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A Cloudy Outlook for Low Climate Sensitivity

Posted on 5 December 2010 by dana1981

One of the largest uncertainties in global climate models (GCMs) is the response of clouds in a warming world.  Determining which types of cloud cover will increase or decrease, whether that will result in a net positive or negative feedback, and how large the feedback will be, are major challenges.  The variation in global climate sensitivity among GCMs is largely attributable to differences in cloud feedbacks, and feedbacks of low-level clouds in particular.

For climate scientists who are skeptical that anthropogenic greenhouse gas emissions will cause a dangerous amount of warming, such as Richard Lindzen and Roy Spencer, their skepticism hinges mainly on this cloud cover uncertainty.  They tend to believe that as the planet warms, low-level cloud cover will increase, thus increasing planetary albedo (overall reflectiveness of the Earth), offsetting the increased greenhouse effect and preventing a dangerous level of global warming from occurring.

Recently some studies have examined the cloud feedback specifically in the eastern Pacific region.  Stowasser et al. (2006) found that:

"In terms of the sensitivity of the global-mean surface temperature, almost all the differences among the models could be attributed to differences in the shortwave cloud feedbacks in the tropical and subtropical regions." 

In order to evaluate this uncertainty, Lauer et al. (2010) used 16 GCMs and the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM) described in Lauer et al. (2009) to simulate clouds and cloud–climate feedbacks in the tropical and subtropical eastern Pacific region.  To investigate cloud–climate feedbacks in iRAM, the authors ran several global warming scenarios with boundary conditions appropriate for late twenty-first-century conditions (specifically, warming signals based on IPCC AR4 SRES A1B simulations).

Figure 1 shows the results of the 16 GCMs, iRAM (bottom center), and satellite observations (bottom right).  A clearer version of this figure can be seen in Figure 1 on Page 6 of Lauer et al. (2010).

"The authors find that the simulation of the present-day mean cloud climatology for this region in the GCMs is very poor and that the cloud–climate feedbacks vary widely among the GCMs. By contrast, iRAM simulates mean clouds and interannual cloud variations that are quite similar to those observed in this region."


Figure 1: Annual average TOA shortwave cloud forcing for present-day conditions from 16 IPCC AR4 models and iRAM (bottom center) compared with CERES satellite observations (bottom right)

Thus the study shows that that iRAM simulates recently observed cloud cover changes in this the eastern Pacific more accurately than the GCMs, and iRAM also successfully simulates the main features of the observed interannual variation of clouds in this region, including the evolution of the clouds through the El Niño Southern Oscillation (ENSO) cycle.  Given these conclusions, the logical assumption is that iRAM will also model future cloud cover changes more accurately.  Operating under this assumption, the authors conclude as follows.

"All the global warming cases simulated with iRAM show a distinct reduction in low-level cloud amount, particularly in the stratocumulus regime, resulting in positive local feedback parameters in these regions in the range of 4–7 W m-2 K-1....The GCM feedbacks vary from -1.0 to +1.3 W m-2 K-1, which are all less than the +1.8 to +1.9 W m-2 K-1 obtained in the comparable iRAM simulations. The iRAM results by themselves cannot be connected definitively to global climate feedbacks, but we have shown that among the GCMs the cloud feedbacks averaged over 30°S–30°N and the equilibrium global climate sensitivity are both correlated strongly with the east Pacific cloud feedback. To the extent that iRAM results for cloud feedbacks in the east Pacific are credible, they provide support for the high end of current estimates of global climate sensitivity."

Lauer et al. (2010) is not alone in its conclusion that the low-level cloud cover feedback will be positive.  Other studies analyzing satellite data from the International Satellite Cloud Climatology Project (ISCCP), the Advanced Very High Resolution Radiometer (AVHRR), and the Clouds and the Earth’s Radiant Energy System (CERES)  such as Chang and Coakley (2007) and Eitzen et al. (2008) have indicated that cloud optical depth of low marine clouds might be expected to decrease with increasing temperature. This suggests a positive shortwave cloud–climate feedback for marine stratocumulus decks.

In another recent paper, Clement et al. (2009) analyzed several decades of ship-based observations of cloud cover along with more recent satellite observations, with a focus on the northeastern Pacific.  They found that there is a negative correlation between cloud cover and sea surface temperature apparent on a long time scale—again suggesting a positive cloud-climate feedback in this region.

In short, while much more research of the cloud-climate feedback is needed, the evidence is stacking up against those who argue that climate sensitivity is low due to a strongly negative cloud feedback.

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Comments 1 to 50 out of 103:

  1. That's a heavy weight paper. Had not seen it yet. You deserve your growing reputation John! Thnx for the post! Dan
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    Response: This post was written by Dana (who does deserve his heavyweight reputation as he's written most of the advanced rebuttals).
  2. Richard Lindzen and Roy Spencer believe that the formation of low level clouds resulting from global warming will result in a negative feedback keeping the warming in check. I just don't get it! How much statistical certainty do they have that this is going to happen? My guess is "not too much" so where is the logic in taking such a chance with our only world. Furthermore, suppose they can give a certainty which is ridiculously high, say 99.99% Does it really make sense to allow the chemistry of the planet to change so drastically considering the unknown and potentially devastating consequences? Like I said "I just don't get it!" Bob
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  3. My name's not John, but thanks dansat! Bob Guercio - I agree, the odds are not too good that Lindzen and Spencer are right. Not nearly good enough to bet the farm on.
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  4. This is a superb article. Here is something that is not dumbed-down. There are enough facts on this topic to allow the readers to make their own conclusions.--R.T. Thomes
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  5. Excellent post Dana. It is so sad that the data look so bad.
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  6. Well done Dana. Of course it's not good news, but it at least gives some support for the "instinct" that low cloud sensitivity was more wishful thinking than anything else. On the bright side, someone else just possibly might do further quality work in this area with a better outlook. (If only.)
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  7. What I have never understood about the Lindzen-Spencer position is what do they see happening in the longer term? The clouds don't stop the CO2 build up, so even if they were right, the rise and rise in greenhouse gas concentrations would overwhelm the cloud effect. Or do they imagine cloud cover getting thicker and thicker for the rest of the century keeping pace with rising CO2? And what would be the effect of that on agriculture and the environment? The only relevant negative feedback would be one that began removing CO2 faster than we could pump it into the atmosphere, and there is, sadly, nothing that can do that. And yet people keep quoting Spencer as if this clouds are some kind of serendipitous mechanism that will providentially save us all, no need to worry, keep burning fossil fuel as fast as you like. Or am I missing something in the logic of all this?
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  8. David, as I understand it, people in denial (of any sort) grasp for anything that will allow them to keep their illusion that "everything's going to be all right". To maintain the fiction is all that matters, even if it means saying the sky is green and the sun rises in the west...
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  9. Thanks all. David, I suspect the logic goes that in the short-term, cloud feedbacks will prevent dangerously rapid warming, and in the long-term we'll eventually move away from our reliance on fossil fuels. After all, they're limited resources anyway (particularly oil). I suspect the (wishful) thinking is that we'll run out of oil before climate change becomes too dangerous. And of course there's always the 'warmer is better' mentality - I'm not sure if the Lindzens and Spencers subscribe to that, but many skeptics do.
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  10. I didn't think it could be that simple, thought I was missing something. These guys really believe that cloud cover will carry us through until fossil fuels run out? And then carry us through a lot longer until CO2 levels fall (how?). You have got to be joking, is the obvious comment, but sadly I know they are not.
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  11. "David, as I understand it, people in denial (of any sort) grasp for anything that will allow them to keep their illusion that "everything's going to be all right". " Compare this to your own view which is that an unknown but dominant effect in the climate cant and wont help despite whatever becomes known about it in the future. This result is born from yet another model. Its made to sound definititive which is little more than a LOL.
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  12. The other thing to remember is that some of the theories and 'preliminary models' from Spencer and Lindzen are 'almost working' on the descendant part of the solar cycle, however will seriously break correlation during the next 5 years on the ascendant part of the TSI curve.
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  13. There is really no evidence one way or another as of yet as to cloud feedback. There are suggestions going in both directions.
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  14. Interestingly, this paper: Deep ocean heat uptake as a major source of spread in transient climate change simulations (found from here) indicates that model climate sensitivity is correlated with the model's depth of mixing in the polar oceans. Taking the two together it seems like modelled tropical cloud cover is correlated with modelled polar ocean mixing depth. Does anybody else find this a bit surprising?
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  15. Re: Camburn (13) The obligatory "we-don't-know-anything-for-sure" drive-by. Despite your baseless assertion, evidence does exist. Unless you have something that shows what we've figured out thus far fails to meet some definition of evidence that you are operating under? The Yooper
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  16. It's interesting how these posts help illuminate the broader context. On this thread (and others), water vapor is the dominant GHG and its warming effect is far more significant than any possible effects of CO2. Here, water vapor (a prime component of clouds) will keep us cool and comfortable as that irrelevant CO2 continues to build for years to come. Note to skeptics: Read some of your own stuff in context. Then make an attempt to be internally consistent every once in a while!
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  17. Daniel Bailey@15: 1. We all know that GCMs do not handle the h20 cycle well at all. 2. When I read a paper, I read the error bars and the certainty of the paper. 3. There are papers on both sides of the cloud sensativity scenerio. Non of the papers show, with a credible certainty, that clouds are well modeled yet. Hence the results are uncertain. Show me this evidence that is verifiable and not modeled, but observed. A small area of the world is a start, but we all know regional variances do NOT make climate.
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  18. Camburn wrote : "There is really no evidence one way or another as of yet as to cloud feedback. There are suggestions going in both directions." Except there is far more evidence going one way than the other, as you can see at AGW OBSERVER : Papers on Cloud Feedback observations Papers on the Iris Hypothesis of Lindzen Anti-AGW papers debunked (including Spencer) But so-called skeptics prefer to stick with the lone voices, don't they ? Why IS that ?
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  19. TimTheToolMan, This is a Model Intercomparison study, linked as usual to the appropriate data.
    Figure 1: Annual average TOA shortwave cloud forcing for present-day conditions from 16 IPCC AR4 models and iRAM (bottom center) compared with CERES satellite observations (bottom right)
    and not presented as definitive. Look at the last line:
    ...while much more research of the cloud-climate feedback is needed, he evidence is stacking up against those who argue that climate sensitivity is low due to a strongly negative cloud feedback.
    No study indicating sensitivity somewhat above 3 rather than below is a LOL unless you just don't care about the next generation, and Model Intercomparison Projects (MIPs) seriously contribute to our understanding of climate. These studies lead the various modeling teams to dig into the internals of their models and keep improving their physics of one climate process after another. I don't see any LOL in this picture.
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  20. J Murphy@18: Thank you for the link. Again, I will state that the level of certainty in the models concerning clouds is not there. If you want to read papers that agree with your ideas as gospel, even tho the papers are 100% honest in their assesment of what has been learned, that is your perogative. After all, GAGW is still in the hypothosis stage and has not advanced to theory stage. There is a huge amount to learn about how clouds act/interact. On a global scale, there is nothing deffinitive as of yet.
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  21. Some people underestimate what climate models can do. But check comment 20 here.
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  22. In the Clement et al abstract: "This observational analysis further indicated that clouds act as a positive feedback in this region on decadal time scales." The observational analysis they refer to is a link from clouds to temperature and changes in large scale circulation. If we call the latter, X, it is obvious that X controls clouds which control temperature over the short run. In Lauer et al, they say "iRAM simulates mean clouds and interannual cloud variations that are quite similar to those observed in this region." How similar is "quite similar"? What about diurnal? And like the other paper, they are not considering X as a control on clouds which control SSTs and temperature in general. To be clear, my statements are about the short run, theirs are about the long run. But if the short run causation is circulation -> clouds -> temperature, then I don't think that a different causation is supported over the long run either. In a-detailed-look-at-galactic-cosmic-rays.html, Dana shows a "minor" influence of GCR on clouds, but shows that GCR doesn't explain warming, both correct conclusions for recent warming. The problem is that GCR can also have a major effect on the clouds in the short run which is always reversed because GCRs always reverse. But what if they don't? Obviously other long term factors can compensate, one of which is GHG. In How-we-know-the-sun-isnt-causing-global-warming.html Dana takes a "divide and conquer" approach to solar influences. That works for recent 30 years or so for which we have good data, but doesn't help for example for explaining the MWP. In short, a variety of natural factors plus CO2 warming explains the past 30 years. But if the solar factors align differently in the next 30 years, we could be looking at a situation in which a combination of solar factors negates or overtakes the GHG warming (e.g. increased GCR causing increased clouds, decreased UV causing more blocking, etc).
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  23. Denialism has been mentioned, and the "deny harder" response noted. But often and by no means only here, I get the impression that people in the climate area think of it largely in 19th century terms. To grasp this modern world one must be acutely aware of denialism as a set of rhetorical tactics. If you bring your concept of denialism up to date it becomes easier to see that we are dealing with industrial strength denialism.
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  24. @Camburn: why are you obsessed with uncertainty? The fact uncertainty exists is no reason for complacency. Say you are playing russian roulette: there is a lot of uncertainty as to whether or not the next shot will be the bullet. Does that diminish the risk in any way? The whole insurace industry is built on managing risk and uncertainty. The life of a professional poker player as well. You have to realize that uncertainty is no reason to dismiss the threat posed by AGW. (Oh, and by the way, what does GAGW stand for? It doesn't seem to be a popular acronym, as I couldn't find much when I googled it. Did you invent it?)
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  25. @Eric: GCRs aren't caused by the sun. You make a lot of long-term suppositions, which is fine, however we have to rely on the current data we have in order to speculate about the future, and the current data doesn't suggest anything but continued warming due to human activity, with a possible increase over that due to increased solar activity. I think skeptics are beggining to grasp at straws. That's the feeling I get reading these comment sections, anyway.
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  26. #22: "GCRs always reverse. But what if they don't?" GCR occurrence on earth is moderated by the interplanetary magnetic field; to a lesser degree by the earth's magnetic field. Thus the frequency of GCRs observed on earth is a 'magnetic field proxy,' which can be correlated with solar intensity. GCR intensity ended 2009 at a record setting peak and "early in 2010 the intensity decreased to 1997-1998 levels," where it remains through October 2010 (date of the report cited). The moral of that story: a solar min corresponds with a GCR max. We measure these things. For GCRs not to reverse (whatever that means) suggests that the sun will no longer cycle? And yet there are already signs of such a 'reverse'. "But if the solar factors align differently in the next 30 years," We can all play 'what if' ... if you like.
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  27. We can play "what if" to our hearts content. I have stated that we don't know a lot about clouds. NASA seems to agree. http://earthobservatory.nasa.gov/Features/BlanketClouds/
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  28. archiesteel@24: I hit the wrong key...it should have been: CAGW. Catastrophic AGW.
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  29. Archiesteel@24: I am not possessed with uncertainty. I read the papers for what they propose. This is a bit on and a bit off topic now: 1. Does co2 play a factor in climate. Yes, to a small extent. 2. Do we need to stop burning fossil fuels? Yes, emphatically. They are a finite resource that can't be replaced. 3. Should we use CAGW as the reason to stop burning fossil fuels? Naw....the error bars with the models etc are not up to snuff to do so. Should we use our heads and stop because fossil fuels are finite?.....YES....it is very easy to see that to any person with an ounce of brains. I look at the science for what is has shown. I see the AGW science as infant and subject to an enormous amount of factors, some known, a lot to be learned. That is why you don't see the public engaged. I have found this forum to be an interesting forum. I also see that some refuse to acknowledge how much we do NOT know. Just as the link I posted from NASA shows. Forget the carbon tax, that is foolish. Even Mr. Hansen thinks that is foolish. Investigate the origin of that idea. Goldman Sachs should come to mind. A money tree for them. There are solutions with proven tech to stop using so much fossil fuels. It is time to impliment them for the right reason: Fossil fuel is finite!
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  30. Camburn # 27 You found a 1997 paper: Zender, C. S., B. Bush, S. K. Pope, A. Bucholtz, W. D. Collins, J. T. Kiehl, F. P. J. Valero, and J. Vitko, Jr. 1997. "This article contributed by NASA's Distributed Active Archive Centers (DAACs.)"
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  31. Yes Pete. And since then we still havne't figured out clouds.
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  32. @Camburn: "I am not possessed with uncertainty." Your posts tell a different story. To you, "uncertainty" seems to be reason for you to think the science is immature. It isn't, and uncertainty wouldn't be a sign it is either. "Does co2 play a factor in climate. Yes, to a small extent." Actually, CO2 is the main control knob of climate. There is ample evidence suggesting this is the case, and very little evidence supporting the idea that CO2 only has a small effect on climate. Or, to put it in a way you will understand, there is much greater uncertainty about CO2 having only a small effect than to the contrary. "Should we use CAGW as the reason to stop burning fossil fuels? Naw....the error bars with the models etc are not up to snuff to do so." That is your opinion, but it is not supported by the overwhelming evidence in favor of AGW. "I look at the science for what is has shown. I see the AGW science as infant and subject to an enormous amount of factors, some known, a lot to be learned. That is why you don't see the public engaged." Actually, the science is far from being in infancy, and the majority of factors are known. Again, you exaggerate how much we don't know. Furthermore, that is not why the public is not engaged. Mostly, the propaganda efforts of Big Oil, who tries to either deny the science, or muddy the issue so much it confuses people about the actual state of the science. As for the Carbon Tax, I think you are confusing this with Cap and Trade. The two are very different. Please explain to me how Goldman Sachs would make money with a Carbon Tax? At least you agree that we should transition away from fossil fuels, but that doesn't mean you are right about the state of the science. It's pretty clear to me that you're not.
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  33. @Camburn: "Yes Pete. And since then we still havne't figured out clouds." Look at it this way: clouds may or may not lessen the impact of CO2 on warming. This does not put into dispute the actual CO2 impact, which is well-known. In other words, the uncertainty here is whether or not clouds will mitigate the warming, not if the warming will take place. To take a gamble on clouds that way isn't a rational position, it's wishful thinking (especially when the science tends to indicate they won't). To take my russian roulette example again, betting that clouds *might* mitigate AGW is like hoping that the gun should jam if you pull the trigger and get the bullet.
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  34. Camburn - you need to be more careful with your terminology. You said in comment #13 "There is really no evidence one way or another as of yet as to cloud feedback". That is flat-out wrong. In this article alone I discussed 4 studies providing evidence for a positive cloud feedback. You may not be convinced by this evidence, and I'm sure there have been a couple of studies providing some evidence for a small positive or negative cloud feedback, but there is most certainly evidence. And nobody is claiming the issue is settled. As Pete noted in comment #19, the article ends by saying "...much more research of the cloud-climate feedback is needed..." And of course a problem with the "don't regulate CO2 because the uncertainty is too large" argument is that uncertainty goes both ways. There's just as much likelihood that AGW will be majorly catastrophic as there is that it will be benign. Is it possible that a negative cloud feedback will save us from catastrophic warming in the near-term? Certainly. But it's also possible that a positive cloud feedback will drive us toward catastrophic warming in the near-term. Failing to act on the possibility of the latter in the hope that the former is true is extremely poor risk management. It's like driving around without auto insurance because you don't think you'll get into a car accident. Uncertainty is not the friend of the skeptic. The only reason not to act to reduce CO2 emissions would be if we are certain that their effects will be benign. As you clearly agree, we don't have that certainty.
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  35. Tim the Toolman #11 Er Tim, the stakes are our future. If clouds will save us, a lot of scientists will have egg on their faces. If clouds don't save us then vaya con Dios civilisation. Do you feel lucky?
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  36. "And of course there's always the 'warmer is better' mentality - I'm not sure if the Lindzens and Spencers subscribe to that, but many skeptics do." I don't know about Spencer, but his co-worker and co-religionist John Christy has definitely argued that warmer will be advantageous anyway.
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  37. dana1981@34 Thank you. I am thinking on a global scale in regard to studies. I have read papers that go both plus and minus as far as sensativity. With that in mind, I have to think that the sensativity issue is still wide open. I agree. Uncertainty goes both ways. I acknowledge the upper end and the lower end when reading.
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  38. I don't know enough about the physics of clouds to be certain, so I might sound a bit naive by saying the following. Isn't the Lindzen/Spencer scenario that the small increase in planetary average temperature from increasing CO2 will cause the relative humidity to increase, primarily in the troposphere - extra water vapour will be present and will lead to the formation of larger, or more numerous, clouds, which will reflect back sunlight thus cooling us back down in a negative feedback? Where I might be naive is that, as a hang glider pilot, I know, because I have felt it (we're right out in the air flow) that in the gaps between the clouds there is plenty of water vapour too. If Lindzen et al speculate that the feedback from increasing water vapour is negative because of the increase in planetary albedo due to more clouds, have they fully taken into account the increased greenhouse effect from the un-condensed water vapour in the space between the clouds, the area of which is usually much greater than that of the clouds themselves... Comments?
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  39. dhogaza@36: Past temp proxies and the advancement of mankind indicate that warmer is better. The Golden Age of the Holocene happened at the optimum. Back to clouds tho.
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  40. Camburn #39 Possibly warmer might be better in the long run once the climate has settled down to the new equilibrium but there might be horrendous instabilities in the interim period. Do you feel lucky?
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  41. Re: Camburn Once upon a time, warmer seems to have been better. Let's look at that graphically, shall we? But with what we know of the radiative forcing of CO2 and its status as the Chief Control Knob of Temperatures, consider this graph carefully: At some point we may have to update the top graph with an arrow with this legend:
    "Agriculture ends"
     Let the good times roll... The Yooper
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  42. Nick@40. I feel very lucky. Yooper@41. Ag isn't going to end. This is the end of my off topic additions. We shall save those for threads that talk about the subjects raised that were off topic. Thank you.
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  43. Great post. Very useful, clear and authoritative. Many thanks.
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  44. As Camburn notes, this really isn't the place to argue whether warmer is better. Nevertheless since the issue has been raised, I will make one comment on the subject. There is quite obviously a point at which warmer is no longer better, otherwise we could live on Venus. I think Daniel Bailey's first graphic in comment #41 illustrates quite nicely that we can no longer use past human civilizations to assess whether "warmer is better" because we're about to move well outside the range of temperatures experienced during those civilizations. "Warmer is better" is a huge oversimplification.
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    Moderator Response: Everybody please comment on whether warming is bad on the thread...wait for it...It’s not bad.
  45. @Camburn: temperatures are already at similar levels to that of the Holocene (if you look at the entire globe). "I have read papers that go both plus and minus as far as sensativity. With that in mind, I have to think that the sensativity issue is still wide open." The fact that there exists two points of view on the same issue does not mean the two points of view are equally valid. The fact is that a preponderance of papers argue for high sensitivity, and are supported by evidence, does count. It's not enough to read papers, you also have to understand them and be able to evaluate their merit. The fact you feel the issue is still "wide open" is a clear sign this isn't what you're doing.
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  46. Camburn wrote : "Show me this evidence that is verifiable and not modeled, but observed." So, I have pointed you towards some observational studies, and the lead article has more, but then you move onto : "Again, I will state that the level of certainty in the models concerning clouds is not there." But what about the observations, then ? You also quickly moved onto : I am thinking on a global scale in regard to studies after first stating : A small area of the world is a start, but we all know regional variances do NOT make climate. What is it, exactly, that you are trying to argue for ? And why do you imagine that "regional variances" have no effect on regional climates ? Camburn wrote : "After all, GAGW is still in the hypothosis stage and has not advanced to theory stage." G(or C)AGW, whatever either of them mean (each so-called skeptic has their own personal views of what they believe AGW means and what they think the 'C' bit means - means nothing to the rest of us) is a strawman. AGW is a theory, as strong as any other theories, e.g. Evolution. Look up the difference between Theory and Hypothesis. Camburn wrote : "Does co2 play a factor in climate. Yes, to a small extent." Can you reveal your sources for that "small extent" ?
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  47. I am not a good poster but hopefully this will work: http://woodfortrees.org/plot/hadcrut3vgl/from:1995/to:2010 The temperature trend for the past 15 years has been flat. That should be a start for "small extent". And this temp chart brings us back to co2 and climate sensativity as a whole. People will want to point to warming since 1970 as evidence of AGW. Yet, they ignore the warmth of the early and mid 20th century. http://woodfortrees.org/plot/hadcrut3vgl/from:1850/to:2010 It took until the early 1980's to recover to levels exibited in the 1870's and 1940's. And this is getting so far off topic. Look at the temperature graphs. I used hadcrut3 gloabal. Hope that is acceptable.
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  48. Camburn, again this isn't the place to argue about temperature trends, but here is the exact same data you plotted, but with a linear trend added. As you can see, it is not flat - not even close, aside from the fact that HadCRUT shows less warming than data sets which account for the Arctic. But back to the subject at hand, while it's true that these are regional studies, they are of a very critical region. As I quoted from the Stowasser 2006 study, "In terms of the sensitivity of the global-mean surface temperature, almost all the differences among the models could be attributed to differences in the shortwave cloud feedbacks in the tropical and subtropical regions." And climate sensitivity studies do not ignore previous warm (or cold) periods. Quite the opposite.
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  49. "Yet, they ignore the warmth of the early and mid 20th century." Camburn, this is patently false. Why dont you read the IPCC WG1 to see how false. "AGW" is not so much a theory as an outcome of the theory of climate - which basically says that climate will respond to whatever is the net value of the forcings present. Funnily enough the forcings (solar, aerosol, GHG) have all varied the last century and climate change has followed our best estimates for what those net forcing were - not just in terms of temperature but also in terms of the fingerprint (summer/winter temps etc) expected. Ignored my foot. Its covered in depth in the published science.
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  50. #25 archiesteel "....with a possible increase over [CO2 warming] due to increased solar activity." and #26 muoncounter "We can all play 'what if' ... if you like." Thank you for your responses. The sensitivity to CO2 warming is by definition a 'what if' game because sensitivity is modulated by external factors (solar and celestial) which are not predictable, they can go either way. A paleo-based calculation of sensitivity has to consider variations in external factors, solar or not (e.g. GCR) which are not adequately represented in the paleo record. When GCRs increase or decrease due to some celestial cause (modulated by the sun), both the paleo temperatures decrease or increase in response and CO2 follows. We see that happen in centuries or longer (not to mention that's all the resolution that we have in ice core and other proxies). So CO2 feedback is a minor and slow feedback factor; paleo or present day (obviously nature is damping man's additional CO2, not amplifying it). Thus the CAGW hypothesis depends on water vapor amplification of CO2 warming. Talking about methane is likewise not convincing since it will take centuries for enough permafrost to melt to get methane feedback. Water vapor is partly covered in the cloud discussion above and partly not. The non-cloud portion of the water vapor feedback depends on the distribution of water vapor (the evenness) which in turn is somewhat dependent on clouds, but also winds, precipitation, soil moisture, etc. The claim that the major uncertainty in sensitivity can be adequately covered by examining a handful of modeled and measured cloud types and a few parameters (optical depth and albedo) is not correct. The amplification of CO2 warming by water vapor is determined by planet-worth of weather such as the oft-trumpeted claim of more numerous storms (a negative feedback). Numerous water vapor modulations must be considered and, more importantly, the external factors like GCR can scramble the entire equation. Thus predictions are by definition a what-if game but those external factors also modulate sensitivity and are not considered at all in the models referenced above.
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