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Opposite Views on Climate Feedbacks (and perhaps the answer lies in the middle)

By Chip Knappenberger -- March 5, 2009

Just how much warming should we expect from rising levels of atmospheric greenhouse gases (GHGs)? The answer largely hinges on how much extra warming might be generated by the initial warming—that is, how strong (and in what direction) are the feedbacks from water vapor and clouds.

By most estimates (including climate model outcomes), these feedbacks are positive and result in about a doubling of the warming that would result from greenhouse gas increases alone. By others, however, the total feedbacks are negative, and imply that the total warming will be less than the warming from greenhouse gas increases alone, and only a fraction of that which is commonly expected.

The ultimate warming experienced across the 21st century will depend on the combination of greenhouse gas emissions and how the climate responds to them. The feedback issue is an essential part of the latter, for it spells the difference between a high climate sensitivity to greenhouse gas doubling (say 3-4ºC) and a much lower one (1-2ºC).

As to where the answer lies, the devil is in the details, and in this instance he is hard at work, as the processes involved are exceedingly complex—difficult to not only to fully understand, but even to adequately measure. In a “Perspectives” piece in a recent issue of Science magazine, Andrew Dessler and Steven Sherwood attempt to put to rest any notion that the feedbacks on global temperatures are anything but positive and significant. Dessler and Sherwood point out that the role of water vapor plays the largest role in the feedback process—higher temperature (from greenhouse gases) lead to more water vapor in the atmosphere which leads to even higher temperatures still (as water vapor, itself, is a strong greenhouse gas).

But actual hard evidence that water vapor is increasing in the atmosphere has been hard to come by. Dessler and Sherwood review the recent literature on the topic, including an important contribution from Dessler et al. published last year, and conclude that there now exists sufficient evidence to conclude that atmospheric water vapor is increasing very much in line with climate model expectations and that this increase produces roughly twice the global temperature rise than does anthropogenic greenhouse gas enhancement alone. Meaning, of course, that all is generally right in model world, or as they put it: “There remain uncertainties in our simulations of the climate, but evidence for the water vapor feedback—and the large future warming it implies—is now strong.”

But apparently Dessler and Sherwood didn’t convince everyone that this is the case. One notable person who was less than impressed that this was the whole story was Roy Spencer, who has himself been working on the feedbacks issue. Spencer points out that water is actually involved in two feedback processes—the first, through water vapor as described by Dessler and Sherwood, and the second, through water droplets, or, more commonly, clouds. Changes in the patterns (horizontal, vertical, and temporal) and characteristics (droplet size, brightness, etc.) of cloud cover play an important role not only in the earth’s climate, but in how the climate responds to changes in the greenhouse effect. And, as you may have guessed from their ephemeral nature, the behavior or clouds is not particularly well-understood, and even less well modeled.

Spencer has been looking into the cloud part of the feedback processes. Over the past several years, during the period when Dessler et al. (2008) finds a positive feedback from increases in water vapor, Spencer, in his investigations, finds that cloud cover changes produce a feedback in the opposite direction. And when he adds these two effects together, he finds that the total feedback from warming-induced changes in water in the atmosphere to be negative (that is, the cloud effect dominates the vapor effect). Granted, Spencer’s investigations are far from complete and even farther from being generally accepted, but they do raise important concerns as to the ability of examinations of short-term behavior to diagnose long-term response (a situation relied on by both Spencer, and Dessler and Sherwood). Spencer concludes that “unless you know both [vapor] and [cloud] feedbacks, you don’t know the sensitivity of the climate system, and so you don’t know how much global warming there will be in the future.” Virtually the opposite sense of things than that put forth by Dessler and Sherwood.

Obviously, the final arbiter will be the earth’s climate itself, as it is the true integrator of all forces imparted upon it. But, still today, we struggle to even accurately observe the finer details of how it is responding to the changes to which it is being continually subjected. And we are further still from understanding the processes involved sufficiently to produce unassailable models of the climate’s behavior, much less future projections of its response response (as evidenced by the recent slowdown in the rate of global temperature increase despite ever-growing greenhouse gas emissions). And so the process of science continues…

[Breaking news: A new peer-reviewed paper has just been published in the journal Theoretical and Applied Climatology, by researchers Garth Paltridge and colleagues which finds that the increase in atmospheric water vapor that, according to Dessler and Sherwood most definitely accompanies the increase in temperature, is absent in one of the primary databases used to study climate behavior—the so-called NCEP reanalysis data. The authors admit that perhaps there are errors contained in this dataset which may explain their results, but as it stands now (and unless some errors are identified) the reanalysis data supports a negative water vapor feedback. Paltridge et al. conclude:

Negative trends in [water vapor] as found in the NCEP data would imply that long-term water vapor feedback is negative—that it would reduce rather than amplify the response of the climate system to external forcing such as that from increasing atmospheric CO2. In this context, it is important to establish what (if any) aspects of the observed trends survive detailed examination of the impact of past changes of radiosonde instrumentation and protocol within the various international networks.

Lead author Garth Paltridge describes the trials and tribulations of trying to get this result (which runs contrary to climate model expectations) published in an enlightening article over at ClimateAudit, including how at least one of the Dessler and Sherwood authors knew of Paltridge’s soon-to-be-published results and yet made no mention of it in their Science piece. Hmmm, so much for an open discussion of the science on this issue.]


Dessler, A.E., and S. C. Sherwood, 2009. A matter of humidity. Science, 323, 1020-1021.

Dessler, A.E., et al., 2008. Water-vapor climate feedback inferred from climate fluctuations, 2003-2008. Geophysical Research Letters, 35, L20704.

Spencer, R., and W.D. Braswell. 2008. Potential biases in feedback diagnosis from observations data: a simple model demonstration. Journal of Climate, 21, 5624-5628.


  1. jae  

    Well, I would add that the fact that temperatures have not increased for over 10 years (air or oceans), despite continually rising CO2 levels, should at least make one question this positive feedback hypothesis. Where’s the beef?


  2. TokyoTom  

    Chip, three questions:

    – what is your view about what the paleodata tell us about long-term sensitivity, including “‘slow” climate feedback processes not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments?

    Jim Hansen has said that “Paleoclimate data show that climate sensitivity is ~3°C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO2 for the range of climate states between glacial conditions and icefree Antarctica.” (see the link at my name above)

    – even if sensitivity is low, aren`t we looking at at least two doublings?

    – why is it that you and Pat never seen to write about ocean pH? Is this just something you guys have never looked at, and so act like it`s safe to ignore?


  3. Andrew  

    Hansen’s estimates aren’t the only game in town, Tom, and some more recent estimates are lower:
    Anyway, this paleoclimate derivations rely on the presupposition that CO2 was a major rather than bit player in the glaciation cycles (the justification for this reminds me of ID: “What else could explain it other than CO2/A Designer?) which maybe rather popular but hardly proven (we just don’t have the data to make such attributions.


  4. TokyoTom  

    Chip, my understanding is that Annan was doing a Bayesian analysis of the short-term sensitivity estimates that are used in the models. If so, then he hasn’t factored in the possibility that Hansen raises of a long-term (centuries/millennia) equilibrium sensitivity, including slower surface albedo feedbacks, of ~6°C for doubled CO2 for the range of climate states between glacial conditions and icefree Antarctica. Doesn’t sound like you address this possiblity either.

    – As noted, its all seems that you are ignoring that, even if short-term sensitivity is low, that we run a very good chance of having two doublings.

    Why speak narrowly on solely “estimates of the strength of the water vapor feedback and on-going research into these estimates” to a lay group that is interested in the big picture? Why not provide a bigger picture?

    – Such a bigger picture would also include the ongoing rapid ocean acidification, which will only increase as we continue to pump up CO2 levels.

    – So: “As to where the answer lies, the devil is in the details, and in this instance he is hard at work, as the processes involved are exceedingly complex—difficult to not only to fully understand, but even to adequately measure” – we don’t really know where we’re headed. BUT, just as for ocean pH, “let’s hope things adapt (as they have in the past)” (ignoring that past adaptation to ocean pH was on the scale of very long periods, and not a few decades/centuries).

    Maybe you can help me to better understand, in the face of all this uncertainty, that you consider the ramping up of our global limate experiment to be the “conservative” thing to do? Or are you encouraging wishful thinking and do-nothingism simply out of resignation?


  5. Rob  

    “Well, I would add that the fact that temperatures have not increased for over 10 years (air or oceans), despite continually rising CO2 levels, should at least make one question this positive feedback hypothesis. Where’s the beef?”

    This is true, as long as you take 1998 (a major El Nino year) as the starting year for your analysis. Looked at over other time frames…. not so much.


  6. Andrew  

    Rob-What time frames are appropriate and which aren’t? After all, it would be rather important to determine the answer to this question, wouldn’t it? Well, the point, as I see it, that the “pause” over this period makes is that warming is not accelerating (as Gore and pals often claim) but proceeding at either the same rate as it has since 1979 or perhaps even slower.

    Tom-why exactly is Hansen’s “long term sensitivity” estimate better than Annan’s short term estimate for policy purposes (or Chylek and Lohmann’s for that matter)? Your accusing Chip of believing in “do nothingism” implies you think something can be done-well, how do you propose to provide energy for the world while drastically cutting emissions? If you are so worried about Ocean pH, perhaps you should poor truck loads of Heartburn medicine into it? Feel good solutions feel good, but they don’t work at all.


  7. russ  

    The Real Problem is Not Tomorrows CO2 But Yesterdays CO2

    EU President says environmentalists want to command human behavior. Hearing the president of the EU frame the context of “global warming” in this context is most welcome. It inspires comment. He is both right and wrong. He is quite correct in his observation that the dark green movement has been and is exploiting global warming and climate change to seize power and run modern society back to some stone age fantasyland. He is utterly wrong to equate this with the notion that there is no problem with fossil CO2. There is a big problem only its not bearing down on us at the slow pace of changing glaciers, it is much near than that.

    While many international leaders debate or work toward emission reduction strategies and carbon capture and storage the real problem is not tomorrows CO2 but yesterdays CO2. Nor is the central problem the role CO2 has in Global Warming.

    We must turn our attention to the 1000+ gigatonne carbon bomb, two centuries of accumulating CO2, still mostly in the air as it takes centuries for airborne CO2 to equilibrate with the rest of the planet. Reports call the alarm of ocean acidification, adding acid flames to the raging fires of fossil CO2. What’s missing is mention of the best, only, means to fight ocean acidification and CO2 in the air.

    Just 500 gigatonnes of yesterdays CO2 has reached the oceans where Revelle’s Rule tells us 80% of CO2 ends up. The first carbon bomb will be exploding in the ocean for more than a century even if we stop the emission of new CO2 today. No amount alternative energies, recycling, bicycling, or “clean coal” will tend to the first carbon bomb. Sure lets reduce the size of the second bomb but first things first. Here’s how.

    ONLY ocean replenishment and restoration can enlist, as allies, the most powerful force of nature – the ocean plants, the bloomin’ plankton. But high and rising CO2 in the air is not only responsible for ocean acidification worse it has fed green plants on land making them greener, bushier, and living longer making them “good ground cover.”

    Ground cover improvements have reduced the amount of dust blowing in the winds by 1/3 in just a few decades. For the oceans dust in the wind brings vital mineral micro-nutrients that terrestrial Yin (dust) is just as important as rain, the Yang, blowing from sea to land nurturing plant life. Since earth and ocean satellites went aloft 30 years ago we’ve measured decimation of ocean plants, 10% are gone from the Southern Ocean, 17% from the N. Atlantic, 26% from the N. Pacific, and 50% from the tropical seas. Just yesterday, a few decades past, ocean pastures grew more verdant consuming 4-5 billion tonnes more CO2 each year than today.

    Today, as stewards of our blue planet, we must replenish ocean micro-nutrients to restore the verdant ocean pastures. If we bring the ocean plankton blooms back to levels seen only 30 years ago those plants will annually convert billions of tonnes of CO2 into ocean life instead of acid ocean death. Those verdant restored ocean pastures will deliver 7 times the CO2 reductions called for by the Kyoto Protocol.

    To begin, and we must without delay, the work requires only tens of millions of dollars, to succeed in a matter of a decade requires only a few billion dollars. In the bargain the restored oceans will feed everything from tiny krill to the great whales and everything and everybody in between – fish, seabirds, penguins, seals and us.

    Replenish and restore the oceans without delay.


  8. Climate Impacts of Waxman-Markey (Part II)—Global Sign-Up — MasterResource  

    […] models, produces the greatest warming by the end of the century—about 4.5ºC (although the world abounds with observations that suggests that this temperature rise is overblown, but that is the subject of […]


  9. Houston Chronicle: Knee-Jerk Climate Alarmism, Policy Activism w/o Economic Analysis — MasterResource  

    […] CO2 emissions, casting the high-sensitivity models in a harsh light? Some very interesting work on feedback effects appears to explain the reasons for the models’ overestimation of anthropogenic warming. And […]


  10. Lana Lang  

    Andrew said:
    “Tom-why exactly is Hansen’s “long term sensitivity” estimate better than Annan’s short term estimate for policy purposes (or Chylek and Lohmann’s for that matter)? Your accusing Chip of believing in “do nothingism” implies you think something can be done-well, how do you propose to provide energy for the world while drastically cutting emissions?”

    Why should the longer-term view including ice-sheet disintegration be included? It is really in order to consider the longer-term effects over the next few hundred years, and consequences for future generations, rather than just your lifetime.

    You ask; “how do you propose to provide energy for the world while drastically cutting emissions?” – and I would infer this point suggests you believe in resignation; that nothing can be done. Obviously there are ways to provide energy while cutting emissions; it’s called renewable energy.


  11. Why is Water Vapour, the Most Important Greenhouse Gas, Ignored? | Dr. Tim Ball  

    […] a positive feedback and is at the center of the debate of climate sensitivity. Evidence shows the positive feedback is wrong and climate sensitivity is overestimated. Negative trends in [water vapor] as found in the […]


  12. Hair Loss  

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  13. cknappenberger  


    My article was obviously not a review of paleo- climate sensitivity estimates, or the inherent difficulties of making useful estimates of the climate sensitivity using any method or datasets (paleo or otherwise). I think, as you know, James Annan and others have had many interesting discussions of this topic. My article was on estimates of the strength of the water vapor feedback and on-going research into these estimates.

    Perhaps the topics you suggested, including ocean pH changes, I will cover at some point. I am not sure what you want me to say about ocean pH…it is declining, let’s hope things adapt (as they have in the past). I think Craig Idso has a lot to say on the topic and will be making a presentation at the climate change conference in New York next week, for those interested.



  14. cknappenberger  


    At MasterResource, we address a variety of topics and a variety of scope. My article was covering the recent publication of basically an opinion article about the strength of the feedbacks from water vapor under increasing anthropogenic greenhouse gas emissions. I pointed out that there are other folks (scientists working on the same issue) who hold different opinions than Dessler and Sherwood. Has the final determination of the climate sensitivity been made? No. Is it one number that holds over a variety of scales? Probably not. The problem with trying to assess very long-term behavior (multiple centuries to millennia) is that humans will probably subject the earth and its climate to all sorts of things during that time period, and so the “natural” evolution that has occurred in the past at these timescales may not be a good predictor of future behavior (ignoring the fact that we aren’t starting from an ice age). I think that our immediate concerns are on what we may expect in the coming decades to perhaps a century—I haven’t much of a clue as to what our technologies will be even at the far end of that period, so I can’t really speculate on how our climate may respond to them. Perhaps we have different opinions from one another as to the timing and size of the “big picture” and where our concerns should lie. But, in my articles that I write for MasterResource, I describe my take on various aspects of my version of the “big picture.” I am sure you do the same thing on your blogspace.



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