar variability that vastly dwarf anything seen at the surface or troposphere. For at least some purposes one can calculate the energy budget of the surface and tropopshere and stratosphere while ignoring anything the thermosphere does.

    Re 42 Sordnay – the most obvious radiative effect is that less atmospheric mass in total means that, for a given composition, there is less mass of any given substance within the atmosphere. There is also (via weight given to mass by gravity, which itself varies among planets) the effect on pressure-broadenning of the lines that make up absorption bands – this alters the optical properties for a given amount of material (temperature also has an effect). The lapse rate(s) that would be found in a troposphere also depend on composition and gravity and (depending on vertical coordinate and whether or not a layer has a dry-adiabatic lapse rate) pressure and temperature.

  17. 67
    Chris Colose says:

    pointer (58-60), and others–

    Please. This consistent talk about “the post is too technical when rasmus wanted to talk to lay audiences” is rather tedious, and it takes away from the possibility of more interesting discussion. IMO it also underestimates the curiosity of those “regular joes” who took the time to find this post at all. For the one-stop readers who wanted an orange, they did not get a persimmon…they got an orange in addition to an apple (complements of rasmus, thanks again) so it should make their day.

    I say this because it is not an overly-demanding post to follow. There is nothing that assumes knowledge like calculus or physics that takes several years to build up, and there are plenty of linked references such as “CO2 problem in 6 easy steps” or “Saturated Gassy Argument” which allows one to explore different approaches to the same issue. There are a few elements (e.g., stratospheric cooling) which are not necessary to bring up right away, and the part about negative feedbacks is a bit of a confused complication, but they can safely be skipped over by a reader without a loss of much information if necessary. Further, it is easy to find slightly less laborious posts on the internet (e.g., wikipedia) which still provide adequate background for building an intuition. If the “regular joe” wants to learn the broad-brush picture he can do so easily if he spends just a few hours on the web.

    In addition to the myriad web sources, we are fortunate that this post appears on a site which allows comments, and happens to be full of commenters who are well familiar with what was written and can answer additional questions people may have. They can also elaborate on points Rasmus already made. Still further, those knowledgeable commenters are a supplement to a handful of experts who can even provide further nitty gritty details if it comes to that. I suppose RC has a nice professor ==> teaching assistant ==> student hierarchy and I find no reason that any of the posts RC does should be beyond the realm of accessibility. Even for those people pretty sketchy with the all that has been written, it will be a great mental exercise (as nicely stated in post #55 by wili) to track through the steps, ask questions in the comments, visit the linked websites, etc.

    Consider this though: Since the discussion surrounds science to public communication, it is necessary to have such intermediate level information of high quality that is accessible for people who are beyond wiki cartoon diagrams but not yet at differential equations. This IMO, is one large gap that requires filling. Those wanting more details will search it out at university and in textbooks, and those seeking less detail can already find it on the web (assuming they can sort out the good stuff from the wingnut stuff, but it’s hard to get rid of that issue in a world where anyone can write anything they want). Accessibility of info is not the issue; the largest audience out there is the “regular joes” who have no desire to learn climate change, in the same way that many here have no desire to learn about medicine. It’s not that they don’t believe in climate change, take wrong sides, or don’t understand it…they just don’t think about it. I don’t claim to know anything about social or psychological sciences to elaborate, but this might just be a consequence of the fact that climate change operates on timescales much larger than a political term or the time it takes to schedule your son’s soccer practice. Experts in a lot of fields want the regular public to understand elements of their field that people just don’t think about…I had an ecology professor a while ago who was angry that I didn’t really know about mercury contamination issues in a local water source. I’m sure that is a big issue for such scientists and has public repercussions, but admittedly I did not have the interest to read up on it, and it’s time we accept the fact that most people aren’t going to unless it personally influences them (whether it be climate change, mercury pollution, mudslides, landfills genocide in Africa, etc etc). C’est la vie. Perhaps the answer to the science==> public communication gap is that it can’t improve except that bloggers, authors, etc have to keep doing what they do. Many of them do it quite well. If the concern of climate scientists continues to go unheard, there will be severe social and ecological repercussions but maybe that undesirable outcome is precisely what will manifest itself. If not, awesome. If so, we tried, and hopefully industrial mistakes will be avoided by people hundreds or thousands of years from now in this grand history lesson.

  18. 68
    Foobear says:

    I like the summary, but you’re going to cause most non-scientific readers to go crosseyed with it. I agree with #2 – there’s other ways of explaining it that are easier for non-scientific people to understand, without misrepresenting the current state of the science.

    When I teach it, I just show a chart that shows watts coming in, watts reflected by the surface and atmosphere, watts trapped by GHG, and then talk about climate sensititivity, do a couple multiplications to show the math matches predictions (also accounting for the ocean buffering effect), and there you go.

    You can also just mention that greenhouse gasses respond differently to different wavelengths of light without asking people to understand black body radiation or use those scary, scary equations.

  19. 69
    Titus says:

    Ref: Spencer #63 gives an analogy with which I can associate:

    “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.”

    “Greenhouse” does not work at all for me and creates confusion. It is not how I know a greenhouse works.

    As a lay person I would recommend the adoption of this analogy as it appears as Spencer says; 100% correct.

  20. 70
    Patrick 027 says:

    For the record, I know someone who read “A Brief History of Time” when he was in elementary school and understood it. (Maybe it helps that young minds are stretchy?)


    It might help to show the spectra of upward and downward LW fluxes at different heights. It should be easy to understand why increased opacity (decreased distances travelled by photons from emission to absorption)generally causes the LW fluxes to change and ultimately approach local blackbody values (thus bringing the net LW fluxes to zero except where there is a sharp temperature discontinuity, such as (relative to optical thickness) the ‘top of the atmosphere’ (TOA) – space acting in this context as a near zero-K blackbody. The increase/decrease of net upward LW flux going from one level to a higher level equals the net cooling/heating of that layer by LW radiation – in equilibrium this must be balanaced by solar heating/cooling + convective/conductive heating/cooling, and those are related to flux variation in height in the same way. (change in forcing from bottom to top of a layer = forcing of that layer; equilibrium temperature response of a layer changes the LW and convective fluxes to restore balance).

    Showing how a change in CO2 amount causes different LW flux changes at different heights would show how stratospheric cooling occurs, and the spectra (overlayed for 0.5 x, 1 x, 2 x, 4 x, 8 x preindustrial CO2) would also show the origin of the logarithic proportionality for tropopause-level forcing once the center of the band is saturated. One could also show how the spectra of LW radiation is affected by the resulting temperature increase and also by the water vapor feedback.

    (the temperature response to a forcing tends to be spread out from the location of that forcing, because the temperature change at one location changes the LW fluxes reaching other areas. Hence, for GHG forcing in general, stratospheric cooling (assuming either that the cooling extends to the base of the stratosphere or that the stratosphere has a sufficient band of wavelengths with significant but not large optical thickness, etc…) reduces the tropopause level forcing downwar, while tropospheric-surface warming reduces the stratospheric cooling. The temperature response also spreads out from the forcing via convection, which is why the surface and various levels of troposphere tend to shift (to a first approximation, setting aside the structure of the circulation and the occurence of stable air masses and the moist lapse rate feedback and surface dryness, etc) about the same amount in response to a tropopause-level forcing.)

  21. 71
    Patrick 027 says:

    Re 63 Spencer – I like that!

  22. 72
    Bob Doppelt says:

    It is true that the majority of the American public does not understand climate science—improving biophysical climate literacy is important. But we have made a major mistake by framing climate change as a scientific and environmental issue. That’s the primary reason why the public doesn’t “get it”. Climate change is merely a symptom of maladaptive human thinking and beliefs that have been translated into maladaptive norms and values, technologies and policies. It is this sphere—in social science climate literacy–that increased understanding and better solutions will be found. My program at the UO has been part of an extensive research program on communicating climate change to the general public. Just one sample of the findings—many people don’t know what a greenhouse is or if they do, they think of its as a good thing. Thus, our long term use of the greenhouse effect term or enhanced greenhouse effect does not resonate with the majority of Americans. Good climate communications is framed in a way that motivates people to alter their thinking and behavior. For the most part we have failed in this task (for other findings and to obtain a copy of our handbook on climate communications and behavioral change see our website: I think a major enhancement of the social science principles of climate literacy is in order.


  23. 73
    PaulD says:

    I don’t think many informed skeptics have a problem with textbook greenhouse gas physics. I think you lose most of them on the issue whether there are strong positive feedbacks that amplify the initial warming caused by CO2. If the net feedbacks in the climate system are close to zero, then the warming from CO2 will be at or below the low-end projections of the IPCC. Your discussion of feedbacks is weak and not convincing.

  24. 74
    Ray Ladbury says:

    Pointer and Richard Thompson, We’ll work on a post communicating the greenhouse effect through monosyllabic grunts just for you.

    Jeez, Dudes, if you didn’t understand something why not ask questions. Or did you really think that you could communicate everything you need to know about the greenhouse effect in 500 words?

  25. 75

    I agree that many posts at RC are beyond the average reader. I teach freshman/sophomore non-science majors and they have difficulty here.

    Having said that, RC posts and the subsequent comments are the best resource for people like me who have a science background (or for people are well read on the topic of climate change) but who are not experts in the many fields associated with climate change. (But who is?) What I learn here I can then bring to others with the confidence that I have the correct science.

    Kudos to RC and the regulars who comment here. Keep it coming. You are providing a very valuable service.

    I took a whack at stratospheric cooling if anybody is interested. I agree that it is difficult to simplify and I admit to oversimplification.

    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

  26. 76
    Kjell Arne Rekaa says:

    The figure 3 confuses me a little. First of all, there seems to be 9 planets, which for the time beeing are allowing Pluto into the gang of planets… ;-)
    Next, it make sense to then expect the upper right planet to be Merkur, earth and Venus is marked and named. Then it make sense that Merkur, Mars and Pluto is more or less with same temperature as expected, while the gas-giants are a bit hotter than expected.
    The strange thing is Venus, which is plotted as expected to be colder than earth?! Even though it is a bit closer. Here the earth is expected to be about 260K while Venus is expected to be 240K (I´ll guess the scale is in Kelvin?), while messured are something like 290K and 720K respectively?

  27. 77
    Ray Ladbury says:

    You need to be careful in trying to interpret scientific results–the solar-thermosphere connection in your article has very little to do with climate. Also, when you say “solar activity”–what activity? Magnetic? Luminous? Coronal Masss Ejections?

    Solar luminosity–which is the main solar influence–has been relatively constant for 50 years or more. And as to your contention that our understanding of Earth’s climate is still primitive–that is complete utter BS. Where on Earth are you getting this idea?

    Iskandar, What you are posting bears zero relation to the real science. Where are you getting your information? Note especially that the 2.7 K temperature of space is a relic of the Big Bang. It is the blackbody temperature of the radiation left over from that event.

  28. 78

    Very good but I think you could improve on the stratospheric cooling part. I have an undergrad degree in Meteorology and that part I had to read twice ;)


  29. 79
    Rich Thompson says:

    Re Ray 69: I’m a scientist who has in the past taught, among other things, the science of climate change to large classes of non-science major undergraduates. It’s not that I didn’t understand the post, it’s that nobody without at least some technical background would. Which is fine, unless you introduce the piece with the message that anybody who can’t understand it is stupid. Like this post did. Which is why a lot of people don’t like or trust scientists.

  30. 80
    Chris Colose says:

    Kjell Arne Rekaa #72,

    Venus is indeed closer to the sun, but it is not just the fraction of solar energy it receives that counts…you have to weight that by the amount it actually absorbs. Venus has a much higher albedo (reflectivity) than Earth because of its thick cloud cover (and would even have a high albedo without the clouds due to Rayleigh scattering from the dense CO2 atmosphere). When this is accounted for it actually turns out Venus absorbs less solar energy than Earth, and thus would be colder than Earth if you fixed the albedo and removed the greenhouse warming. It is common in these simple radiative balance calculations to hold the albedo at present day values and compare from there. The famous “255 K” value for no greenhouse effect on Earth is an example of this, although in reality if we got that cold you would expect a snowball-like Earth and a much higher albedo from the increased brightness of the surface…and thus the “no-greenhouse temperature” would be even colder than 255 K. You can’t model these feedbacks on the back of a napkin though so it’s not really an important point for simple descriptions.

  31. 81
    Jack says:

    Hello RealClimate folks,

    I was delighted to read the second paragraph of this post: “… 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…”.

    In other words, the fundamental reason scientists think atmospheric CO2 strongly affects the global temperature is not climate model output – it’s just *basic radiative physics*! Now, if you are an atmospheric scientist, this is taken for granted. But among the general educated public, it is my anecdotal impression that this has been largely missed, and that the public discourse gives the “climate models predict that…” wording rather than the “physics says…” wording.

    So, as a result, it seems to me (again informally) that almost all educated people, including technically-educated people who have had considerable college physics courses, needlessly lack confidence in the pronouncements of atmosphere and earth scientists about the reality of the global-warming threat. Now, here’s my point: to remedy this, I find that you rarely need to explain the whole nine yards laid out in this post! You merely have to say that it’s *physics*, and if they are technically-educated explain the general thrust of this physics (e.g. triatomic or larger molecules, which are scarce in the atmosphere, generally have vibrational modes that will be active in the IR, where Earth emits due to Wien’s law, and thus will act as a blanket or lid, and Earth would be 255K = -18C = 0F by Stefan-Boltzmann if this weren’t the case) and they go, “Oh, ok! Why didn’t anyone say that before?”

    If on the other hand they don’t know much physics, then it’s even easier… they know the power and track record of physics (e.g. electricity, nuclear power, gadgets, modern life, etc.) so they have confidence if it is said that “physics” is the reason for the CO2-climate connection.

    To illustrate this, could you imagine if during the effort to convince the U.S. government to embark on the Manhattan Project, the word “physics” was hardly ever used by the advocates of atom bomb development (who were simply known in this alternate reality as “nuclear *scientists*”), to such an extent that many well-placed non-physicists didn’t even realize that the claims of destructive power were based on it? I imagine they would have had a harder time convincing the government that this might actually work.

    So I guess I agree with the spirit of this post, but in some way I think you’re trying too hard. It’s just a simple matter of stating what we take for granted, but is not at all obvious to those outside our discipline–the *physical* (and chemical) nature of the science we do. “Earth science” or “climate science” does not have the same connotation, and sounds vague or untrustworthy or ungrounded to a lot of people.

    [I am a graduate student in the atmospheric sciences department of a large state university in the USA. Your contributors eric and raypierre know me.]



  32. 82
    sue says:

    The Stanford research supports the other two sources AND provides a simple, reasonable (to a sociologist) explanation for the small decline based on analysis of hte data. I quote: ‘”Our surveys reveal a small decline in the proportion of people who believe global warming has been happening, from 84 percent in 2007 to 74 percent today,” Krosnick said. “Statistical analysis of our data revealed that this decline is attributable to perceptions of recent weather changes by the minority of Americans who have been skeptical about climate scientists.”‘, but still shows high levels of support.

  33. 83
    John E. Pearson says:

    74: Kjell. Please read the text and the caption. The predicted temperatures for the planets were calculated by neglecting the greenhouse effect. The Venusian temperature is not at all strange.

  34. 84
    CStack says:

    Keep it simple, use the approach that has already worked = CFC emissions. (a) state your objective clearly = GHG causes X, which is bad for humanity because of Y. In the case of CFCs, they cause X = UV increases at the earth’s surface, and Y = skin cancer & cataracts.

    This approach resulted in the Montreal Protocol, which is one of the most successful global efforts in history. We are at a disadvantage, since the short-term effects of climate change aren’t as nasty. Humanity has a hard time concentrating on the long-term.

    I often focus on oceanic acidification as a primary, here/now concern that most people can identify with. Very straight-forward, just pull out an Alka-Seltzer or equivalent & drop in a glass of water.

  35. 85
    Jim Eager says:

    Kjell Arne Rekaa @72, although Venus is closer to the sun it has a much higher albedo than Earth, so it reflects more incoming solar energy back out to space before it can be absorbed.

  36. 86
    John Atkeison says:

    We have lots of opportunities to tell the story different ways, because there are lots of different people out there with differnt points of view and differnt notions of what is happening in the world. (One culd also say that there are many “audience segments” out there. )

    There is one set of facts that best expresses our current understanding of the physical processes, but there are a whole bunch of wildly differing sets of assumptions and mindsetsamong people that we need to communicate with.

    Please remember that most people do not have a college degree, but that this does not mean they are stupid.

  37. 87
    Eli Rabett says:

    A minor point:

    “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.”

    could be improved as

    “because they can both absorb and emit infra-red light. When they absorb IR light they rotate or vibrate faster. When they emit it they lose the excess energy. Excited molecules can also lose energy by collisions, heating the atmosphere.”

  38. 88
    Eli Rabett says:

    Those looking for simple explanations of the greenhouse effect might try this java applet

    Or this equation free (two graphs) one or if you are not math averse, this one from Chris Colose

  39. 89
    Ron DeWitt says:

    As you point out, many of the negative feedback effects that have been proposed would tend to decrease not only the effect of increased CO2, but also of whatever natural variability there may be. But this is also true of the positive feedback mechanisms that have been proposed, at least those that work through temperature as an intermediary. This would seem to pose a problem to any model that includes only positive feedback mechanism (and no “controversial” negative feedback mechanisms), for the climate has remained relatively stable despite the occurrence of natural variations such as volcanic veiling, solar variability, etc. Without negative feedback, a system will tend to have a dynamic much like that of a billiard ball balanced on a knife edge; any minor perturbation will send it hurtling off in one direction or the other. Thus, in view of the past stability of the climate system, the existence of negative feedback is not controversial, but rather an observed fact. This does not rule out the possibility that the negative feedback can be overwhelmed by other effects, but if we are to believe that, some explanation would be required. Your discussion leaves the impression that negative feedback is merely being dismissed as “controversial” and is not being addressed at all.

  40. 90
    ccpo says:

    It’s not in the presentation, gents, it’s in the false dichotomy. It’s in the lying. It’s in the spinning.

    So long as doubt is allowed to be presented as equal to truth, you haven’t a prayer. Until there is a large, undeniable response to denialism such that only that 30% will believe a shred of the lies and BS, you haven’t a prayer.

    Take legal action when the opportunity arises.

    Get the President on the TV and do a week-long series on the climate science and make the denialists look as bad as they are.

    I’m a teacher. Trust me, this isn’t an issue of presentation. They can see the melting Arctic, they have noticed the warmer winters, the earlier springs. This is about overcoming ideology. And fear.

    Doubt is easy to sow, hard to root out. If you don’t collectively hit back really, really hard, doubt wins.

    It’s that simple.

    But, hey, pretty pictures. An “A” for presentation. Once the seeds of doubt have been ripped out of the soil, they should be quite useful.


  41. 91
    ccpo says:

    From # 82

    “Statistical analysis of our data revealed that this decline is attributable to perceptions of recent weather changes by the minority of Americans who have been skeptical about climate scientists.”

    I rest my case.

  42. 92
    Chris Dudley says:

    Figures 3 and 5 should give the temperature units (K and C respectively) along the vertical axis. It would be good to see units for the vertical axis of fig. 1 since there are numbers. The animation in Fig. 4 is quite good but I think there is a problem with the description. If the atmosphere is less transparent, that is increased optical depth rather than reduced. A perfectly transparent atmosphere has zero optical depth.

    This abstract which seems to simply state that BAU means everybody dies is intriguing: A post on it would be appreciated. Thanks and keep up the good work.

  43. 93
    Chris Colose says:

    Thanks Eli :-)

    In addition, there is another blog by a physical chemist which has a multi-series post about radiation, absorption by gases, the GHE, etc which is very good but seems to have escaped the attention of the blogosphere. It is written at a fairly technical level but I encourage readers to have a look

  44. 94
    Guy Walton says:

    Guy says:

    Hey Gavin,

    This is my first reply to any Real Climate post. I became convinced that AGW was a major problem for the future of the planet back in the late 1980’s. I am one of the co-authors of the Records Study posted under the following credentials:

    “Relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S.”, was in Geophysical Research Letters.

    The complete citation is

    Meehl, G. A., C. Tebaldi, G. Walton, D. Easterling, and L. McDaniel (2009), Relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S., Geophys. Res. Lett., 36, L23701, doi:10.1029/2009GL040736.

    The public, especially in the U.S. has lost interest in the global warming issue mainly due to the fact that there has been a lack of extreme heat and major U.S. hurricane landfalls in the last few years. Our last major heat wave occurred in August 2007. Also, there has not been a major hurricane hitting the U.S. since Katrina, Rita and Wilma in 2005. Odds are that our “luck” is about to run out. The records study that I was a part of demonstrates, statistically, that deadly heat waves during the summer are becoming increasingly more likely. The study indicates that the ratio of record highs to record lows is increasing for every month of the calendar year: thus, during the summer months in the future record heat will be a more likely phenomenon than weather patterns that produce relatively cool conditions. The Northeast is currently in the throes of a major heat wave. The weather pattern in the Atlantic this season is conducive for above average tropical activity. I’m a meteorologist, not a climatologist, although I have taken a great interest in that field of science in the last decade. I think that it’s important to relate day to day and monthly weather with long term climate trends. Only then will be public become more inclined to pay attention to the issue of AGW.

    Thanks for keeping up the Real Climate site. I’ve enjoyed reading posts from the site for roughly the last five years.

  45. 95
    Patrick 027 says:

    Re 89 Ron DeWitt and everyone – I think Ron DeWitt’s comment illustrates perhaps a common misunderstanding. Based on the definitions many would use, they would describe the climate system, as understood by climate scientists in general including the author of this post, as have a net negative feedback to forcings in general (that are not too idiosyncratic).

    In the convention of climate change jargon, one very important negative feedback is not counted as a ‘feedback’ because it is taken more as a ‘given’. This is the ‘Planck response’ – the change in emission of radiation as a direct function of temperature. When climate scientists state that there is likely a net positive feedback, they mean ‘besides the Planck response’. The Planck response is included in the basic physics and in conceptual, simple, and complex models. If this were not the case, then absent other sufficient negative feedbacks, climate sensitivity would be infinite. Given only the Planck response, climate sensitivity for a doubling of CO2 would be around 1 K. If the other feedbacks combine to be a net positive feedback as expected, then the (Charney**) climate sensitivity for a doubling of CO2 would be around 3 K, give or take 1 K. This is larger than 1 K, but it is still finite, indicating a stable climate (including the Planck response as a feedback, the net feedback is negative, but smaller than if it were only the Planck response).

  46. 96
    Patrick 027 says:

    Regarding the graph of planets – unnecessary to the main point, but it might help to know how the ‘surface’ temperature of the gas giants is defined. For something roughly analogous to terrestrial planets, that might be the temperature at a depth between where solar heating dominates and where the geothermal flux dominates.

  47. 97
    Ron DeWitt says:

    I think I understand, Patrick. 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?

  48. 98
    Titus says:

    Ray Ladbury @77

    Sun activity here (there are shed loads more if you take a mo to look):

    You ask where I get the idea that climate science is primitive (your words): Well, it appears that the more powerful the computers the more we know we don’t know. I’m not trying to be discourteous but please take a wider look at what’s going on and you may understand a bit more about why folks have lost faith.

    [Response: I’d recommend a reading of ‘A Vast Machine’ (2010; MIT press) by Paul N. Edwards. -rasmus]

  49. 99
    Chris Colose says:

    Ron DeWitt (#89),

    Your comment is very confused, although admittedly, the discussion of feedbacks in the main post left way too much room for confusion unless the reader already knew what to make of it beforehand. Perhaps RC could do a related post that deals specifically with this issue. I will do a general outline here for anyone interested who has happened to stumble this far into the comments. Apologies for length…

    There are many radiative feedbacks: some positive, some negative. To be clear, rasmus was generally talking about the net effect. When we say “positive” and “negative” feedbacks in the sense of radiation (so I’m not talking about carbon-cycle responses such as methane release from the oceans or such) we’re referring to temperature-sensitive variables which themselves affect the radiation budget of the planet. Examples for clarification to come briefly. A “baseline” no-feedback scenario can be shown, through taking the derivative of the Stefan-Boltzmann law, to have a sensitivity of about 0.25 degrees Kelvin per W/m2 forcing. A doubling of CO2 (or close to a 2% change in solar irradiance) corresponds to a forcing of about 4 W/m2 and so the no-feedback response should be on the order of a degree temperature change. Whether the net effect of feedbacks is to be positive or negative depends on the temperature rise relative to this baseline. If the temperature increases by only 0.1 degrees for a 1 W/m2 forcing or 4 degrees per 1 W/m2 forcing, then you know the system is dominated by negative and positive feedbacks, respectively. We don’t generally call the Planck response (i.e., the increase in radiation with an increase in temperature) to be a “feedback” in the popular sense, because as I’ve just said, the “feedbacks” modify the Planck response (which is necessary to come back to equilibrium anyway). Part of the misunderstanding may come from the jargon used by the community, but we’ll have to deal with it. So, net positive feedback does not imply any runaway scenario (at least not a priori), it just means the temperature response is larger than what you’d get with just the radiative forcing acting.

    A net positive or negative feedback does not in itself say much about how prone the system is to being temperature-stable or prone to “running away.” The system can have a net negative feedback and still change very much provided a radiative forcing from sunlight or CO2 is sufficiently large, although for typical changes in these variables that Earth encounters, one would indeed expect only relatively small climate changes to occur if negative feedbacks did in fact dominate. Although the sensitivity of climate does change itself as the boundary conditions change, the past (PETM, glacial-interglacial cycles, etc) does not support sensitivities as low as 1 degree per doubling of CO2, and it doesn’t support very high ones (like 10 degrees per doubling) either. It does show that positive feedbacks are dominant, and for timescales of anthropogenic global warming about 2 to 4.5 degrees Celsius per doubling, and a bit higher if you include century-timescale “slower feedbacks” such as ice sheets. The primary radiative feedbacks are as follows–


    Water Vapor is a very important greenhouse gas, and the amount of it in the atmosphere is also strongly coupled to the temperature. We expect, through the Clausius-Clapeyron equation, that the specific humidity will increase roughly 20% in response to 3 degrees of warming provided the temperature and humidity vary in such a way as to keep the global relative humidity roughly constant. This is the most important feedback in terms of magnitude and it makes the Earth much more sensitive to climate changes of all sorts by making the OLR vs. T curve a bit more linear than T^4. This alone essentially doubles the sensitivity from the Planck-only radiation response. The extreme end of this, at very large optical depth throughout a deep part of the atmosphere, is when the OLR slope flattens out to a horizontal line and the outgoing emission becomes completely decoupled from the surface temperature, which is when a runaway greenhouse can kick in. Earth is quite far from this limit.


    Suppose the planet in consideration has a surface of high albedo surrounded (or on top of) a surface of lower albedo, and the extent of such a surface is temperature-dependent. This is particularly the case for snow and ice on Earth which tend to be surrounded by relatively dark ocean or vegetation. Decreasing the ratio of the high albedo to low albedo surface increases the solar absorption. Such an issue provides a positive feedback and also underlies the amplification of temperature in the Arctic, which has recently clearly emerged in observations. The loss of sea ice cover (dominated by extent and only slightly influenced by thickness) allows for a change in the heat fluxes between ocean and atmosphere (heat absorbed by the ocean in summer is released back to the atmosphere, growing heat transfer in the colder months as decades pass). It’s not very strong in the summer when energy is used for evaporation or melting but is particularly pronounced in the Autumn and should emerge even stronger in the winter as time progresses.


    As Rasmus noted, the strength of the greenhouse effect is sensitive to the vertical temperature structure of the atmosphere. Moist adiabats themelves change with temperature (see figure: changing the moist stability of the atmosphere. In the tropics which are prone to deep convection, the water vapor response to warmer temperature also promotes a less steep lapse rate owing to latent heat effects. This is the popular upper troposphere amplification or “hotspot” as is often discussed in connection with observation vs. models in the tropics. With a reduced lapse rate comes a reduced greenhouse effect (the situation is opposite at the poles but tends to be a negative feedback globally). This partially offsets the water vapor feedback and because of the strong coupling it is typical the combined WV+LR feedback instead of the two individually (interesting the model spread for WV+LR is smaller than either of the two feedbacks considered on their own). See figure:


    This is a big uncertainty in the climate system response and I don’t personally have a great background in all of the various model/observation/theory developments of the last few years. In brief, it’s complicated because clouds have different impacts (generally lower clouds cool since the albedo influence dominates and high clouds warm since their greenhouse role dominates) and also depends upon latitude and other things. Clouds exert a profound influence on the shortwave and longwave parts of the energy budget and trying to find the difference between two large and competing effects is problematic.

    The longwave component of cloud feedbacks being positive tends to be a robust result of models, and model spread is primarily from the shortwave part of the response. A compelling argument for the positive longwave response is a leading alternate to Lindzen’s IRIS although it receives less attention, and is known as the FAT hypothesis (from Dennis Hartmann) and arises from the fundamental physics of convection only heating the atmosphere where radiative cooling is efficient, and thus the temperature at the top of convective cloudiness should be near constant as it becomes warmer. There is some evidence (e.g., Clement et al, Science) for regional positive feedbacks from albedo, but there is no widespread agreement in the community or amongst models as to the size and sign of this influence. A related issue is that clouds are not readily resolved in GCM’s but must be parameterized, leaving room for a wide variety of plausible feedbacks.

  50. 100
    EL says:

    [All suggestions welcome. – gavin]

    Who is your target audience?