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Infrared Iris Never Bloomed

Posted on 13 December 2011 by dana1981

In 2001, Lindzen et al. published Does the Earth Have an Adaptive Infrared Iris?.  The infrared iris hypothesis suggets that increased sea surface temperature in the tropics would result in reduced cirrus clouds and thus more infrared radiation leakage from Earth's atmosphere.  This radiation leakage in turn would have a cooling effect, dampening global warming as a negative feedback.  NASA explains the hypothesis and why it's called the iris effect:

"Much like the iris in a human eye contracts to allow less light to pass through the pupil in a brightly lit environment, Lindzen suggests that the area covered by high cirrus clouds contracts to allow more heat to escape into outer space from a very warm environment."

Lindzen et al. was published over a decade ago, so how has the iris hypothesis withstood the test of time?

Subsequent Research

In a very short timeframe, a number of other studies had investigated the iris hypothesis.  Approximately 6 months after Lindzen et al. was published, Fu et al. (2001) (revised in early 2002) published a paper which found evidence of an iris effect, but that it was significantly smaller than Lindzen et al. suggested:

"We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback should be small. Although not mentioned by Lindzen et al, tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al., we obtain a feedback factor in the range of −0.15 to −0.51, compared to their larger negative feedback factor of −0.45 to −1.03 [W m-2 K-1]."

A few months later, Lin et al. (2002) published another study examining the iris hypothesis. Lin et al. took observational data from the Clouds and the Earth's Radiant Energy System (CERES) over the tropical oceans and plugged them into the same model that Lindzen used.  This observational data dramatically changed the iris hypothesis, because it showed that the clouds in the tropics are significantly more reflective (a.k.a. higher albedo) and have a weaker warming effect than in the Lindzen model. 

Clouds have two competing effects on global temperature - cooling by reflecting solar radiation, and warming by increasing the greenhouse effect.  Which of these effects is larger depends on the type of cloud. 


The model used by Lindzen et al. had concluded that for clouds in the tropics, the warming effect was greater.  Thus the decrease in cloudcover hypothetically caused by the iris effect would result in less cloud warming, and thus a negative feedback.

Using the CERES data, Lin et al. concluded that the cooling effect is actually larger for tropical clouds, and thus Lindzen's iris effect (if it existed, which this study didn't investigate) would actually result in a weak positive feedback:

"The observations show that the clouds have much higher albedos and moderately larger longwave fluxes than those assumed by Lindzen et al. As a result, decreases in these clouds would cause a significant but weak positive feedback to the climate system, instead of providing a strong negative feedback."

Less than a year after Lindzen et al., a third response paper, Hartmann and Michelsen (2002), was published.  They analysed the same sea surface temperature (SST) and cloud data as Lindzen et al., but concluded that the changes could not be attributed to an iris effect:

"It is unreasonable to interpret these changes as evidence that deep tropical convective anvils contract in response to SST increases. Moreover, the nature of the cloudweighted SST statistic is such that any variation in cloud fraction over the coldest water must produce a negative correlation with cloud fraction, a fact that has no useful interpretation in climate sensitivity analysis.  Therefore, the observational analysis in [Lindzen et al.] lends no support to the hypothesis that increased SST decreases the area covered by tropical anvil cloud."

One year after the publication of the iris hypothesis, Del Genio & Kovari (2002) also found errors in the Lindzen analysis which dramatically changed the conclusions:

"A clustering algorithm is used to define the radiative, hydrological, and microphysical properties of precipitating convective events in the equatorial region observed by the Tropical Rainfall Measuring Mission (TRMM) satellite....The adaptive iris hypothesis (clouds thinning with warming) is clearly not supported by the TRMM data. TRMM storms become larger and cover a greater fractional area, and the largest of them become somewhat brighter, at higher SST. Several flaws in reasoning lead Lindzen et al. (2001) to their conclusion."

So within a year of the publication of Lindzen et al., there was one paper concluding they had significantly overestimated the iris effect, a second concluding that if the iris effect existed, it would lead to increased warming, and a third and fourth papers finding no evidence for the iris effect.

Like Lin et al., Chambers et al. (2002) examined data from CERES to look for evidence of the iris effect.  As with previous results, they found that the feedback effect is much smaller than proposed in Lindzen et al., and probably slightly positive.

"Regardless of definition, the radiative properties are found to be different from those assigned in the original Iris hypothesis. As a result, the strength of the feedback effect is reduced by a factor of 10 or more. Contrary to the initial Iris hypothesis, most of the definitions tested in this paper result in a small positive feedback. Thus, the existence of an effective infrared iris to counter greenhouse warming is not supported by the CERES data."

Lin et al. (2004) compared Earth Radiation Budget Satellite (ERBS) decadal observational data with the predictions of the iris hypothesis using 3.5-box model, also replacing the modeled radiative properties in Lindzen's paper with CERES data, as Lin et al. (2002) had previously done.  The study concluded as follows.

"On the decadal time scale, the predicted tropical mean radiative flux anomalies are generally significantly different from those of the ERBS measurements, suggesting that the decadal ERBS nonscanner radiative energy budget measurements do not support the strong negative feedback of the Iris effect. Poor agreements between the satellite data and model predictions even when the tropical radiative properties from CERES observations (LaRC parameters) are used imply that besides the Iris-modeled tropical radiative properties, the unrealistic variations of tropical high cloud generated from the detrainment of deep convection with SST assumed by the Iris hypothesis are likely to be another major factor for causing the deviation between the predictions and observations."

Rapp et al. (2005) similarly found little evidence for the iris effect in their research:

"this study addresses some of the criticisms of the Lindzen et al. study by eliminating their more controversial method of relating bulk changes of cloud amount and SST across a large domain in the Tropics. The current analysis does not show any significant SST dependence of the ratio of cloud area to surface rainfall for deep convection in the tropical western and central Pacific....the interaction between the SST and effective cloud size may even have a slight positive relationship, not the inverse relationship suggested by [Lindzen et al.]"

Spencer et al. (2007) did find a short-term reduction in cloudcover which is at least nominally consistent with the iris hypothesis, with some caveats.  This study examined  the daily evolution of tropical intraseasonal oscillations in satellite-observed tropospheric temperature, precipitation, radiative fluxes, and cloud properties, and found:

"The decrease in ice cloud coverage is conceptually consistent with the ‘‘infrared iris’’ hypothesized by Lindzen et al. [2001]....We caution, though, that the ice cloud reduction with tropospheric warming reported here is on a time scale of weeks; it is not obvious whether similar behavior would occur on the longer time scales associated with global warming."

However, Dessler (2010) did not find evidence of a significant negative cloud feedback, as was suggested by Spencer et al. (2007).

"There have been inferences of a large negative cloud feedback in response to short-term climate variations that can substantially cancel the other feedbacks operating in our climate system. This would require the cloud feedback to be in the range of –1.0 to –1.5 W/m2/K or larger, and I see no evidence to support such a large negative cloud feedback"

Dessler concluded that the short-term global cloudcover feedback is probably positive, and unlikely to be strongly negative, with a 95% confidence range at 0.54 ± 0.74 W m-2 K-1.  Therefore, even if the iris effect exists in the tropics, it won't be able to offset very much (if any) global warming, according to Dessler's results.

Iris Hypothesis Never Got off the Ground

In short, much research has focused on Lindzen's iris hypothesis, but very little supporting evidence has been uncovered.  On the contrary, studies have consistenly shown that Lindzen dramatically overestimated the iris effect in his initial study, and that if the effect exists, it may even amplify warming as opposed to dampening it.  There certainly isn't any evidence that the infrared iris will result in enough of a negative feedback to significantly slow down global warming.

Note: this is the Intermediate rebuttal to Infrared Iris will reduce global warming

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Comments 1 to 20:

  1. Great science. Thanks for the information dana1981. However, how long are we going to tolerate Homeostasis arguments?? What evidence is there that the phase space of the climate system has "very" stable points that pertubations out of those points will dissipate and the system will relax to the same point? e.g. warming will cause the Iris to open and the system will return to its previous stable state? Given the vast time that the system has existed, such stable points would have been "explored" by the system and the climate would have settled to a stable state. In the same sentence they claim chaotic dynamic systems that we have no prayer in understanding and Homeostasis. Am I missing something?
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  2. DrTsk "What evidence is there that the phase space of the climate system has "very" stable points that pertubations out of those points will dissipate and the system will relax to the same point?" I'd say that the short answer is "none". On the contrary, we may anticipate that the system has instabilities, only we don't know where.
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  3. We've often remarked that arguments for low climate sensitivity (and climate stability in general) seem to conflict with the planet's many large climate shifts in the past (i.e. see our discussion with Pielke here and here). We have yet to receive any sort of explanation as to how these two arguments gel. Frankly, paleoclimatology is a bit of a thorn in the side of the low sensitivity crowd, which they seem to prefer to ignore.
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  4. Very interesting. One point is unclear in my understanding: the hypothetical "Iris" feedback seems to be nearly instantaneous as it is derived from clouds (relaxation time of the atmosphere is fast, nucleation's change follows change in water vapour concentration) and so, it should be observed at each warming phase (forced or by natural oscillation) in the tropical oceanic basins. No reason to wait for years or decades before the Iris "opens" and "closes". But if it is the case, there would no warming signal at all in the Tropics, just short term variations around a zero trend, with negative feedbacks compensating near immediately the warming. Should we consider the very basic fact that there is a multidecadal warming trend in the Tropics (and elsewhere of course) as a contradiction of Iris hypothesis?
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  5. The illustrated example of difference in low and high cloud feedback is interesting but unexplained. Why should higher cloud have lower albedo and higher radiative forcing than lower cloud? Is there evidence that this is so? As SST rises would the presence of low cloud be expected to increase over time and increase more rapidly than high cloud? Does low cloud reflect more sunlight or is this perceived to be the case because on average low cloud is more abundant than high cloud?
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  6. Good article. Dana #3, that is one question I would really like to hear an answer to from so-called skeptics, and one that should be raised every single time skeptics claim that climate sensitivity is low.
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  7. skept - no warming in the tropics would assume that the iris effect offsets all of the local warming. It would depend on the magnitude of the effect; it might only dampen the tropical warming, for example. Agnostic - I don't know much about clouds, but I believe the type which tend to reside at higher altitudes tend to have more of a warming effect. In other words, it's not that they're at high altitudes that makes them less reflective, but merely that the type of clouds which exist at high altitudes also happen to be less reflective. The figure is just a simplification illustrating that general correlation. But maybe somebody who knows about clouds can speak more intelligently about this.
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  8. We've been warming at close to the estimated rate, and the climate warmed during past epochs. The Iris Effect would have to have taken the equivalent of a "union 10" for current warming and been AWOL for past warming. Nature isn't an in and out runner. It runs to form every time.
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  9. Ah but previous warming and current warming is due to natural factors. And nature is very tolerant and happy with natural efforts to change the climate and turns a blind iris to such changes. But as soon as man dares be arrogant enough to think that he can beat nature in the climate control game, the iris is whipped out to stare down man into submission and show us who is really boss of the climate. And of course the Iris will not only stabilise planety temperature for us through changes in the amount of clouds, these changes to clouds will occur without any possible changes to rainfall and drought patterns on the planet.
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  10. 5. Agnostic, the difference between high and low clouds is mainly dominated by the infrared effect, iirc. Clouds near the surface are warmer, and the cloud-tops radiate upwards at near the surface temperature - their 'greenhouse effect' is very small. Clouds high up are a lot cooler. So they block outgoing heat, but they don't radiate as much of it upwards, so they have a large effective greenhouse effect. Both of them reflect sunlight, which cools us. You might wonder how upper clouds could be a net cooling when sunlight is pretty intense, but remember that the greenhouse effect works day AND night! This rough model is what's quoted in PhysicsWorld here.
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  11. 5. Agnostic, added to the point above, you can see the diagram in the post shows a smaller red up arrow from the high cloud, which tallies with what I was saying. It's suggesting a higher optical albedo for low clouds, which I'd have to check. My speculation is that droplets in clouds tend to be Mie scatterers, so larger particles mean more scattering. If you check here, it suggests smaller optical radii and lower density of droplets for stratus (higher) clouds versus cumulus (lower) ones - I expect that higher droplet density and bigger droplets would encourage more scattering and ultimately higher albedo. I normally leave the atmospheric work to someone else, so perhaps I will be corrected!
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  12. 5. Agnostic, with regards to cloud albedo. Water droplets are more effective reflectors than ice crystals, which should explain why higher clouds have lower albedo than lower clouds.
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  13. Excellent job, Dana.
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  14. Just a general observation about deniers such as Lindzen et al and their psychology. (in Kiwi English) If you investigate the philosophical ancestry of the Corporate Right in America, from which of course the bulk of New Right thinking elsewhere has migrated, one finds it comes from the Austrian conservatives who exported themselves to Chicago. (a point I think that Norman Mailer missed in his "Siege of Chicago"). Anyway it fell on fertile soil as the Mid-West beef industry is about as brutal and conservative as it gets, not to mention truck loads of manure!. Mailer got that bit right! It gave the red-necked conservatism of the beef barons some philosophical respectability. This transplant became the Chicago school which produced Friedman and co. and it exported its economic proselytisers to universities around the world. The formative influence from Austria was Ludwig Von Mises who adapted and reformulated the principle of praxeology which rejects empiricism (i.e. evidence) in favour of a priori thinking with deductive conclusions. This approach in turn has an interesting ancestry deriving as it does from the doctrine of ideas (or philosophical representationalism) which claims mental ideas are the proximal objects of perception. From here we have classical skepticism about external reality. Their conclusion? The Cartesion claim that only the perception of your own ideas and their deductive derivatives are irrefutable. From this point Weber, Hayek and Popper formulated the doctrine of methodological individualism, the claim that all valid reasoning and rational action must be reducible to the thoughts and intentions of rational individuals. Microeconomics is sacrosanct, macroeconomics and societies are collectivist fictions! Ayn Rand, Ronald Reagan and Alan Greenspan were also disciples. What has this got to do with the infrared iris hypothesis I hear you chorus? This hypothesis and many others wheeled out by the denial camp are quintessentially a priori claims that either ignore or dispute all empirical evidence. One is tempted to see parallels with religion and in fact many in the denial camp are men of faith. In my experience, walking about with your nose in the Good Book is likely to result in you walking off a cliff or under a bus! This little historical tiki tour (Kiwi neologism) is worth bearing in mind if you are tempted to think the denial camp can be persuaded by scientific arguments.
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  15. FYI Dana, Lindzen begs to differ with your assessment, and has a powerpoint presentation with counterarguments for at least the early-2000s papers. The argument-counterargument volleys are over my head, at least as much of same as I want to devote to this. So, 2 Qs - First, what heuristic does the non-expert use in a situation like this? I think the answer is to ask: do unrelated scientists looking into your hypothesis think that the evidence from their research supports it? (If I'm the scientific bystander, and the hypothesis-holding scientist stands his ground, is there any other way I can go about forming an informed view?) And second, not using a heuristic, could someone please (of the expert ilk) address some of Lindzen's counterarguments, e.g. that Lin et al 2002 were "ignoring the fact that we were taking Ac(260) as a surrogate for all tropical upper cirrus, and instead asserting that the clouds defined by Ac(260) were essentially the only clouds we were considering.")
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  16. From Reisman - ""Science is at its core about reproducibility." If the result is reproducible or replicated by multiple scientific organizations or scientists, then it is more acceptable than that which is not. Both Santer and Mann seemed to agree that the Iris Hypothesis did not stand up to scientific scrutiny."
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  17. "However, how long are we going to tolerate Homeostasis arguments?? What evidence is there that the phase space of the climate system has 'very' stable points that pertubations out of those points will dissipate and the system will relax to the same point?" Well, I think it obvious that the system must have some moderating feedbacks on geologic timescales to have remained in a fairly narrow range of habitability for the last several hundred million years. Else we wouldn't have come back from things like snowball Earth, major meteorite strikes and volcanic eruptions, a faint young Sun, and so on. On multidecadal to millennial timescales, however, positive feedbacks must dominate for things like the glacial/interglacial cycle, D-O events, and the PETM to occur.
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  18. ahaynes @15 - I'm not sure what timeframe your reference about Lindzen defending his iris theory is from. However in an article in SEED magazine, An MIT climatologist's quixotic struggle against global warming science Lindzen's attitude was interesting: In 2001, Lindzen published a paper speculating that as the Earth warmed, water vapor would decrease in the upper atmosphere, allowing heat to escape back into space more efficiently, and thereby reducing overall temperature. The paper met with vigorous criticism. Eventually, he disavowed the idea. “That was an old view,” Lindzen said about his five-year-old hypothesis. “I find it insane that I am still forced to explain this.”
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  19. EOttawa @18 – That Seed article is very relevant. Gavin’s comment hit the nail – Lindzen sounds like a contrarian but he actually agrees with most of the mainstream science. Like many others that seek to sew doubt, he likes to throw stuff in the fan in the hopes…
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  20. Has anyone used a 'modern' version of John Tyndall's 1861 experiment to demonstrate the effects of CO2? More specifically, is there any laboratory data relating to the effect of the additional 75ppm of CO2 added to the atmosphere since 1958? Any comments would be much appreciated - thank you.
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