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What is the net feedback from clouds?

What the science says...

Select a level... Basic Intermediate

Evidence is building that net cloud feedback is likely positive and unlikely to be strongly negative.

Climate Myth...

Clouds provide negative feedback

"Climate models used by the International Panel on Climate Change (IPCC) assume that clouds provide a large positive feedback, greatly amplifying the small warming effect of increasing CO2 content in air. Clouds have made fools of climate modelers. A detailed analysis of cloud behavior from satellite data by Dr. Roy Spencer of the University of Alabama in Huntsville shows that clouds actually provide a strong negative feedback, the opposite of that assumed by the climate modelers. The modelers confused cause and effect, thereby getting the feedback in the wrong direction." (Ken Gregory)

The effect of clouds in a warming world is complicated. One challenge is that clouds cause both warming and cooling. Low-level clouds tend to cool by reflecting sunlight. High-level clouds tend to warm by trapping heat.

clouds

As the planet warms, clouds have a cooling effect if there are more low-level clouds or less high-level clouds.  Clouds would cause more warming if the opposite is true.  To work out the overall effect, scientists need to know which types of clouds are increasing or decreasing. 

Some climate scientists, such as Richard Lindzen and Roy Spencer, are skeptical that greenhouse gas emissions will cause dangerous warming. Their skepticism is based mainly on uncertainty related to clouds.  They believe that when it warms, low-level cloud cover increases. This would mean the Earth's overall reflectiveness would increase. This causes cooling, which would cancel out some of the warming from an increased greenhouse effect. 

However, recent evidence indicates this is not the case. Two separate studies have looked at cloud changes in the tropics and subtropics using a combination of ship-based cloud observations, satellite observations and climate models. Both found that cloud feedback in this region appears to be positive, meaning more warming.

Dessler (2010) used satellite measurements of cloud cover over the entire planet to measure cloud feedback.  Although a very small negative feedback (cooling) could not be ruled out, the overall short-term global cloud feedback was probably positive (warming).  It is very unlikely that the cloud feedback will cause enough cooling to offset much of human-caused global warming.

Other studies have found that the climate models that best simulate cloud changes are the ones that find it to be a positive feedback, and thus have higher climate sensitivities.  Steven Sherwood explains one such study:

While clouds remain an uncertainty, the evidence is building that clouds will probably cause the planet to warm even further, and are very unlikely to cancel out much of human-caused global warming.  It's also important to remember that there many other feedbacks besides clouds. There is a large amount of evidence that the net feedback is positive and will amplify global warming.

Basic rebuttal written by dana1981


Update July 2015:

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial

This rebuttal was updated by Kyle Pressler in September 2021 to replace broken links. The updates are a result of our call for help published in May 2021.

Last updated on 25 July 2017 by skeptickev. View Archives

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Comments 101 to 125 out of 261:

  1. Sphaerica (RE: 98), "How do you get "half?" Citation, please." Trenberth actually has it being less than half - meaning more of the surface emitted energy absorbed by the atmosphere is emitted up out to space than is emitted down toward the surface (specifically, he has 157 W/m^2 emitted down and 169 W/m^2 emitted up). So using his numbers for this component would actually result in even more energy reflected away than retained. That aside, when the atmosphere (clouds or clear sky) absorbs surface emitted LW infrared, it re-emits it equally in all directions, which means the net effect through the whole atmosphere is half goes up out to space and half goes down toward the surface. Using Trenberth's numbers, he has the surface emitting 396 W/m^2. Of this, he has 70 W/m^2 passing through unabsorbed straight out to space (40 W/m^2 through the clear sky and 30 W/m^2 through the cloudy sky). 396 - 70 = 326 W/m^2 absorbed by the atmosphere. He then has 169 W/m^2 designated as being emitted up out to space, so 326 - 169 = 157 W/m^2, which is the remainder and is the amount emitted down toward the surface. 70 + 169 + 157 = 396 W/m^2 total emitted by the surface. 70 + 169 = 239 W/m^2 emitted to space. 239 coming in from the Sun + 157 emitted down from the atmosphere = 396 W/m^2 at the surface. A lot of confusion lies in Trenberth having 78 W/m^2 of the incoming energy from the Sun absorbed by the atmosphere and brought to the surface as 'back radiation'. Only it's not 'back radiation', it's 'forward radiation' yet to reach the surface that last originated from the sun - as opposed to 'back radiation' being energy that last originated from the surface. These distinctions are absolutely critical to understanding the constraints Conservation of Energy puts on the system relative to all the energy flows.
    Response: [DB] "it's 'forward radiation' yet to reach the surface that last originated from the sun" Parse:Fail
  2. RW1 - I begin to see the problem. You are not distinguishing between the current state of affairs (from the Trenberth diagram) and the feedbacks, or how those values will change with temperature changes. All of the studies show a mean estimate of slightly positive feedback for clouds, with Dessler's including a small possibility of cloud feedback being negative. Do you have any issues with the referenced papers mentioned in this thread? I do not recall seeing any such in this thread so far. If you don't, then you are not actually addressing feedback.
  3. 100, RW1,
    ...one piece of evidence among many in support of negative cloud feedback...
    Aside from your calculations (which we're still trying to work through), what other evidence do you have of negative cloud feedback?
  4. Sphaerica (RE: 93), "5. You think the reduction is so great that 1.9 isn't a problem -- it's not a dangerously high amount of warming. I have not made this claim, though I'm not sure a 1.9 C rise would be 'dangerous'. "6. You discount the fact that the lapse rate feedback might not be as great as estimated, or that CO2 and albedo feedbacks might be greater or kick in sooner than estimated, or that anthropogenic CO2 additions could actually increase with human population growth and expanding industrialization. 7. You discount the fact that other studies point to a climate sensitivity of 3+, which imply that that the cloud feedback estimates are either correct, or that any error is offset by underestimations of other positive feedbacks (or over-estimation of the lapse rate negative feedback). 8. You discount the fact that we've already seen the climate warm by 0.8˚C this century without a noticeable negative cloud feedback, i.e. that the climate is obviously sensitive enough to swing 0.8˚C in a mere 100 years (0.6˚C of that in only the last 30 years) despite your proposed fast acting negative cloud feedback." You're putting a lot of words in my mouth and making a lot of assumptions. I have not discounted anything entirely. Lots of things or combinations of things are theoretically possible. All the evidence has to be weighed very carefully. There are many lines of major contradictory evidence against net positive cloud feedback. I have presented many here, but the simplest and perhaps most significant is positive cloud feedback for GHG 'forcing' at the surface is contradictory to the system's response to solar 'forcing' at the surface. For additional GHG 'forcing' it's nearly a factor of three greater. Ultimately, for net positive cloud feedback to be supported, this has to be explained by quantifying specifically how the feedback causes this much change for the next 3.7 W/m^2 at the surface from GHG 'forcing' and why it does not for the original 98+% (239 W/m^2) incident on the surface from the Sun.
  5. 101, RW1,
    Trenberth actually has it being less than half...
    This is your interpretation, and it's incorrect, as you will see below.
    ...which means the net effect through the whole atmosphere is half goes up out to space and half goes down toward the surface.
    This is incorrect (probably because it is oversimplified).
    That aside...
    This entire paragraph is a misinterpretation of the diagram. You are reading too much into it and seeing things that are not there. 40 pass through the atmosphere (cloudy or not) directly from the surface into space. 356 are absorbed by either the clouds or the atmosphere. His diagram does not distinguish. 30 are emitted from clouds into space. You cannot say this is part of the 396 (some of it will be from thermals and evapostranspiration/latent heat, and some is the energy absorbed by the atmosphere from inbound radiation... the individual sources are not relevant, and cannot really be separated). 169 is emitted into space from the atmosphere. Again, like the 30 from clouds, you cannot separate this by source. 333 is radiated back down the surface, again from the atmosphere and clouds, without distinction. Specifically, in your calculations:
    326 - 169 = 157 W/m^2
    You can't do this, because the 169 is from the atmosphere, which is also heated by solar input, thermals and evapostranpiration/latent heat. You can't just allocate all of that to the radiation from the surface.
    157 emitted down from the atmosphere...
    No. You can't say this. 333 emitted down from the atmosphere + clouds, that's as much as you can specify.
    A lot of confusion lies in Trenberth having 78 W/m^2 of the incoming energy from the Sun absorbed by the atmosphere and brought to the surface as 'back radiation'.
    But he doesn't have that. In his diagram (which is already a simplification) it is absorbed by the atmosphere and you can't then say where it goes, any more than you can identify the molecules from a cup of water after you've dumped it into a boiling pot. The atmosphere is heated from above by the sun (78), and from below through the processes of radiation (356), thermals (17), and evapostranspiration/latent heat (80). It sheds heat by radiating upward (199, 169 atmosphere + 30 clouds) and downward (333). You cannot do from these numbers what you are trying to do, which is to allocate some measure of this specifically to clouds. Which isn't to say that it can't be done from appropriate sources, or that the information isn't available, only that your calculations here do not and can not do it.
  6. 104, RW1,
    positive cloud feedback for GHG 'forcing' at the surface is contradictory to the system's response to solar 'forcing' at the surface.
    Please explain. This makes no sense.
    For additional GHG 'forcing' it's nearly a factor of three greater.
    Please explain. This makes no sense.
    Ultimately, for net positive cloud feedback to be supported, this has to be explained by quantifying specifically how the feedback causes this much change for the next 3.7 W/m^2 at the surface from GHG 'forcing' and why it does not for the original 98+% (239 W/m^2) incident on the surface from the Sun.
    Please explain. This makes no sense to me whatsoever.
  7. Sphaerica (RE: 103), "Aside from your calculations (which we're still trying to work through), what other evidence do you have of negative cloud feedback?" I don't think you've been paying attention. I've presented much more evidence than just my initial calculations (I suggest you go back and reread the thread from the beginning), but burden of proof is on those claiming a positive cloud feedback, which subsequently leads to an enhanced warming warming of 3 C, rather than a more modest 1 C in line with the system's directly measured response to surface incident energy.
  8. 107, RW1,
    I've presented much more evidence than just my initial calculations...
    Like what? I remember nothing of the sort. I've seen nothing of any weight or importance in anything you've said so far. And no, I'm not going back to re-read the convoluted thread. If you have valid points to make, it is easy enough to state them. An unwillingness to do so speaks volumes.
    ...but burden of proof is on those claiming a positive cloud feedback...
    Huh? And why would that be? The burden of proof falls on the person trying to convince other people. I'm not trying to convince you of anything. You're trying to convince me. And so far you have a set of calculations that are invalid, logic that is insufficient, and hand waving supported by "I don't think you've been paying attention."
  9. Sphaerica (RE: 105), "This entire paragraph is a misinterpretation of the diagram. You are reading too much into it and seeing things that are not there. 40 pass through the atmosphere (cloudy or not) directly from the surface into space. 356 are absorbed by either the clouds or the atmosphere. His diagram does not distinguish." Have you read the paper? The diagram does distinguish. If you look carefully, it shows that of the 396 W/m^2 emitted at the surface, 40 W/m^2 of it passes straight through the atmosphere to space and 30 W/m^2 of the 396 is emitted through the clouds for a total transmittance of 70 W/m^2 - meaning 70 W/m^2 of the surface emitted 396 goes straight out to space without being absorbed by the atmosphere. "169 is emitted into space from the atmosphere. Again, like the 30 from clouds, you cannot separate this by source." This isn't correct because the clear sky only absorbs a total of 131 W/m^2 (0.33 x 396 = 131). If the clear sky is emitting a total of 169 W/m^2 up - that's more than the energy it absorbs, which is impossible. The 169 W/m^2 represents the total amount the atmosphere, both cloudy and clear sky, is emitting up out to space.
  10. 109, RW1,
    ...the clear sky only absorbs a total of 131 W/m^2 (0.33 x 396 = 131)...
    Where do you get this from?
    If the clear sky is emitting a total of 169 W/m^2 up - that's more than the energy it absorbs, which is impossible.
    But you can't tell how much the "clear sky" absorbs. The "clear sky" does not absorb separately from the clouds. The two are not distinct. Clouds form and dissipate very quickly. There are only three components to the ("Trenberth") system; space, atmosphere, and surface. Different components and interactions are represented in each (land/ocean at the surface, sky/clouds in the atmosphere), but you can't separate them for the purposes of Watts bookkeeping (except where he has explicitly done so). The atmosphere absorbs 78 from inbound solar radiation, 17 from thermals, 80 from evapostranspiration/latent heat, and 356 from surface radiation, for a total of 531. It radiates 169 to space from the atmosphere, 30 to space from clouds, and 333 back to the surface, for a total of 532. So the entire system gets 290 in from space, and sends 290 out to space. The atmosphere gets 531/532 in, and sends 531/532 out. The surface gets 517/516 in, and sends 516/517 back out... Each is in balance. You cannot say that the clear sky absorbs 131.
    Response: [DB] FYI, Chris Colose is explaining something along these lines to Kevin McKinney over at Open Mind as we speak type. Very understandable.
  11. RW1 - "...40 W/m^2 of it passes straight through the atmosphere to space and 30 W/m^2 of the 396 is emitted through the clouds..." No - 30 W/m^2 is emitted by the clouds, heated by radiation, convection, and latent heat. This is not a direct re-radiation or window through the clouds.
  12. RW1 - "This isn't correct because the clear sky only absorbs a total of 131 W/m^2 (0.33 x 396 = 131). If the clear sky is emitting a total of 169 W/m^2 up - that's more than the energy it absorbs, which is impossible." As Sphaerica quite clearly states, this is completely incorrect. The atmosphere receives about 532 W/m^2 from various sources, and puts out about 532 W/m^2, thus conserving energy. 131 of the input is direct solar energy. 169 of the output is IR to space. Each is just a portion of the energy flowing through the atmosphere, no impossibilities whatsoever. You are displaying a serious misunderstanding the Trenberth energy diagrams, which explains many of the (incorrect) issues you have raised.
  13. Sphaerica (RE: 110) "RW1, ...the clear sky only absorbs a total of 131 W/m^2 (0.33 x 396 = 131)... Where do you get this from?" From the ISCCP data, which says that clouds cover 2/3rds of the surface. This means 1/3rd of the surface is clear sky (i.e. cloudless). "But you can't tell how much the "clear sky" absorbs. The "clear sky" does not absorb separately from the clouds. The two are not distinct. Clouds form and dissipate very quickly." The average coverage is what matters to the energy flows. If the surface is 1/3rd clear sky, this means that 1/3 or 0.33 of the average emitted surface power passes through the clear sky. If my calculations are in error, why do they accurately predict the correct brightness temperature of 255K?
  14. Sphaerica (RE: 110) "The atmosphere absorbs 78 from inbound solar radiation, 17 from thermals, 80 from evapostranspiration/latent heat, and 356 from surface radiation, for a total of 531. It radiates 169 to space from the atmosphere, 30 to space from clouds, and 333 back to the surface, for a total of 532. So the entire system gets 290 in from space, and sends 290 out to space. The atmosphere gets 531/532 in, and sends 531/532 out. The surface gets 517/516 in, and sends 516/517 back out... Each is in balance." Show me the energy in = energy out calculations that demonstrate COE is being satisfied? There is 239 W/m^2 coming in and 239 W/m^2 leaving. The surface cannot be "getting" 517 watts in, as it's only emitting 396 W/m^2.
  15. 113, RW1,
    From the ISCCP data, which says that clouds cover 2/3rds of the surface. This means 1/3rd of the surface is clear sky (i.e. cloudless).
    No. The ability of clouds to absorb IR is different from "clear sky" (i.e. the atmosphere). One cannot simply take a percentage. It's a meaningless estimation.
    If my calculations are in error, why do they accurately predict the correct brightness temperature of 255K?
    Where do you do that, and how?
  16. 115, RW1,
    The surface cannot be "getting" 517 watts in, as it's only emitting 396 W/m^2.
    You are ignoring the 23 reflected, 17 transported through thermals and 80 transported through evapotranspiration (396 + 23 + 17 + 80 = 516).
    Show me the energy in = energy out calculations...
    Apologies... I mistyped the 290. The number is for space is 341 in and 341 out (79 reflected by clouds + 23 reflected by the surface + and 239 emitted by the atmosphere/clouds). The 239 comes from ignoring the reflected incoming radiation, which for all intents and purposes never affects the system. So 341 in - 102 reflected = 239. Similarly, 341 out - 102 reflected = 239. Everything balances.
  17. RW1, Now that we've identified the flaws in your calculations and can do away with them... 1) Do you have any actual evidence at all that cloud feedbacks would or should be negative? 2) Do you have any response to the question that I've posed 3 times (posts 27, 71, 90) and KR once (post 94)? For the fifth time, do you have any response to the fact that multiple studies, using a wide variety of methods, all point to a climate sensitivity of 3 or greater, and so the chance of cloud feedbacks being negative or neutral is slim to none?
  18. RW1 - In reference to Sphaerica's question, on cloud feedback, keep in mind that both Sphaerica and I (as in my post here) are asking about changes in cloud response from the current state, which are what will amplify or (as you seem to be arguing) damp temperature changes.
  19. 113, RW1, From the ISCCP data, which says that clouds cover 2/3rds of the surface. This means 1/3rd of the surface is clear sky (i.e. cloudless). What about cloud pressure? What about optical thickness? What about cloud albedo? How much is low cloud? How much is high cloud? What is the moisture content in the air column? What are the particulate concentrations in the air column? And there's more. Not quite so black and white is it?
  20. #92 Sphaerica at 21:57 PM on 21 April, 2011 I'd suggest that of that 341, since 78 is absorbed by the atmosphere, only 263 is available to be reflected (although this is a gross estimate, since it's more complex than that). If one assumes a cloud cover of .66 then 174 of that 263 is subject to cloud cover. 79 reflected from 174 gives .45, which is well within the ranges given by Hansen 1998 -- even at the upper end. No, that's not correct. Trenberth's figure clearly shows that according to him from the average 341 W/m2 incoming shortwave solar radiation at ToA 78 W/m2 is "Absorbed by Atmosphere" and 79 W/m2 is "Reflected by Clouds and Atmosphere" (right back to space, yes). Therefore a fraction of that 79 W/m2 is reflected by the atmosphere, not clouds, so somewhat less than 79 W/m2 remains to be reflected by global cloud cover. If cloud fraction is 0.66 as you say and no short wave radiation is reflected by the atmosphere from cloud free regions, then average cloud albedo is 79/341/0.66 (=0.35) which is way too small (should be more than 0.42). On the other hand if there is some reflection from the atmosphere in cloud free regions, average cloud albedo comes out even smaller. It may be the case that average cloud fraction projected to a plane perpendicular to incoming solar rays is much smaller than 0.66. It should be close to 0.5 to bring cloud albedo back to a reasonable range. It would simply mean cloud fraction around polar regions is much higher than the global average while it is lower above the rest of the globe. I do not know if it is the case or not, I have not seen data on average latitudinal distribution of cloud cover. If anyone knows a link to such measurements, that could help a lot. However, if cloud fraction is so high in polar regions indeed, it would diminish ice-albedo feedback tremendously, because, unlike in glacial times, there is not much ice to be melted elsewhere.
  21. 120, BP, I disagree with your logic. You cannot count the 78 absorbed by the atmosphere as eligible to be reflected. So it's not 79 of 341, it's 79 if 341-78, or 79 of 263, so it's not 79/(341*0.66)=0.35, it's 79/(263*0.66)=0.455. And, again, the 0.66 is a gross estimate. After all, what matters isn't how much of the earth's surface is under cloud cover, but how much of the daylight surface, at what angle of incidence (as you said, latitudinal distribution, as well as proximity to the day/night boundary), and what type of clouds. All in all, to me it's a total non-issue. I see no reason to doubt the numbers. Especially since, in the scheme of things, this is a bird's-eye view over-simplification diagram meant to help people better understand how energy moves through the earth's climate system, and what the term "energy budget" actually means. It's not like this is the foundation for all AOGCMs and all of climate science.
  22. Sphaerica is quite correct - the Trenberth diagrams are summaries of a great deal of data, not a climate model themselves. Which is why attempting to model cloud feedback directly and solely from the Trenberth energy 'budget' without understanding the underlying physical processes and their response to changes is rather quixotic.
  23. #122 KR at 03:55 AM on 23 April, 2011 Sphaerica is quite correct Sphaerica is not correct and you know it. It is math, not a matter of opinion.
  24. Berényi - Indeed, it's a matter of math. And physics. Tied to observations. However, Sphaerica is correct in stating that the Trenberth numbers are summaries, not a climate model, and playing with the numbers does not replicate actually considering the physical interactions of climate elements! As to the cloud effects, I suggest you look at the Trenberth 2009 paper for details. The term "cloud" occurs ~30 times in the text (not counting references to CloudSat), and in >12 bibliography titles; clouds were recognized as a major source of inter-estimate variability when that was written, and Trenberth et. al. put a lot of work into it. In other words, if you disagree with the numbers, address the paper that presented the numbers!
  25. What problems do you all have with the figure? Reflected solar radiation (101.9)/Incoming solar radiation (341.3) = 0.298 (global albedo) What is that figure doing here? I don't see you are going to get any of it as it looks pretty "constant", don't you think? No detail about low and high clouds can be got from there. Then no delta something or delta nothing, as you like. It looks quite an off-topic in this post. Just the same old song played the last few days with Trenberth being hiding behind everything because, he got mail!

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