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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 101 to 103 out of 103:

  1. Eric @ 100... 3C globally means more like 8-10C with arctic amplification. Do you have a reference for the study your talking about?
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  2. Yes, what Rob said. Melting ice decreases both local and overall planetary albedo. I don't think it's possible to justify the claim that melting ice will not have a significant impact on global temperature. Particularly if significant methane deposits are released. The other issue is that we're currently on pace to double atmospheric CO2 levels in the next 75 years or so. That's doubled from current levels, not pre-industrial levels. So with a 3°C sensitivity to 2xCO2, you're talking about 3°C warming from now, 4+°C warming from pre-industrial levels. Or if you choose to believe the low end of the sensitivity range - which according to the article I just wrote, you probably shouldn't - it's 2°C warming between now and the end of the century, 3°C above pre-industrial. And remember, the 'danger limit' is 2°C above pre-industrial.
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  3. #101, Rob, it was the Tim Lenton article discussed at http://www.realclimate.org/index.php/archives/2007/08/musings-about-models/ After reading it I pointed out that the tipping points were local and had essentially no chance of "tipping" the global temperature (my comments are #17, #65, and later). There are some good responses to those as well. The article link seems to be broken, I will try to find it later. #102 Dana, thanks for the correction. I see now that 0.2 per decade (if that is sustained decade by decade) would actually be a 3C sensitivity. Your CO2 as a pollutant thread has some interesting discussion: legal definitions, the contribution of science to policy, etc. I think BP has a special knack in those areas.
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  4. #100: "the high degree of blocking from local stratospheric warming from GCR spikes." You've totally lost me with that idea, but I am taking this comment over to the cosmic ray thread.
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  5. #102: "we're currently on pace to double atmospheric CO2 levels in the next 75 years or so. That's doubled from current levels," Dana, even at 3ppm per year, 75 years is 'only' another 225 ppm. I'm sure we'll hit 390 this spring, but it'll take a while longer to double than 75 years. Besides, I'm thinking by 450ppm, things will get downright ugly.
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  6. global warming was last on the list of early 2010 priorities. now still deeper into the recession with still super high unemployment, hostile north korea, nuclear capable iran, extremely sensitive confidential us documents leaked...i suspect that global warming importance has slipped to number 32,966 on a list of top 100. i commend this website for it's continued passion for keeping global warming alive. though it may seem like a losing battle...it is the vocal minority that actually elicit change...whether for good or for bad.
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  7. Re: jasonk (106) Global warming won't stop because people in general in the US (or anywhere else, for that matter) don't currently place a high priority on it. They longer they wait, the worse it will get. And the less that can be done about it. More's the pity. Meanwhile, we vocal few here still try to talk about the science, not electrical flights of fancy or iris effect pixie dust. It's what we've decided to do with the time we have. While the time to make a difference still has not completely elapsed. The Yooper
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  8. Under are comments authors to this paper: Boyce et al., 2010.: “But their numbers have dwindled since the dawn of the 20th century, with unknown consequences for ocean ecosystems and the planet's carbon cycle., … the global population of phytoplankton has fallen about 40 percent since 1950., That translates to an annual drop of about 1 percent of the average plankton population between 1899 and 2008., It's very disturbing to think about the potential implications of a century-long decline of the base of the food chain ..., They include disruption to the marine food web and effects on the world's carbon cycle., Phytoplankton productivity is the base of the food web, and all life in the sea depends on it., In addition to consuming CO2, phytoplankton can influence how much heat is absorbed by the world's oceans, and some species emit sulfate molecules that promote cloud formation.
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  9. muoncounter #105 - by "current pace" I was referring to an accelerating, not linear increase. For example see the upper IPCC emissions scenarios, which are what we're currently on pace with.
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  10. Camburn @64 1. Statistically, and this is important unless you want to throw statistical analysis out with the wash, we have not warmed for the past 15 years. "Because the temperature has random ups and downs on top of the long term warming trend, you need about 16 years to conclude (with 95% confidence) that a trend in those 16 years is not a result of chance. So if the only temperature data we had was 1995-2009 (15 years!) we couldn't conclude with 95% confidence that there was a trend. But we also couldn't conclude...that there is no trend -- there simply isn't enough data for a firm conclusion. But we have more data than just 1995-2009, so we can conclude (with greater than 95% confidence) that there is a warming trend. "In fact, since we have data for 2010 now, it is even now true that we have statistically signficant warming since 1995." I'd like to join archiesteel in asking you to acknowledge your error.
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  11. Arkadiusz Semczyszak @108, It would be helpful if, instead of simply posting quotes from papers, you provided some context or explanation for them. What point are you trying to make?
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  12. Through July 2010, were the warmest 14 months in the temp records, and through May -the warmest 12 months. With less than a month left of 2010, it is still not known if 2010 will set a record, despite the fact that La Nina has already started. 1998 was nearer the top of the 11 year solar cycle and 2010 is closer to the bottom. The 2009-2010 El Nino was nowhere near as strong as 1998-99. And yet - Every year since 2001 has been warmer than any year in the records before 1998. I really don't get why the "warming has stopped" argument keeps getting repeated.
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  13. It looks like Spencer is pounding the table again today that "hen cloud changes cause temperature changes, it gives the illusion of positive cloud feedback – even if strongly negative cloud feedback is really operating!" I think Spencer's all wet, but I haven't seen a thorough analysis of this line of argument. Spencer basically argues that climate scientists have cause and effect all mixed up. Unlikely, but an interesting line of thought. I have not seen a really good response to e.g. Spencer & Braswell- is there one?
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  14. Hot off the press: Dessler (2010) "Estimates of Earth's climate sensitivity are uncertain, largely because of uncertainty in the long-term cloud feedback. I estimated the magnitude of the cloud feedback in response to short-term climate variations by analyzing the top-of-atmosphere radiation budget from March 2000 to February 2010. Over this period, the short-term cloud feedback had a magnitude of 0.54 T 0.74 (2s) watts per square meter per kelvin, meaning that it is likely positive. A small negative feedback is possible,but one large enough to cancel the climate’s positive feedbacks is not supported by these observations. Both long- and short-wave components of short-term cloud feedback are also likely positive. Calculations of short-term cloud feedback in climate models yield a similar feedback. I find no correlation in the models between the short- and long-term cloud feedbacks."
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  15. Ha! And here's some additional commentary from Dessler, addressing Spencer directly. My cup runneth over. Especially this part:
    And as far as my interest in influencing the policy debate goes, I’ll just say that I’m in College Station this week, while Dr. Spencer is in Cancun. In fact, Dr. Spencer had a press conference in Cancun — about my paper. I didn’t have a press conference about my paper. Draw your own conclusion.
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  16. Camburn @64:"1. Statistically, and this is important unless you want to throw statistical analysis out with the wash, we have not warmed for the past 15 years." Leaving aside the disingenuousness of totally ignoring a *93%* level of significance for said warming: Start from when Phil Jones made that comment, and go back to 1995 -- *16* years instead of 15. Voila: warming, at or better than the 95% significance level. Or: Start from when Phil Jones made that comment. Go back to 1995 (15 years) Add the rest of 2010 since he made that comment. Voila: warming at or better than 95% significance level. Still feeling lucky, Camburn? Btw, I can see that at this point, having lost on the substance of Dana post, you're just robotically going down the same old well-worn list of 'skeptic' talking points. The Jones quote; the Trenberth quote; can 'hide the decline' be far behind?
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  17. Damn, I wish Dessler's paper had come out a week earlier! Maybe I'll have to do a new blog post as an addendum to this one...
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  18. This paper addresses cloud feedback: http://www.esrl.noaa.gov/psd/people/dezheng.sun/dspapers/Sun-Yu-Zhang-JC-revised.pdf
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  19. Steven: As far as staticistical temps.....2010 is not over. I still stand by my statement. Also, 93% does NOT cut it.
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  20. Re: Camburn (118) Generally it is both good form to provide some insights (if you have any) into what readers of a linked source can expect to find therein and to provide a link to the published version of the paper, not a submitted version. The Zhang paper is fairly long in the tooth (submitted in 2007, accepted in 2008, published in 2009) for a cloud paper. Compare and contrast it to this recent paper by Dessler et al (to which Albatross has already linked to previously in 114 and to which dana1981 has then referred to above at 117) which find a net positive contribution from clouds. (119) Enough data has come in on 2010 to make even the portion available for 1995-2010 (inclusive) statistically significant. So you are wrong on that. The Yooper
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  21. Daniel@ 120: I posted the url so that people could read the paper. The url you posted takes you to the publisher, and is not readable unless you are a member. The paper deals with observed cloud feedback verses modeled cloud feedback. Rather than inflect my opinion, I present the literature for each to interpret. As far as 119: We shall see.
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  22. Camburn: As far as staticistical temps.....2010 is not over. I still stand by my statement. Also, 93% does NOT cut it. 2010 will be over in about three weeks. Perhaps you'll concede the point then? I'm sure there are other people here who will remind you, if I forget.
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  23. #115 syphax, I went to the link you posted (RealClimate). I read the posts below and it is the same I read everywhere including this site. Many questions and a lot of opinions but still no real solid undebatable answers. CO2 will produce warming of the Earth. How much is the big unknown. The cloud issue seems to be the most mysterious of all the factors. No clouds or water vapor feedback and a doubling of CO2 gives a 1.2 C warmup. That is about the only number I see given that most agree to (both camps). On the cloud issue I believe Spencer's view has a logical basis. The observation is less clouds and a warmer ocean. One conclusion feels a warmer ocean produces less clouds, another feels that the fewer clouds warm the ocean (more direct solar radiation gets absorbed). If not for clouds the Earth's total albedo would be around 0.15 and the Earth would be much warmer (The albedo calcualtor indicates about 20F). With clouds the Earth's albedo is 0.3. So some types of clouds may warm the Earth depending upon the thickness, the location and the time of cloud development (night clouds keep the night from cooling as much, thick day clouds in the summer greatly reduce the high temperature, anyone can get this information from personal observations), but overall clouds must cool the earth because of their overall effect on albedo.
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  24. Norman the disagreement is specifically on ENSO, not generally on warm/cold ocean less/more clouds. It's stated explicitly by both Spencer and Dessler, it's ENSO that causes less clouds or less clouds that produce ENSO? The former mechanism is straightforward, the latter is really misterious and not even guessed by Spencer himself.
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  25. Camburn, In addition to what others have posted, if you are going to use statistics, you might as well try to understand what they mean. A 93% confidence means that there is a 93% chance that the observed pattern is 0.93 likely to have not been caused by the inherent variation of the data. That leaves 0.07 for the chance that it is, which in this case, means that at the time the test was made, your statement that "we have not warmed for the past 15 years" had a chance of being correct of 0.07. So, if you are not willing to choose a course of action based on a 0.93 chance of warming, I don't see why you would expect anyone to choose a course of action based on a 0.07 chance of no warming. Yet, that is what you are proposing. Why is that? Nevermind that in the larger context, the window of time that you have chosen to take a stand on is only a fraction of the time for which we have plentiful information about, and the rest of it supports the 0.93 probability position. Lastly, the 5% rule is not really golden; its pretty arbitrary.
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  26. Camburn... Please inform us as to what 93% does not cut it and why? I mean, if you're going to cut down the data set to present a conclusion that falls just short of statistical significance you need to back that up with a meaningful reason why you do so. If there is no specific reason for choosing the 1995 to present data set then, since is falls short of statistical significance for either warming or cooling, then you need to pull back and add data to the set. Pull back until you get a statistically significant data set. Back to say, 1990. Then what do you find? If you absolutely must have 95% confidence or better then you need to use enough data to get to that confidence level. Do this and you absolutely will find unequivocal warming.
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  27. I think it's a waste of time arguing with Camburn over the statistics here. He's clearly demonstrably wrong in many ways, and he's clearly not going to admit that he's wrong. I know we try to avoid the term on this site, but I'm sorry, that's denial. It's not relevant to this article anyway, so I suggest we just let Camburn have his denial and move on. For the record though, Chris G is correct that the generally accepted confidence level of 95% is quite arbitrary, not that it matters since the temperature trend is statistically significant at this level, whether Camburn will admit it or not. Spencer's hypothesis seems rather bizarre to me. Cloud cover is what controls ENSO cycles, yet ENSO cycles generally happen on a pretty regular basis. So what then is causing cloud cover to change on this regular basis? Wait, let me guess - 'natural cycles'?
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  28. Thank you dana1981. If 2010 ends with temps of statistical importance, I will agree. It is pretty plain and simple. Good question as to what causes cloud cover to change during the ENSO cycles. There is no question that it does. Finding the cause is more difficult. Has anyone read Sun's paper?
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  29. "The cloud issue seems to be the most mysterious of all the factors. No clouds or water vapor feedback and a doubling of CO2 gives a 1.2 C warmup." However the water vapor feedback is well-established, with solid observational data from space backing it up. There's nothing going on with temp trends to make one believe that we're observing a significant negative feedback today from clouds, and the arguments that we will magically see such a negative feedback kick in at just the right time to stop further warming, based on dubious speculation, seems like wishful thinking to me.
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  30. Camburn: "Thank you dana1981. If 2010 ends with temps of statistical importance, I will agree. It is pretty plain and simple." What, in your mind, makes 1995 as a starting point more meaningful than 1994? Other than the fact that cherry-picking 1995 allows for the claim that since that date, significance onlyr reaches 93% rather than 95%. So, what's your non-cherry picking scientific reason for picking that date? Also, 95% significance for publication etc has been more or less picked out of a hat by statisticians, there's no theoretical basis for that number, and indeed some fields typically accept less significance, while others require much higher significance. Statistics are a tool, and the 95% level is a guideline, nothing more. Fisher (the Godfather) talked about this quite a bit, he'd be the last in the world to insist that >=95% confidence means "true" while <95% means "false". Such claims are just bullshit.
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  31. dhogaza #129 - Yes, Dessler discussed the well-established water vapor feedback in his paper, as opposed to the much more uncertain cloud feedback. We'll discuss that in the blog post on his paper. Besides which, there is a large body of empirical evidence that climate sensitivity is 2 to 4.5°C for 2xCO2. I agree, the speculation that there will miraculously appear a negative feedback just in time to prevent dangerous warming levels is wishful thinking.
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  32. dana1981: I look forward to discussing Dessler's paper. dhogaza: There are 30 year cycles to climate. 1/2 of that cycle is 15 years. That is why the 15 year temp criteria. The reason for the 95% is that is the significance level of HadCrut temp data. That is stated in the literature. I did not pull that out of thin air. Back to clouds again. No comment on Sun's paper at all?
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    Moderator Response: [Daniel Bailey] If you are going to respond to archiesteel at 133 below with links to sources, per the Comments Policy, please provide a summary of what you think the linked reference means in support of your position. Thank you!
  33. @Camburn: "There are 30 year cycles to climate." Really? I'm skeptical. Do you have any evidence to back this claim up?
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