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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 51 to 75 out of 261:

  1. RW1 - Re: your claim that "A very large amount (if not most) of the enhanced warming from the climate models comes from positive cloud feedback." From your link: "In AOGCMs, the water vapour feedback constitutes by far the strongest feedback" - followed by the lapse rate, and then surface albedo and clouds. "Water vapor and clouds act on time scales of hours to days." Absolutely. Which why they are strictly feedbacks, not forcings. They cannot stay out of balance long enough to affect any other feedbacks on their own.
  2. Sphaerica (RE 48), "Water vapor does not equal clouds." I never claimed that it did. "Again, no, clouds come in a distant fourth, at best, behind H2O feedbacks (water vapor), CO2 feedbacks, and albedo feedbacks." Not according the latest IPCC report, which says: "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%. The surface albedo feedback amplifies the basic response by about 10%, and the cloud feedback does so by 10 to 50% depending on the GCM." Clouds can be up to 50%, where as surface albedo is only about 10%.
  3. KR (RE: 51), "From your link: "In AOGCMs, the water vapour feedback constitutes by far the strongest feedback" - followed by the lapse rate, and then surface albedo and clouds." Are you not actually reading the whole section? This not what it says (or implies). The surface albedo is the smallest feedback - closer to a third of the average cloud feedback (0.26 W m-2 °C–1 vs. 0.69 W m–2 °C–1 for clouds). The water vapor feedback is directly tied to and offset by the lapse rate feedback (1.80 W m-2 °C–1 vs. -0.84 W m-2 °C–1 for the lapse rate). " 'Water vapor and clouds act on time scales of hours to days.' Absolutely. Which why they are strictly feedbacks, not forcings." Define specifically what you mean by a 'forcing'? The only true 'forcing' of the climate system is the Sun. All the other components, such as water vapor, clouds, and precipitation, are really just responding directly or indirectly to the Sun's forcing, the net effect of all of which dictate the equilibrium surface temperature. "They cannot stay out of balance long enough to affect any other feedbacks on their own." Why not? What's keeping them from staying "out of balance"?
  4. muoncounter (RE: 50), "Huh? What does that have to do with the rate at which CO2 radiative forcing increases global temperature?" I don't understand the question. Is 100 years not a significantly slower rate than hours to days?
  5. See the IPCC report for formal definition of forcing, but it is basically something can change the radiative balance independent of temperature. Only a change in forcing can change climate. The forcing in the system are solar, GHGs (which can also be a feedback, but are a forcing if changed independently of temperature -eg by release of fossil fuel), and aerosols. On a larger time scale, changes in continent distribution and in the nature of the biosphere can alter albedo so that it is also a forcing.
  6. KR (RE: 51), If water vapor is not a 'forcing' in the climate system, then how can CO2 be a 'forcing'? Are you claiming that increased water vapor in the atmosphere is not a 'forcing', but increased CO2 is a 'forcing'? This is the problem. They are both 'forcings' in the way you're using the term. The main difference is water vapor acts on much shorter time scales, but it is still a 'forcing' none the less.
  7. 52, RW1, Clouds come in ahead of albedo, and behind water vapor. I'm still not quite sure how you turn this into "A very large amount (if not most) of the enhanced warming" but I'll concede the point. Clouds are an important positive feedback in the models (but not "most"). So what's your point? 53, RW1,
    The only true 'forcing' of the climate system is the Sun.
    No. If anything, solar output is the single strongest constant in the entire system. Changes in CO2 concentrations in the atmosphere are almost certainly the primary driver as far as total change. They are tied to every major climate swing in some way, and global temperature closely tracks CO2 concentrations. Albedo is probably the primary driver as far as getting the ball rolling (and that can come from orbital forcings or aerosols -- volcanism). Water vapor and clouds are fast acting feedbacks that do not force anything on their own. But we're drifting. What is your point about clouds again? Now that you've proven that they are a strong positive feedback, why are we discussing them?
  8. 56, RW1,
    ...but increased CO2 is a 'forcing'?
    Yes, because water vapor responds quickly to changes in temperature. There's nothing anyone or anything can do to inject water vapor into the atmosphere and keep it there. The temperature will drop, and the water vapor will condense and things will return to normal. This is not the case with CO2, whether it is added anthropogenically or geologically. No matter how it gets there, once it does get there, it stays there for a very long time and it's effect forces the climate to follow suit.
  9. RW1 - "If water vapor is not a 'forcing' in the climate system, then how can CO2 be a 'forcing'? Are you claiming that increased water vapor in the atmosphere is not a 'forcing', but increased CO2 is a 'forcing'?" Not just claiming that, but stating that with plenty of evidence. CO2 has been changing due to anthropogenic emissions, while water vapor and clouds have been changing strictly due to temperature changes. This is part of the grand scheme of Cause -> Effect, RSVP; CO2 (due to our actions) is a recent cause of climate change, water vapor and clouds respond as an effect.
  10. RW1#54: "I don't understand the question." Well, you stated "Anthropogenic CO2 forcing is very gradual," I asked how you to substantiate this; you responded that "it's claimed to take about 100 years to double CO2." The time it takes to double CO2 through anthropogenic input has nothing to do with the time it takes for the forcing of CO2 already in the atmosphere to increase temperature, which is, of course, already in progress. I don't understand how you mixed up the two. See the 40 year lag thread I linked earlier for discussion of this.
  11. Sphaerica (RE: 57), "What is your point about clouds again? Now that you've proven that they are a strong positive feedback, why are we discussing them?" Because if a lot of the enhanced warming comes from positive cloud feedback, and the cloud feedback is NOT really positive - but negative (even slightly negative), it is going to reduce the projected amount of warming significantly. The IPCC even says that if the cloud feedback is neutral, it would reduce the average sensitivity to 1.9 C instead of 3 C. That's a reduction of over half of the enhanced warming. If the cloud feedback was even moderately negative, the average sensitivity could easily come down to 1 C or less. In short, the cloud feedback is huge.
  12. muoncounter (RE: 60), "The time it takes to double CO2 through anthropogenic input has nothing to do with the time it takes for the forcing of CO2 already in the atmosphere to increase temperature, which is, of course, already in progress. I don't understand how you mixed up the two. See the 40 year lag thread I linked earlier for discussion of this." Sorry for the misunderstanding, but my question then is why doesn't it take 40 years for the forcing of water vapor in the atmosphere to increase (and decrease) temperature?
  13. RW1#62: "why doesn't it take 40 years for the forcing of water vapor in the atmosphere" I thought there was agreement that water vapor doesn't stay in the atmosphere that long. I don't know how things are where you live, but I wipe a lot of that water vapor off my car windows every morning. #61: "the average sensitivity could easily come down to 1 C or less." Ah, we've come full circle, as you've said that before: It's also inline with the sensitivity only being about 0.6 C . Of course, the temperature record doesn't support that contention, as we've already seen 0.8C with far less than a doubling of CO2.
  14. KR (RE: 59), "Not just claiming that, but stating that with plenty of evidence. CO2 has been changing due to anthropogenic emissions, while water vapor and clouds have been changing strictly due to temperature changes." Not necessarily strictly temperature changes, but even so, I don't see how that excludes them from being a 'forcing'. Do water vapor changes not also cause temperature changes? Do cloud changes not also cause temperature changes? Surely they do.
  15. muoncounter (RE: 62), "I thought there was agreement that water vapor doesn't stay in the atmosphere that long." It doesn't, but it also doesn't take 40 years for changes in water vapor concentration to effect changes in temperature. For example, a sunny humid day is generally warmer than a sunny dry day, all other things being equal. "Ah, we've come full circle, as you've said that before: It's also inline with the sensitivity only being about 0.6 C" That was assuming only half of the 3.7 W/m^2 from 2xCO2 is incident on the surface. For the purposes of this discussion and elsewhere here, I've accepted that the full 3.7 W/m^2 affects the surface (at least for now).
  16. RW1 - At the risk of repeating myself: Cause => Effect CO2 is a cause, changing (primarly and from anthropogenic actions) independent of temperature, while water vapor and clouds respond promptly to temperature and don't change on their own, and are hence amplifying effects of temperature change. Water vapor and clouds change in response to temperature. If you have any evidence supporting water vapor or cloud changes independent of temperature, I suggest you publish it. Nobody else has found any such evidence - I will (I believe correctly) take assertions to that effect as just wishful thinking without such evidence.
  17. RW1 - clouds are not forcing because unless you have something like GCR changing clouds, there is no way to produce a long term change in cloud cover without something else being responsible for changing the temperature. If your vision of reality is right, then you would have world with no change to GHG, solar, or aerosols, going through climate change (ie a long term change in radiative balance). Now plenty of that kind of internal variability in short time scales - weather. But no evidence whatsoever of any such change on long term.
  18. Sphaerica (RE: 58), "Yes, because water vapor responds quickly to changes in temperature. There's nothing anyone or anything can do to inject water vapor into the atmosphere and keep it there. The temperature will drop, and the water vapor will condense and things will return to normal." But what causes the temperature to drop and the water vapor to be removed from the atmosphere if water vapor is the primary amplifier of warming? "This is not the case with CO2, whether it is added anthropogenically or geologically. No matter how it gets there, once it does get there, it stays there for a very long time and it's effect forces the climate to follow suit." Yes, but I don't see how CO2's effect is fundamentally different than water vapor, especially if water vapor is the primary amplifier of warming (CO2 induced or otherwise). In other words, why would the response to water vapor warming in the system be any different than warming caused by CO2? Why would the same forces that modulate or control water vapor's radiative forcing, not modulate and control CO2's radiative forcing? The surface has no way of distinguishing where the radiative 'forcing' originated from - water vapor or CO2. All the surface 'knows' is its total energy flux, as it determines the surface temperature.
  19. The atmosphere has a temperature gradient. At a certain height, water condenses out. CO2 does not. Maximum water content in atmosphere is temperature-dependent. Maximum CO2 is not. Note that in our current AGW-world, CO2 is not a feedback. The mechanisms are too slow to have produced much GHG feedback yet.
  20. scaddenp (RE: 69), "The atmosphere has a temperature gradient. At a certain height, water condenses out. CO2 does not." I know. "Note that in our current AGW-world, CO2 is not a feedback." Agreed.
  21. RW1,
    In short, the cloud feedback is huge.
    You love to exaggerate things. The cloud feedback is important, not huge. It's more important if it is neutral or negative, but you've shown no evidence other than that you think common sense says so, while hundreds of climate scientists think otherwise. But even if you proved clouds to be a weak negative feedback, it would reduce sensitivity to anywhere from 1.9 to 3.4 (versus 3 to 4.5), given that 3 is the current best estimate, but also at the low end of the range. Even 1.9 is very, very bad, especially since we're currently taking no action to avoid it. But first you need to submit some evidence beyond your "plain, everyman logic" to prove that clouds are even a neutral feedback, let alone negative. And that evidence has to contradict all of this evidence to the contrary. I'm afraid a sensitivity below 3˚C is very, very unlikely.
    But what causes the temperature to drop and the water vapor to be removed from the atmosphere if water vapor is the primary amplifier of warming?
    This question is evidence that you don't understand how things work. You need to go study more. If this were the case, the planet would never, ever cool, no matter what.
    ...but I don't see how CO2's effect is fundamentally different than water vapor..
    Because water vapor will increase or decrease in the atmosphere fairly quickly in response to temperature. Raise the temperature, raise the water vapor. Lower the temperature, lower the water vapor. CO2, on the other hand, will stay in the atmosphere for hundreds of years, even if, for example, a large volcanic eruption temporarily lowers temperatures.
    ...why would the response to water vapor warming in the system be any different than warming caused by CO2?
    There's no difference in the warming. What is different is that the CO2 won't drop out of the atmosphere when the temperature drops (for instance, during the winter).
    Why would the same forces that modulate or control water vapor's radiative forcing, not modulate and control CO2's radiative forcing?
    There are no such forces for either. This isn't a human designed system with controls and balances. It's nature, and it's (fortunately) got a simple balance to it, and one that should be very hard to shove, but we've found a way to do it. The point is not how each one (water vapor vs. CO2) affects temperature. The point is that water vapor content is itself affected by temperature on short time scales, while CO2 is only affected on very, very long time scales. And, in fact, there is a positive CO2 feedback (such as outgassing from the ocean) that will, in the long term, increase CO2 levels even further.
  22. KR (RE: 66), "Cause => Effect CO2 is a cause, changing (primarly and from anthropogenic actions) independent of temperature, while water vapor and clouds respond promptly to temperature and don't change on their own, and are hence amplifying effects of temperature change." I think the confusion here lies somewhere in between the definition of 'forcing' and that there are many other things in the climate system, other than anthropogenic CO2 (and GHGs), that are changing and subsequently inducing new 'forcings' independent of temperature. As just one example, take the fluctuations of Arctic and Antarctic sea ice extents, which we know are largely driven by factors other than temperature (wind patterns, ocean currents, etc,). Yes, anthropogenic CO2 'forcing' is a cause and not an effect of temperature, but even without anthropogenic CO2, the climate is frequently perturbed by new 'forcings' - not all of which are due to temperature changes, yet the globally averaged temperature remains very, very stable.
  23. RW1 - the answer then to "Yes, but I don't see how CO2's effect is fundamentally different than water vapor, especially if water vapor is the primary amplifier of warming (CO2 induced or otherwise)." CO2 is non-condensated gas. That is why the same forces that modulate or control water vapor's radiative forcing, do not modulate and control CO2's radiative forcing.
  24. "As just one example, take the fluctuations of Arctic and Antarctic sea ice extents, which we know are largely driven by factors other than temperature (wind patterns, ocean currents, etc,). " Splorff! Long term (30 year) change in arctic albedo is driven ultimately by change in temperature. Short term variation from year to year depends on wind/ocean etc. Oh and what is changing ocean/wind? I think it is time to come up with some evidence for unforced climate change if you are arguing about the definition of forcing.
  25. RW1 wrote: "I think the confusion here lies somewhere in between the definition of 'forcing' and that there are many other things in the climate system, other than anthropogenic CO2 (and GHGs), that are changing and subsequently inducing new 'forcings' independent of temperature. As just one example, take the fluctuations of Arctic and Antarctic sea ice extents, which we know are largely driven by factors other than temperature (wind patterns, ocean currents, etc,)."
    What boils to: - I know it is a system - I know CO2 is a factor - But the cause must be elsewhere, darn. - Let me uncouple the system and I'll tell why. Let also that the uncoupling makes Temperature an irrelevant variable to the climate system. "Why would you stop when you can rev up?" (signed: Thelma & Louise)

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