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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 176 to 200 out of 261:

  1. 174, lancelot,
    So while RW1 has not proposed a mechanism, others appear to have.
    I'm afraid I don't see that. Can you be more specific? The question at hand is: "Given warming that results from anthropogenic CO2, what will be the resulting cloud feedback?" While the work you discuss is interesting, it does not in any way affect the topic at hand -- which is the direction and degree of feedback from clouds in response to warming. That cloud formation is impacted by many factors is obvious. That temperatures are impacted by unexpected but measurable factors such as cosmic rays, through cloud formation, is what is under debate in current science (and on other threads here at SkS) but at the current time is not in any way supported by the evidence available. But in any event, it is not relevant to this thread, which discusses cloud feedbacks, not clouds as a primary forcing.
  2. lancelot, So while RW1 has not proposed a mechanism," The mechanism is the whole of the atmospheric water cycle (ground state water -> evaporation -> water vapor -> clouds -> precipitation -> ground state water), which is the primary way the planet's energy balance and ultimately the globally averaged surface temperature is maintained despite such a large degree of local, regional, seasonal hemispheric and even sometimes globally averaged variability. In general, mechanistically, when clouds are increasing the climate is too warm and trying to cool (more or denser clouds are exposed to space, reflecting more of the Sun's energy) and when clouds are decreasing the climate is too cool and trying to warm (fewer clouds lets in more of the Sun's energy). If water vapor is the primary amplifier of warming, as claimed, what then is the controller? If not clouds via there ability to reflect incoming solar energy and precipitate out the water from the atmosphere, then what? Does anyone think it's just a coincidence that energy from the Sun drives evaporation of water? Is it just coincidence that evaporated water removes heat from the surface, condenses to form clouds and the clouds reflect the sun’s energy? Is it just a coincidence that the water precipitated out of the atmosphere emanates from clouds?
  3. That's a rather medieval style of argumentation RW1 - leading questions and anthropomorphism! So no, the climate isn't "trying" to do anything. It responds to forcings according to the laws of physics. Likewise the answer to your repeated "is it just coincidence?" is, in each case, "no, it's physics". RW1, the "controller" is very well understood. It's temperature. The water vapour content of the atmosphere rises as the atmospheric temperature rises. That is very well understood physics, and has been measured repeatedly in the real world. No leading questions about "coincidences" required! Your "mechanism" lacks both physics and evidence. One really needs to consider the response of the atmosphere in terms of humidity. I'm sure you agree that the absolute humidity of the atmosphere has risen as a result of greenhoue-induced warming. That's uncontestable - we can measure this. How about "relative humidity"? There is some evidence that the water vapour content of the atmosphere might maintain approximately constant relative humidity in a warming world. In such a situation there's no reason to expect the levels of clouds to increase since the atmosphere can maintain increased water vapour without enhanced condensation into clouds. If the water vapour content of the atmosphere rises (i.e. the observed increase in absolute humidity) at a level that is somewhat lower than required to maintain constant relative humidity, then the formation of clouds is more likely to be suppressed. That's consistent with the so far rather limited real world evidence. There's zero real world evidence that clouds increase in a warming world. On the contrary, the limited evidence supports a small positive cloud feedback in a warming world (see the top article of this thread).
  4. 177, RW1, Your "intelligent design" approach which requires that the earth's climate system be constructed and constrained like an electrical engineering project imposes any number of requirements (such as the idea that there be a single controller) which are quite simply fabricated. Your "coincidences" are mere observations of known physics. You imply that because they exist, they must be primary factors in the system, but you provide no evidence of such. You imply that because you haven't bothered to list any other mechanisms here, they cannot exist or must be inconsequential. Your position amounts to argument from ignorance. Your argument boils down to "I can only envision things happening this one way, so how can it be anything else?" Your position amounts to recognizing that you cannot touch the sun but it moves through the sky, therefore it must be the flaming wheel of the sky god's chariot. How could it be anything else?
  5. The climate system's energy balance is extremely dynamic. It's constantly changing spatially and in time - all the time, yet long-term averaged it's very tightly constrained. The global temperature anomaly barely moves by more than a couple tenths of a degree per year despite such variability. This is hardly consistent with net positive feedback on imbalances, let alone net positive feedback of 300%. Water vapor and clouds are also not a homogeneous distribution - they are by far most dynamic component of the entire atmosphere. The notion that the feedbacks of water vapor and clouds are both strong net positive cannot be reconciled with physical process and feedbacks that so tightly constrain planet's energy balance as a whole.
    Response:

    [DB] Fixed text.

  6. RW1 given an incoming heat flux, it is the emission of radiation that mainly constrains temperature. Why on earth the Stefan-Boltzman law, the most important negative feedback, is so often forgotten?
  7. Riccardo, I assume you're talking about the so-called 'Planck' response of about 3.3 W/m^2 per 1 degree of warming?
  8. RW1 wrote: "The global temperature anomaly barely moves by more than a couple tenths of a degree per year..." Which, is a massive amount of heat. You dismiss the idea of positive feedbacks (which BTW are estimated at a 200% increase on the forcings - 300% would be the combined total) because the average annual temperature anomaly is so 'constrained', but that is a completely groundless argument. I could just as easily claim that the massive average annual temperature anomaly shows how large the positive feedbacks must be. In reality, there is no direct connection between the two and thus the argument is meaningless. We know feedbacks are positive because every past and current warming forcing which we can estimate with any degree of accuracy has been enhanced by positive feedback effects. Frankly, it is ridiculous to claim that feedback effects will not be positive given that measurements of forcing and feedbacks from every El Nino, every Summer, and indeed every sunrise prove otherwise. The sun comes up, temperatures rise, dew evaporates, atmospheric humidity increases, that water vapor causes further warming... positive feedback. Happens all the time and not remotely controversial. The idea that this would suddenly just 'stop' happening if the warming comes from increasing atmospheric CO2 levels is without any justification at all.
  9. RW1 the so-called Plank sensitivity or response is derived from the Stefan-Bolzman law.
  10. CBDunkerson, Are you aware that the system is almost always in a state of energy imbalance to some degree? Have you noticed that even when the system as whole warms to a significant degree like during an El Nino event, it always seems to revert to its pre-equilibrium state fairly quickly afterward? This is hardly consistent with net positive feedback acting on perturbations. Also, the temperature rising as the Sun comes up each day is not the result of net positive feedback acting on the increased incident energy. You seem to be confusing a causative positive response to an increase in 'forcing' with positive feedback in response to the causative change. They are two different things. No one disputes that increased energy from GHGs or the Sun will cause warming.
  11. Riccardo (RE: 185), "the so-called Plank sensitivity or response is derived from the Stefan-Bolzman law." OK, explain to me how it's derived? Why the 'Planck sensitivity' 3.3 W/m^2 per 1 degree of warming?
    Response:

    [DB] "OK, explain to me how it's derived?"

    Try here:

    http://scienceofdoom.com/2010/10/24/planck-stefan-boltzmann-kirchhoff-and-lte/

  12. RW1 try here, but let's not go offtopic here and choose a more appropiate place to discuss sensitivity.
  13. Riccardo (RE: 187), What I'm asking is why specifically is it 3.3 W/m^2 per 1 degree of warming? Why not 0.33 W/m^2 or 33 W/m^2 (some other number), for example?
  14. 188 RW1 The earth radiates energy as E=epsilon*sigma*T^4, where epsilon is the emissivity (0.61) and sigma is the Stefan-Boltzman constant. If you increase the average earth surface temperature (287K) by one degrees you'll increase E by 3.3 W/m^2
  15. RW1 - "What I'm asking is why specifically is it 3.3 W/m^2 per 1 degree of warming? Why not 0.33 W/m^2 or 33 W/m^2 (some other number), for example?" I'm more than a bit appalled by this question. You look at the physics, you run the numbers, and you get a result. The only reason the number would be different is if the physics were somehow different, RW1. But reality is a harsh critic.
  16. RW1, You have now demonstrated extreme ignorance on climate science (a complete failure to understand Stefan-Boltzmann, the Planck response as a feedback, and the way that we know that this response is 3.3 W/m2 per degree K). At this point, you must recognize the need to study and learn a whole lot more before you "authoritatively" comment any further on things that you clearly do not grasp.
  17. RW1 wrote: "Have you noticed that even when the system as whole warms to a significant degree like during an El Nino event, it always seems to revert to its pre-equilibrium state fairly quickly afterward? This is hardly consistent with net positive feedback acting on perturbations." So... because the daily cycle of temperature fluxes due to sunrise and sunset returns to its 'equilibrium' state in short order this is evidence against Summer temperatures being warmer than Winter temperatures? That is essentially the 'logic' you are presenting... 'if A shows steady fluctuations around a flat baseline then B cannot possibly show long term increases'. "Also, the temperature rising as the Sun comes up each day is not the result of net positive feedback acting on the increased incident energy." In part, yes... it is. Most of the temperature increase is due to the increased radiation. However, water vapor feedback also plays a part... as anyone who has lived in a very humid area would be able to tell you. A climate 'forcing' is just a change from a baseline. Most often we choose baselines from on yearly averages. However, it is perfectly valid to look at the incoming solar radiation over the course of a day with the daily average as the baseline. In such case, sunrise would represent a massive warming forcing. We can then look at feedback responses to this forcing... specifically, that warming forcing from sunlight will cause water to evaporate. Water vapor is a greenhouse gas. Ergo, this will create a positive feedback. Further, it is a positive feedback that we have known about and studied for centuries. We use the term 'relative humidity' because the amount of water vapor the atmosphere can hold increases as the temperature does... so the relative humidity is the percentage of the possible maximum water vapor content of the atmosphere for the current temperature. If the temperature goes up the amount of water vapor the atmosphere can hold increases as well. A long understood and thoroughly measured phenomenon. Now, if the temperature of the entire planet goes up... why wouldn't the same feedback apply? If the planet's atmosphere is warmer it will hold more water vapor. That water vapor will produce more greenhouse warming. You argue that 'the notion' of positive feedback from water vapor cannot be reconciled with 'tightly constrained' temperature variations in various other cycles (e.g. ENSO, seasons, day/night, 11 years solar cycle, et cetera). I am saying that there is no logical way to conclude that water vapor will not be a positive feedback, because it is always a positive feedback to warming.
  18. lanC (RE: #189), "The earth radiates energy as E=epsilon*sigma*T^4, where epsilon is the emissivity (0.61) and sigma is the Stefan-Boltzman constant. If you increase the average earth surface temperature (287K) by one degrees you'll increase E by 3.3 W/m^2" Correct. Now, why specifically is the emissivity 0.61?
  19. CBDunkerson (RE: #192), "You argue that 'the notion' of positive feedback from water vapor cannot be reconciled with 'tightly constrained' temperature variations in various other cycles (e.g. ENSO, seasons, day/night, 11 years solar cycle, et cetera)." No, I'm arguing that positive water vapor feedback in conjunction with positive cloud feedback cannot be reconciled with how the planet's energy balance is so tightly - but dynamically, maintained.
    Response:

    [DB] "I'm arguing that positive water vapor feedback in conjunction with positive cloud feedback cannot be reconciled with how the planet's energy balance is so tightly - but dynamically, maintained."

    The casual reader will note that, in the absence of doing the physics and showing your work, your statement is devoid of substance and amounts to empty assertion without merit.

  20. DB, Is net negative feedback required for basic stability and maintenance of the current energy balance or not?
    Response:

    [DB] Your question presumes that there is a current energy balance at the TOA; the best understanding of the science indicates otherwise. 

    Energy in ≠ energy out (energy in exceeds energy out).

    You also do not define what physical process you refer to that is providing the "net negative feedback required for basic stability and maintenance".  Energy balance is the sum of physical processes, not hypothetical ones.

  21. DB, "Your question presumes that there is a current energy balance at the TOA; the best understanding of the science indicates otherwise." No, my question presumes the opposite - that there is virtually always an energy imbalance of some degree, especially over the more shorter time scales that the dynamic physical processes of water vapor and clouds operate. The imbalance is either energy in > energy out or energy in < energy out. You are aware that the system is almost never in perfect equilibrium at any given time or over any given time period, right? "You also do not define what physical process you refer to that is providing the "net negative feedback required for basic stability and maintenance". Energy balance is the sum of physical processes, not hypothetical ones." Yes, the planet's energy balance is the net result of all the physical processes (and feedbacks) in the system. Do you agree the net feedback that operates on all the physical processes in the system - whatever they actually are, is negative?
    Response:

    [DB] With all due respect, the last time we discussed this several threads ago, you first need to learn more about the physics of climate science before trying to bend them to your will.  To that end I also recommend Raymond Pierrehumbert's textbook "Principles of Planetary Climate".

    In any event, given the nature of your questions here, it is evident that you have not yet pursued that option.  That is your choice.  But that leaves you on your pre-existent orbit of asking the same questions repeatedly in the hopes that one day you might find an answer more to your liking.  All in all a non-effective path to greater understanding and a waste of other's time.  And indicative of your earlier statement that you were "not here to learn anything"

  22. RW1, you seem rather desperate to demonstrate a net negative feedback for the whole climate system, not just clouds, yet how do you explain palaeoclimatic variations with a net negative feedback? Leprechauns? Positive feedback need not equal runaway feedback.
  23. DB, I already have Ray's book and have read a lot of it. I guess you're not interested in answering my questions?
    Response:

    [DB] If full understanding of the text was not achieved then perhaps you should place those questions over at RC where Ray has a presence.

  24. RW1, Net positive feedback does not lead to runaway warming (Venus result) anymore than net negative feedback leads to Snowball Earth. So, the answer to your question is no. If you do not understand why, you did not understand what you read.
  25. Bibliovermis, I know net positive feedback does not lead to runaway warming. "So, the answer to your question is no." Which question?

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