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

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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 151 to 175 out of 261:

  1. RW1#149: "net effect of clouds at night is to warm (or slow heat loss)... Again, this is not in dispute, nor does it conflict with net negative feedback for clouds." Oh, that makes it all perfectly clear. A mechanism of warming, 'not in dispute,' doesn't conflict with net negative feedback? Do you have any evidence to support this astounding statement? Or does your model simply require that you assume a net negative feedback and thus do not need to bother with any evidence?
  2. 150, RW1, What else does Dessler say in his paper? Let me give you a hint... your logic that if the effect of clouds is a net negative in the current climate, then a changed climate must have more clouds with the same net negative effect, only more so, is simplistic. It matters what types of clouds form, where they form, and when they form. More clouds in northern latitudes in winter will not nave a net cooling effect. High clouds made of ice crystals that are transparent to visible light will not have a net cooling effect. Clouds that build beyond a certain critical depth will not have a net cooling effect. It's more complicated than your simplistic logic makes it sound.
  3. muoncounter (RE: 151), "Oh, that makes it all perfectly clear. A mechanism of warming, 'not in dispute,' doesn't conflict with net negative feedback? Do you have any evidence to support this astounding statement?" I'm not sure I understand your objection. The net effect of clouds, globally averaged (night and day combined), is to cool. The fact that clouds are much better at trapping outgoing surface emitted radiation than the clear sky, which makes their net effect at night to retain more energy than the clear sky, would only be significant it was more than the energy the clouds reflected away during the day.
  4. Sphaerica (RE: 15), "your logic that if the effect of clouds is a net negative in the current climate, then a changed climate must have more clouds with the same net negative effect, only more so, is simplistic." No, not really. My point is there is really no physical or logical reason why the net cloud feedback would suddenly switch from negative (cooling) to positive (warming) on the next few watts incident on the surface, especially to cause a net warming effect so much greater than the response of the system to the original 98+% (239 W/m^2) incident on the surface from the Sun.
  5. Sphaerica (RE: 152), "It matters what types of clouds form, where they form, and when they form. More clouds in northern latitudes in winter will not nave a net cooling effect. High clouds made of ice crystals that are transparent to visible light will not have a net cooling effect. Clouds that build beyond a certain critical depth will not have a net cooling effect. OK, explain specifically how all the various cloud types will respond in a way that results in a net warming effect on the next few watts at the surface and specifically why all different cloud types don't respond this way to the original 239 W/m^2 incident on the surface from the Sun. "It's more complicated than your simplistic logic makes it sound." I know the balance is complicated. It doesn't change the fundamentals. Besides, Dessler doesn't get into different types of clouds in his analysis.
  6. 154, RW1,
    ...there is really no physical or logical reason why the net cloud feedback would suddenly switch...
    But there is. You're just denying that it exists. Hint: Go to the top of this web page and actually read the post.
  7. Sphaerica (RE: 156), "But there is. You're just denying that it exists. Hint: Go to the top of this web page and actually read the post." I have read the post. I do not see where the issues I've raised has been addressed or answered.
  8. RW1 My point is there is really no physical or logical reason why the net cloud feedback would suddenly switch from negative (cooling) to positive (warming) on the next few watts incident on the surface It doesn't need to switch to a net positive effect in order to be a positive feedback. If the total effect is currently negative, and a warmer climate causes structural changes in cloud cover that makes the total effect less negative, then that will result in additional warming beyond the original forcing. In other words less cooling is equivalent to warming.
  9. 157, RW1,
    I do not see where the issues I've raised has been addressed or answered.
    That's because you ignore the statements that do address them. 1. Your theory is inconsistent with all of the lines of evidence which point to a climate sensitivity of 3˚C or greater. You have seen this presented to you now at least 6 times, and you keep dodging it. How does your theory account for this? Until you answer that question, your theory fails. 2. Your theory is inconsistent with the observational evidence (Dessler 2010) that demonstrates a positive, not negative, feedback in response to short-term warming. While this cannot necessarily directly support a long-term positive effect, it directly refutes your "was negative before, so must be negative in the future" theory. 3. Your argument that models "assume" and require a positive cloud feedback is wrong. While clouds do represent a large area of uncertainty in the models, it is incorrect to think that the scientists who have done the modeling have not thought things through a little more carefully, and in more detail, than you have. You cast aspersions by generalizing their work into the word "assume," and yet provide no direct evidence (other than the general positive/negative thing) that their work is not well considered. 4. Your argument that current warming "depends" on the positive cloud feedback is exaggerated, as has been demonstrated. It reduces potential warming from 3˚C to 2.5˚C or 2˚C, which while helpful is not an inconsequential amount of warming. Like many deniers, you exaggerate one point to try to make it the single, decisive, "AGW killing" argument. 5. Your argument that cloud effect is negative and would not "switch" to positive is at its root flawed and too simplistic. It's rather like saying that May was warmer than April, and June was warmer than May, so every month from now on will get warmer and warmer forever. It takes a too simple premise, and draws an invalid conclusion, because it avoids the complexities of the system. Instead of Occam' razor, your theory uses Occam's guillotine. It is based on a very broad, general, simplistic approach to the problem. It does not consider any details in the issue, such as why the current effect would be negative, or how clouds might or will change. It assumes that all effects are linear and additive. It simply takes the childishly simple view that if current net effect is negative, then a warmer climate must mean more clouds, which must mean more negative. You may feel that this logic is persuasive, and for simple minded people who like to stop thinking as soon as they see an argument that supports their predetermined beliefs, this might well be persuasive. But it doesn't persuade me in the slightest. It matters very, very much where additional clouds form in a warming world (high latitudes, or near the equator), when they exist (during daylight, summer hours, or nighttime or winter hours), and what kind of clouds form (i.e. low, reflective clouds or high, heat trapping clouds).
  10. Sphaerica (RE: 159) "1. Your theory is inconsistent with all of the lines of evidence which point to a climate sensitivity of 3˚C or greater. You have seen this presented to you now at least 6 times, and you keep dodging it. How does your theory account for this? Until you answer that question, your theory fails." I addressed this already HERE. To delve into all those things here would be off topic. Moreover, it is absurd to think that I (or anyone else) is obligated to explain each line of evidence they present in the context of every other line of evidence in the whole subject of climate science.
  11. 106, RW1,
    I addressed this already HERE.
    Really? You call that "addressed?" This was what you said:
    Look, I can only deal with one thing at a time.
    So, for the sixth time you've dodged the question, as well as my other points. You made the following statement:
    I do not see where the issues I've raised has been addressed or answered.
    I pointed out:
    That's because you ignore the statements that do address them.
    I then proceeded to itemize where and how the issues you've raised have been addressed and answered, and I took you to task to yourself address the discrepancies. And you ignored them. Instead your response now is basically that you already have addressed them (you haven't), and that you don't have to... because justifying your personal theories about net cloud feedbacks would be off topic on a thread about cloud feedbacks (although that didn't stop 50 meandering posts about Trenberth's energy budget diagram when you thought that in some way buttressed your personal theories).
  12. Sphaerica (RE: 161), "So, for the sixth time you've dodged the question, as well as my other points." That's what you think. "You made the following statement: 'I do not see where the issues I've raised has been addressed or answered.' I pointed out: That's because you ignore the statements that do address them. I then proceeded to itemize where and how the issues you've raised have been addressed and answered, and I took you to task to yourself address the discrepancies. And you ignored them." No, I've largely ignored your declarations that the issues I've raised are incorrect. Declarations are not scientific discussion.
    Response: [DB] Actually, hand-waving aside, it is indeed pretty clear you've been dodging questions. And not just Sphaerica's.
  13. DB, Which specific questions have I been 'dodging'?
    Response: [DB] Start with Sphaerica's.
  14. Sphaerica (RE: 159), "2. Your theory is inconsistent with the observational evidence (Dessler 2010) that demonstrates a positive, not negative, feedback in response to short-term warming. While this cannot necessarily directly support a long-term positive effect, it directly refutes your "was negative before, so must be negative in the future" theory." Dessler 2010 is primarily just looking at TOA net fluxes and temperatures – he’s made little (if any) attempt to carefully discern cause and effect, and has not come up with any physical reasons or mechanisms behind his interpretation of the data. He admits in the paper at the beginning that the net effect of clouds in the current climate is to cool by about 20 W/m^2, yet doesn’t seem to ascribe much significance to this or express any curiosity as to why this is the case when clearly it needs to be explained. Furthermore, I notice that the SW component is also positive, which seems to be consistent with decreasing clouds causing the warming rather than warming causing decreasing clouds. Even if the claim is warming causes decreasing clouds for positive feedback, how is this consistent with increasing water vapor from warming? Does increasing water vapor from warming cause decreasing clouds? That doesn’t make any sense since water vapor concentration drives cloud formation. It seems to me that unless Dessler can explain all of this and corroborate it with all the other data and system behavior, he really doesn’t have a case.
  15. Sphaerica (RE: 159), "3. Your argument that models "assume" and require a positive cloud feedback is wrong." It's not really "my argument". The IPCC report specifically says this in the fine print, which I have posted here. "While clouds do represent a large area of uncertainty in the models, it is incorrect to think that the scientists who have done the modeling have not thought things through a little more carefully, and in more detail, than you have. You cast aspersions by generalizing their work into the word "assume," and yet provide no direct evidence (other than the general positive/negative thing) that their work is not well considered." No, I have not seen the evidence their work on the cloud feedback has been carefully considered at all, especially because it seems to ignore the basic physics of water vapor and clouds.
  16. RW1 - You might be interested in Wylie 2005. They looked at various satellite measures of cloud cover over the last 20 years. The High Resolution Infrared Radiometer Sounder (HIRS) found no significant cloud trend, while the International Satellite Cloud Climatology Project (ISCCP), with slightly different instruments, found a slight decrease in cloudiness over that period. Keep in mind that while specific humidity (total amount of H2O) in the atmosphere may be rising with temperature, relative humidity (relative to the total amount air can hold at any temperature) may remain steady or even decrease. So - steady or decreasing clouds with rising temperatures, neutral or positive feedback to warming.
    Response:

    [DB] Fixed Link.

  17. RW1,
    Even if the claim is warming causes decreasing clouds for positive feedback, how is this consistent with increasing water vapor from warming? Does increasing water vapor from warming cause decreasing clouds? That doesn’t make any sense since water vapor concentration drives cloud formation.
    Your overly simplistic model of the system completely fails. It doesn't properly consider how clouds form, it doesn't understand that clouds do not need to decrease to provide a positive feedback, it doesn't account for the many varieties, locations (in space, meaning 3 dimensions, and time) of clouds, it fails on many, many other levels. At the same time, your interpretation of Dessler 2010 is flawed. You should probably read the paper several more times before commenting on it again. My advice would be to read more on the subject, and post less. Kitchen table science may make perfect sense to a lot of people, but it's still wrong, and your analysis is kitchen table science.
  18. Sphaerica (RE: 167), "Your overly simplistic model of the system completely fails. It doesn't properly consider how clouds form, it doesn't understand that clouds do not need to decrease to provide a positive feedback, it doesn't account for the many varieties, locations (in space, meaning 3 dimensions, and time) of clouds, it fails on many, many other levels. At the same time, your interpretation of Dessler 2010 is flawed. You should probably read the paper several more times before commenting on it again." I have read the paper multiple times. I even emailed Dessler for clarification on a few things. He admits outright that his analysis doesn't analyze any mechanisms, just data. That the net effect of clouds is to cool by 20 W/m^2 would have to be explained. Just claiming the derivative to changes in surface temperature is positive, as Dessler does, even though the net entire effect is to cool is not good enough.
  19. KR (RE: 166), "Keep in mind that while specific humidity (total amount of H2O) in the atmosphere may be rising with temperature, relative humidity (relative to the total amount air can hold at any temperature) may remain steady or even decrease. So - steady or decreasing clouds with rising temperatures, neutral or positive feedback to warming." This really doesn't make sense. The fundamental problem is that clouds are controlling the water vapor concentration in a highly dynamic manner. The evaporated water vapor condenses in the atmosphere to form clouds, and then the formed clouds precipitate out the water from the atmosphere. As the clouds form and remain in the atmosphere, they reflect more solar energy away because the clouds are more reflective than the surface beneath them. This is why the net measured effect of clouds is to cool rather than to warm. Dessler's analysis is essentially claiming that all of the sudden the incremental effect of clouds on the next little bit of warming will be the warm rather than cool.
  20. 168, RW1, No single paper needs to cover everything, all at once, to completely define something. He doesn't "admit" (and yes, I noticed your cleverly pejorative word choice) that he doesn't cover mechanisms, he simply "tells" you as much. You shouldn't have needed to bother him and ask such an obvious thing, which you would certainly have clearly known if you comprehended the paper. That said, I don't care how many times you've read the paper, you don't seem to be able to comprehend it or, rather, you don't seem to be able to separate what you believe (without adequate logic or evidence) from what current science says. The mechanisms have been explained to you several times. Clouds are not a uniform, one dimensional entity for which more clouds = more of the same. A large number of factors come in to play.
    • Are there more clouds at night, where their albedo is irrelevant?
    • Are there more clouds at the poles or northern hemisphere in winter, where they merely shield reflective snow and ice, and so the difference in albedo is nil, while their radiative warming effects are increased?
    • Are the clouds that form high altitude clouds of ice, that are pretty much transparent to visible light, but still have a strong GHG effect?
    • Do the excess clouds form over or under existing cloud layers, again having little net effect on albedo, but definitely adding their radiative effects?
    Your simplistic view that net cloud impact on temperatures is normally mildly negative, and therefore any increase in clouds due to warming must be comparably more negative, is inadequate. I think you'd be well served to study the topic more before commenting further. I also think you'd be better served by demonstrating some respect (not merely in the words you post, but in your attitude) to the men and women that are actively engaged in studying the science. Believing that you can out-think them when you can't even get a decent grasp on Trenberth's energy budget diagram is just a little bit of a Dunning-Kruger.
  21. Sphaerica (RE: 170), "The mechanisms have been explained to you several times. Clouds are not a uniform, one dimensional entity for which more clouds = more of the same. A large number of factors come in to play." I've never claimed clouds are uniform or a one dimensional entity. "Your simplistic view that net cloud impact on temperatures is normally mildly negative, and therefore any increase in clouds due to warming must be comparably more negative, is inadequate." A -20 W/m^2 net effect is not "mildy negative" - it is very strongly negative (or strongly cooling). In the absence of specifically identified and corroborated physical mechanisms to the contrary, this is an extraordinary claim requiring extraordinary proof. Ultimately, Dessler doesn't have it.
  22. Sphaerica (RE: 170) "Believing that you can out-think them when you can't even get a decent grasp on Trenberth's energy budget diagram..." Find the appropriate thread and I'll be happy to discuss/debate Trenberth's energy budget with you or anyone. Every time I've tried, my posts seem to end up getting deleted for being off topic. Moderators, is there a thread where we can discuss this? If not, can you start one?
  23. 171, RW1, Look. Your comments contain nothing of substance other than to deride Dessler without foundation. He did a study and found that the net change in cloud feedback was positive, which supports the contention of many, many current climate scientists. Make a point and support it, or stop commenting. If you have a valid mechanism by which this may not be so, submit it and support it. If you have data that refutes Dessler's claim, submit it and support it. If all you have to say is "Negative could feedback! Negative cloud feedback! Dessler must be wrong!" then you're wasting everyone's time.
    Response:

    [DB] Agreed.  No one wishes to waste their time by dialoguing with RW1 due to his unwillingness to learn and to properly support a position.

  24. Interesting thread. Is it still active? On mechanisms: Svensmark, Shaviv, J Kirkby et al all suggest a mechanism for cloud nucleation (and thus possibly formation) independent of temperature. I was surprised not to see this mentioned. Other possible mechanisms have been proposed, such as micro organisms prompting nucleation. http://www.wired.com/wiredscience/2011/05/microbes-make-rain/ So while RW1 has not proposed a mechanism, others appear to have. No hard evidence for those mechanisms, but they should perhaps be considered when assessing the evidence for other mechanisms such as co2 forcing.
    Response:

    [DB] "Interesting thread. Is it still active?"

    All threads here at SkS (all 4,000+) are active.  Many are temporarily dormant but any are free to be reawakened at any time.  Regular participants here follow the Recent Comments thread, so they will see any new comment made regardless of the thread it is posted on.

  25. lancelot#174: "Svensmark, Shaviv, J Kirkby et al all suggest a mechanism" Those gentlemen are all part of the 'it's cosmic rays' camp. There are a number of threads addressing their data here at SkS (search: cosmic rays), as well as this excellent RealClimate piece exploding their suggested mechanism.

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