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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 26 to 50 out of 119:

  1. Sphaerica (RE: 25), "They are not "greatly amplifying" small temperature changes. They do have some impact, but there's no reason to exaggerate things with "great" and "small" to try to score points." OK, fair enough. The point is the AGW theory claims the net effect of clouds is to amplify warming rather than attenuate warming. The mechanisms available for altering climate are: * Solar insolation (barely changes) * Orbital forcings (changes predictably on huge time scales) * CO2 (changes on any of 3 time scales, geologic=very slow, natural feedback=medium, anthropogenic=very fast * H2O (changes relatively quickly in direct proportion to temperature, and so it is the primary amplifier in power, but not a controller since it exerts no independent control of its own) * Albedo (can change relatively quickly, or slowly, but almost always as a response to other factors) * Clouds (change almost instantaneously, and positive/negative effects are arguable, but relatively inconsequential compared to the bigger factors)." If the Sun is the source energy and ultimately heat in the system, and if H2O changes so quickly in response to temperature - specifically warming, and water is the primary amplifier but not a controller, what is the controller? If the Sun is the only real heat source, and water concentration (the primary amplifier) is driven by heat from the Sun, the primary controller would need to modulate the amount of the Sun's energy allowed to enter the system. Clouds definitely do this since they make up about 3/4ths of the albedo and are constantly changing spatially and in time - all the time. Is it also a coincidence that clouds are made up of H2O and water vapor concentrations drive cloud formation? It is also another coincidence that water vapor is removed from the atmosphere from precipitation that emanates from clouds? If the net cloud feedback was positive in conjunction with positive water vapor feedback, what would prevent the temperature from rising significantly higher and higher from even just a few days or few weeks of abnormally warm weather? Yet this never happens - abnormally warm weather periods end and normal or colder weather inevitably commences. This is probably because the forces that drive evaporation/water vapor are not as strong as the combined forces of clouds and precipitation. This is fundamentally why net positive cloud feedback is so illogical. "* Land mass dispositions (which greatly affect albedo and the results of orbital forcings, but which only themselves vary on massively long timescales) Which is primary? At the onset and termination of glacial periods, the orbital forcings, but only through an albedo feedback, and in conjunction with a strong CO2 feedback, which in turn operates in conjunction with the strong H2O feedback. Outside of those periods of orbital forcings, under natural conditions, CO2 is the main long term driver, amplified by albedo, cloud and H2O feedbacks. During anthropogenic pollution, CO2 is the only control mechanism that operates on the time scales that we are witnessing, again amplified by all of the usual feedbacks." But clouds operate on even shorter time scales than CO2, and temperature fluctuations occur on much shorter time scales too. Let's not forget that the surface of the Earth is over 2/3rds water. The cycle of water -> water vapor -> clouds -> precipitation -> water is enormous.
  2. RW1,
    Is it also a coincidence that clouds are made up of H2O and water vapor concentrations drive cloud formation? It is also another coincidence that water vapor is removed from the atmosphere from precipitation that emanates from clouds?
    Yes, it is coincidental. That is, both are true, but neither mean anything.
    ...what would prevent the temperature from rising significantly higher and higher from even just a few days or few weeks of abnormally warm weather? Yet this never happens...
    Yes it does, every time the seasons change. That is, the change in temperature in the temperate zones is greater than could be accounted for by the mere change in hours of insolation and angle of incidence of the sun (and change in albedo from winter snow to bare ground). Humidity also rises, and the GHG effect from this amplifies the temperature. This happens, any you experience it, every spring. But to give your question a more direct answer, you seem to be describing a runaway. As long as the feedback is less than the original forcing, it is positive, but not a runaway. This is the simple case of a convergent versus a divergent series in mathematics.
    This is probably because the forces...
    No. This is an assumption without foundation (as evidenced by the word "probably"). Clouds form or dissipate in a matter of hours. What would be needed to change climate would be a major change in the pattern in which clouds form or dissipate -- where, or when, or how much, by a significant degree, over a long period of time. There is no firm logical reason to think that this would happen (Lindzen argues that it should, but I and most of climate science find his arguments very unconvincing, and observational evidence to date is indecisive, which by itself tells you that it's not a "major" control, or else it would be obvious and unarguable). The fact that you think it could happen easily is in no way evidence that it does happen.
    But clouds operate on even shorter time scales than CO2, and temperature fluctuations occur on much shorter time scales too.
    So if they had a moderating effect they would be a serious damper in the system. But they don't. And if they were, because they operate so quickly, you would see their effect instantly. Which is exactly why clouds cannot be a major factor. On the time scales over which clouds vary, if in fact they were a moderating influence, then the temperature of the earth would never, ever vary by more than a fraction of a degree (or the clouds would quickly step in and stop it). You'd never have ice ages, Medieval Warm Periods, Little Ice Ages, or any notable variations. And yet over the history of the earth the temperature has varied by as much as 10˚C. Anything which operates on that fast a time scale is either the primary control, or it's not a control at all. All evidence points to "not at all." It may be a slightly positive or slightly negative feedback, but it's not a control, and not even a major feedback (compared to H2O, CO2, and albedo). Look here for further information on how we know that climate sensitivity is not low, so any moderating influence of clouds cannot be a major, fast acting factor. Oh, and as an aside, I did forget one control from my previous list:
    • Aerosols (due to volcanic activity, these can act very, very quickly, but by themselves are normally short lived, but gain "traction" by reducing the planet's albedo long enough to instigate other feedbacks which can be longer lived).
  3. Sphaerica (RE: 27), "Yes, it is coincidental. That is, both are true, but neither mean anything." The AGW theory seems to require an awful lot of coincidences. "Yes it does, every time the seasons change. That is, the change in temperature in the temperate zones is greater than could be accounted for by the mere change in hours of insolation and angle of incidence of the sun (and change in albedo from winter snow to bare ground). Humidity also rises, and the GHG effect from this amplifies the temperature. This happens, any you experience it, every spring." I generally meant globally - not seasonal or more local temperature change. I'm well aware that seasonal change is due to significant hemispheric insolation change. "But to give your question a more direct answer, you seem to be describing a runaway. As long as the feedback is less than the original forcing, it is positive, but not a runaway. This is the simple case of a convergent versus a divergent series in mathematics." I'm not describing a runaway effect. "So if they had a moderating effect they would be a serious damper in the system. But they don't. And if they were, because they operate so quickly, you would see their effect instantly. Which is exactly why clouds cannot be a major factor. On the time scales over which clouds vary, if in fact they were a moderating influence, then the temperature of the earth would never, ever vary by more than a fraction of a degree (or the clouds would quickly step in and stop it)." But it generally does not vary by much at all. The global mean average temperature tracks very, very tightly - rarely is there a difference from year to year more than 2 or 3 tenths of degree. "Anything which operates on that fast a time scale is either the primary control, or it's not a control at all. All evidence points to "not at all." OK, so then what is the mechanism controlling the energy balance? What is keeping the global average temperatures so tightly in check from year to year? From decade to decade? From century to century? Why don't we see much larger variation - like multiple degree C per year, per decade, per century? The climate system as a whole is remarkably stable. "Which is exactly why clouds cannot be a major factor. On the time scales over which clouds vary, if in fact they were a moderating influence, then the temperature of the earth would never, ever vary by more than a fraction of a degree (or the clouds would quickly step in and stop it). You'd never have ice ages, Medieval Warm Periods, Little Ice Ages, or any notable variations. And yet over the history of the earth the temperature has varied by as much as 10˚C" Most of these changes, except the MVP and LIA, are much longer term and driven by very large external forces - not relatively small internal forcings. External influences aside, just topographical differences alone due to plate tectonics had a huge impact on surface albedo over the Earth's history, which in turn had a huge influence on temperature and climate. The ice ages and interglacials are driven by changes in the Earth's orbit, etc. "It may be a slightly positive or slightly negative feedback, but it's not a control, and not even a major feedback (compared to H2O, CO2, and albedo)." I don't know how you can say this. A very large amount (if not most) of the enhanced warming from the climate models comes from positive cloud feedback. "Look here for further information on how we know that climate sensitivity is not low, so any moderating influence of clouds cannot be a major, fast acting factor. 'For example, between glacial and interglacial periods, the planet's average temperature changes on the order of 6°C (more like 8-10°C in the Antarctic). If the climate sensitivity is low, for example due to increasing low-lying cloud cover reflecting more sunlight as a response to global warming, then how can these large past climate changes be explained?'" Very simply. The large change and increase the distribution of the Sun's energy in the northern hemisphere combined with a positive surface albedo feedback is enough to overcome the negative cloud feedback.
  4. RW1 - "OK, so then what is the mechanism controlling the energy balance? What is keeping the global average temperatures so tightly in check from year to year? From decade to decade? From century to century? Why don't we see much larger variation - like multiple degree C per year, per decade, per century? The climate system as a whole is remarkably stable." To put things briefly: Power radiated to space scales with T^4. This extremely powerful negative feedback keeps the temperature within the ranges you are discussing here. Unless the rate of energy input (insolation changes, large scale albedo from, say, ice changes) or output back to space (emissivity changes due to GHG's) change over an extended period of time (which tends to rule out clouds given their very fast response rate), temperatures will stay near the range determined by the energy boundary conditions.
  5. RW1 - Sorry, looking at my last post I may have left some things unclear. Temperature averages will always, over the long term average, stay near the limits determined by the energy boundary conditions. Long term temperature averages change when those boundary conditions change.
  6. KR, What's controlling the 'energy boundary conditions'?
  7. The boundary condition is that climate energy in = climate energy out when averaged over the long term (i.e., longer than internal variations such as ENSO and yearly seasons, which is apparently about 30 years or so). As to what changes those conditions? Everything Sphaerica listed here. One critical thing is that if the response time of any influence on the energy flows to temperature is considerably shorter than the climate internal variation time frames, it can only be a feedback (responding to changes) rather than a forcing causing long term changes. Water vapor and clouds have response times << ENSO and seasons, and any internal variation in water vapor and/or clouds will get overridden by those variations. Water vapor and clouds respond to climate changes, but cannot cause them.
    Response: [DB] Removed "of one" text string per request.
  8. KR, "As to what changes those conditions?" I asked what's primarily controlling those conditions? What's keeping the 'boundary conditions' (power in = power out) so stable from year to year, given all those influences Sphaerica listed - specifically the water vapor, the "primary amplifier"??? "This extremely powerful negative feedback keeps the temperature within the ranges you are discussing here. What's the mechanism driving the 'extremely powerful negative feedback'??? If it's not primarily clouds through their ability to reflect sunlight and precipitate water out of the atmosphere, then what is it?
  9. KR, Better said: What's the mechanism driving the 'extremely powerful negative feedback'??? If it's not primarily clouds through their ability to modulate incoming solar energy and precipitate water out of the atmosphere, then what is it?
  10. RW1 - Water vapor is an amplifier, a positive feedback. Clouds are a less well understood feedback; most estimates are that they are slightly positive, but not overwhelmingly so. Neither can be a driver of climate, however, since they respond so quickly to temperatures. They cannot stay consistently out of balance long enough to push anything else out of balance. Keep in mind that seasonal and aperiodic variations such as ENSO make water vapor and clouds vary hugely - they are really more amplifiers of internal variance on those time scales. As to the stability of the other possible modifiers of energy in/energy out - look to the variances and speeds there of of insolation/solar cycles, Milankovitch cycles, volcanic and other aerosol sources, ice coverage change speeds, etc. And don't forget the speed of change of anthropogenic forcings, primarily CO2. Which is changing faster than any of the other forcings, in fact faster than all the other forcings are changing combined. And which naturally responds as a feedback to temperature changes on perhaps a 500 year schedule - meaning CO2 imbalances last long enough to shift the climate energy balance and average temperatures.
  11. RW1, You keep creating your own conditions.
    What's the mechanism driving the 'extremely powerful negative feedback'???
    There is none. There doesn't have to be. Insolation is 290W/m2. The planet radiates 290W/m2. Everything (normally) stays in pretty good balance as easily as that. We as humans experience weather, which include daily, seasonal, annual and decadal fluctuations. These are caused by a variety of factors. But the bottom line is 290 in, 290 out. If the temperature of the planet appears to cool, then there is less out for a while. This can only go on for only so long before it catches up and the balance is restored. No clouds or other moderator are needed. The system does not need to behave like a human designed EE system, or contain the same components (i.e. a primary "control"). You're basically making it way more complicated than it needs to be -- or is.
  12. RW1, From an earlier post:
    The AGW theory seems to require an awful lot of coincidences.
    This is a foolish and disingenuous thing to say. It's an appeal rather than a fact, and has no substance whatsoever. To me, your theories are the ones that require an awful lot of coincidences. So avoid sweeping, casual and incorrect statements like this. Appeals to emotion are debate tactics, not science, and if the conversation goes there, I leave. You should also avoid referring to "AGW theory." There is no such thing. There is a GHG theory, which is a proven fraction of climate science. There is all of climate science, which is advancing and changing every day. There is no single "AGW theory," and attempts to cast it as such are merely backhanded attempts to imply that there's something simple there that can easily be ignored or dismissed. It doesn't work that way. The volume of information behind current climate science is enormous, and it's not going to be easily overturned by someone on a blog who has his own ideas about how things work.
  13. RW1,
    A very large amount (if not most) of the enhanced warming from the climate models comes from positive cloud feedback.
    This is not true. Citation, please.
  14. RW1,
    Very simply. The large change and increase the distribution of the Sun's energy in the northern hemisphere combined with a positive surface albedo feedback is enough to overcome the negative cloud feedback.
    This is not true. Citation, please.
  15. Sphaerica (RE: 38), 'A very large amount (if not most) of the enhanced warming from the climate models comes from positive cloud feedback.' "This is not true. Citation, please." Here "In AOGCMs, the water vapour feedback constitutes by far the strongest feedback, with a multi-model mean and standard deviation for the MMD at PCMDI of 1.80 ± 0.18 W m–2 °C–1, followed by the (negative) lapse rate feedback (–0.84 ± 0.26 W m–2 °C–1) and the surface albedo feedback (0.26 ± 0.08 W m–2 °C–1). The cloud feedback mean is 0.69 W m–2 °C–1 with a very large inter-model spread of ±0.38 W m–2 °C–1 (Soden and Held, 2006)." "In the idealised situation that the climate response to a doubling of atmospheric CO2 consisted of a uniform temperature change only, with no feedbacks operating (but allowing for the enhanced radiative cooling resulting from the temperature increase), the global warming from GCMs would be around 1.2°C (Hansen et al., 1984; Bony et al., 2006). The water vapour feedback, operating alone on top of this, would at least double the response.[6] The water vapour feedback is, however, closely related to the lapse rate feedback (see above), and the two combined result in a feedback parameter of approximately 1 W m–2 °C–1, corresponding to an amplification of the basic temperature response by approximately 50%" When you factor in that the water vapor and lapse rate feedback are tied directly together and serve to directly offset each other, the net effect of the cloud feedback, with some of the models at least, becomes as significant - if not more so: "Because the water vapour and temperature responses are tightly coupled in the troposphere (see Section 8.6.3.1), models with a larger (negative) lapse rate feedback also have a larger (positive) water vapour feedback. These act to offset each other (see Box 8.1). As a result, it is more reasonable to consider the sum of water vapour and lapse rate feedbacks as a single quantity when analysing the causes of inter-model variability in climate sensitivity." The bottom line is a lot of the enhanced warming comes from positive cloud feedback.
  16. Sphaerica (RE: 37), "To me, your theories are the ones that require an awful lot of coincidences." Which ones and why? "You should also avoid referring to "AGW theory." There is no such thing. There is a GHG theory, which is a proven fraction of climate science. There is all of climate science, which is advancing and changing every day. There is no single "AGW theory," and attempts to cast it as such are merely backhanded attempts to imply that there's something simple there that can easily be ignored or dismissed." Sorry for the confusion. When I refer to the "AGW theory", I just mean the catastrophic or enhanced warming of around 3C predicted by the models, the IPCC, etc. That's all.
  17. Sphaerica (RE: 36), "There is none. There doesn't have to be. Insolation is 290W/m2. The planet radiates 290W/m2. Everything (normally) stays in pretty good balance as easily as that. But the bottom line is 290 in, 290 out." I know energy has to be conserved (energy in = energy out) - that isn't the point. The point is the equilibrium global surface temperature from year to year fluctuates very, very little. This has nothing to do with energy always needing to being conserved. "If the temperature of the planet appears to cool, then there is less out for a while. This can only go on for only so long before it catches up and the balance is restored." Actually, when the planet cools (or is cooling), there is more going out than coming in for a while, but think you're confusing equilibrium being restored and temperature being restored. They are two totally different things.
  18. KR (RE: 35), "And don't forget the speed of change of anthropogenic forcings, primarily CO2. Which is changing faster than any of the other forcings, in fact faster than all the other forcings are changing combined." I don't see how this is so. Anthropogenic CO2 forcing is very gradual, taking decades and centuries. Water vapor and clouds act on time scales of hours to days.
  19. RW1#43: "CO2 forcing is very gradual, taking decades and centuries." Centuries? How do you know that? See the thread 40 Year Delay, which suggests that the thermal inertia of the oceans results in a lag to full warming from CO2 forcing on the order of 40 years.
  20. (RE: my #2), "Clouds cover 2/3rds of the surface, so 341 W/m^2 x 0.67 = 228 W/m^2 average incident on the clouds. 79 W/m^2 divided by 228 W/m^2 = 0.34 average reflectivity of clouds. 1/3rd of the surface is cloudless, so 341 W/m^2 x 0.33 = 113 W/m^2 average incident on the cloudless surface. 23 W/m^2 divided by 113 W/m^2 = 0.20 average reflectivity of the cloudless surface. 0.34 - 0.20 = 0.14. 341 W/m^2 x 0.14 = 48 W/m^2 loss for each additional m^2 of cloud cover." There is slight error in this calculation, which I would like to correct for the record. The average reflectivity of clouds is actually about 0.35 (not 0.34), which corresponds to about 51 W/m^2 loss for each additional m^2 of cloud cover instead of 48 W/m^2, resulting in a net loss about 12 W/m^2 instead of 10 W/m^2.
  21. muoncounter (RE 44), " 'CO2 forcing is very gradual, taking decades and centuries.' Centuries? How do you know that?" Because it's claimed to take about 100 years to double CO2.
  22. 40, RW1,
    "In AOGCMs, the water vapour feedback constitutes by far the strongest feedback..."
    Water vapor does not equal clouds. The two are very distinct and different. Water vapor involves a GHG effect which is stronger than that of CO2, and is listed in my feedbacks (as H2O).
    The bottom line is a lot of the enhanced warming comes from positive cloud feedback.
    Again, no, clouds come in a distant fourth, at best, behind H2O feedbacks (water vapor), CO2 feedbacks, and albedo feedbacks.
  23. 40, RW1,
    Actually, when the planet cools (or is cooling), there is more going out than coming in for a while...
    When the planet cools, it radiates less, not more.
    The point is the equilibrium global surface temperature from year to year fluctuates very, very little. This has nothing to do with energy always needing to being conserved.
    No, it doesn't, but it also has nothing to do with clouds. It has to do with the fact that the only mechanism for actual cooling the planet is to either reflect or radiate heat, and the only mechanism for warming is to trap heat. As such, the planet has a fairly stable climate because very little will naturally change the chemical balance of the atmosphere or the albedo of the surface except over very, very long time frames. None of this says that clouds have to be some powerful control knob that keeps everything mellow. The planet stays where it is because it is very, very hard to shift it out of balance (as evidenced by the small changes in climate despite large changes in orbital forcings, except for that perfect case that initiates or terminates an interglacial). You can't say that clouds must be a mitigating factor because you figure they must be. You're going to need much stronger evidence than that.
  24. 40, RW1,
    I don't see how this is so. Anthropogenic CO2 forcing is very gradual, taking decades and centuries. Water vapor and clouds act on time scales of hours to days.
    Normal climate change takes millenia. The 0.3˚C swings in global temperature that have been normal for the past two thousand years are not climate change. And clouds have nothing to do with anything. Please begin providing evidence instead of mere assertions.
  25. RW1#:46 "it's claimed to take about 100 years to double CO2." Huh? What does that have to do with the rate at which CO2 radiative forcing increases global temperature?

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