by Judith Curry

It’s nice to see that our ‘discredited’ theory doesn’t seem to go away. – Richard Lindzen

The new Mauritsen and Stevens paper (discussed in the companion post by Rud Istvan) is breathing new life into Richard Lindzen’s iris hypothesis.

Basic mechanism of the iris hypothesis

The basic mechanism of the iris hypothesis is described by text excerpted from the Mauritsen and Stevens paper:

The tropical atmosphere consists of moist and cloudy regions associated with large-scale rising motion, convective storms and pronounced precipitation on the one hand, and dry and clear regions with subsiding motion on the other hand (Fig. 1). The atmospheric circulation maintains an approximate balance between radiative cooling, which occurs preferentially in the dry and clear regions, and latent heating from the condensation of water vapour in precipitating clouds. In the tropics, radiative cooling predominantly occurs in dry and clear subsiding parts of the atmosphere. The radiative cooling is balanced mainly by latent heat released in precipitating deep convective clouds (Fig. 1).

Processes that may change the balance in favour of dry and clear regions in warmer climates have been proposed to constitute a possible negative feedback not represented by climate models. This potential feedback has been termed the iris effect, in analogy to the enlargement of the eye’s iris as its pupil contracts under the influence of more light. The controversial ‘iris hypothesis’ proposes that the fraction of the dry and clear regions could increase with warming and exert a negative feedback: a larger extent of the dry and clear regions would lead to a less cloudy upper troposphere and hence an increase in OLR.

Such an effect could mitigate against climate change [by reducing climate sensitivity]. But a drier upper troposphere would also allow more solar radiation to be absorbed by the Earth and atmosphere, rather than reflected back to space by the clouds, so that the net effect of reducing high clouds is not obvious. On balance, the effect is thought to be negative. The estimate of climate sensitivity with an iris effect, however, depends not only on the rate of reduction of high-level clouds, but also on the cloud optical properties of the most sensitive clouds. If the thinnest clouds are preferentially removed, the effect on outgoing longwave radiation is stronger than that on reflectivity, and the iris effect is stronger. On the other hand, if the reduction in cloud cover affects thicker clouds more strongly, the loss in reflectivity plays a more important role, and the iris effect is less pronounced.

Early history of the iris hypothesis

Richard Lindzen has provided me with 2004 ppt presentation [Lindzen iris] that provides a good overview of his ideas re the iris hypothesis, and also the critiques of this hypothesis.

This caught my attention:

So, what was the result of these criticisms that were clearly made in the absence of any serious attempt to understand LCH [the original iris hypothesis paper]? In the environmental literature as well as in papers by scientists like Jim Hansen and Steve Schneider, LCH is accompanied by the adjective discredited.

discredit (from American Heritage Dictionary 4th Edition)

TRANSITIVE VERB: Inflected forms: dis-cred-it-ed, dis-cred–it-ing, dis- cred-its 1. To damage in reputation; disgrace. 2. To cause to be doubted or distrusted. 3. To refuse to believe. NOUN: 1. Loss of or damage to one’s reputation. 2. Lack or loss of trust or belief; doubt. 3. Something damaging to one’s reputation or stature.

I would suggest that this is an unusual word to use in connection with a serious paper.

The vociferous objections to the iris hypothesis arises not so much from the hypothesis per se, but rather its implications for climate sensitivity. In 2001, around the time of the IPCC TAR, there was a great deal of animosity towards any scientist or paper that argued for a low value of climate sensitivity (e.g. Pat Michaels’ 2002 paper).

In Lindzen’s ppt, he states:

This leads to a very important question: Namely, what will happen to the exponentially growing climate change community if the sensitivity of climate to global forcing is small? Could the wish to avoid this question be the reason why our ‘official’ estimates of climate sensitivity have not improved since the Charney Report of 1979?

Early observational support for the iris hypothesis

Observational evidence for an iris effect is found in observations of tropical variability of upper-level cloud cover, precipitation, and the radiation balance, co-varying with natural variations of the surface temperature.

From Lindzen’s ppt:

Finally, there are a number of papers which simply report on satellite results for the radiation balance in the tropics over the 1990’s.

Chen, J., B.E. Carlson, and A.D. Del Genio, 2002: Evidence for strengthening of the tropical general circulation in the 1990s. Science, 295, 838-841.

Wielicki, B.A., T. Wong, ….., 2002: Evidence for large decadal variability in the tropical mean radiative energy budget. Science, 295, 841-844.

Cess, R.D. and P.M. Udelhofen, 2003: Climate change during 1985–1999: Cloud interactions determined from satellite measurements. Geophys. Res. Ltrs., 30, No. 1, 1019, doi:10.1029/2002GL016128.

These papers all report anomalously increased OLR associated with tropical surface warming. Models, by contrast, fail to show such an increase. Presumably that is why it is referred to as anomalous. One can (though the authors don’t) infer a negative feedback factor from the observations, and it is at the high end of what LCH estimate. Moreover, the ‘anomaly’ is associated with reduced upper level cirrus. However, the same papers insist that what they see is not the Iris Effect, but rather the result of a change in the large scale circulation. However, there is something seriously wrong with this attribution.

In brief, these papers are most likely displaying the Iris Effect.

Convective aggregation as a possible mechanism

From the Mauritsen and Stevens paper:

One objection to the idea of an iris effect is that it is not clear what the physical mechanism might be. An iris effect could result if the efficiency of precipitation within deep convective cloud towers increased with warming, leading to less detrainment into their anvils. This could occur if aggregation of convective clouds into large clusters is temperature-dependent. Aggregation is due to an instability of radiative-convective equilibrium, whereby relatively dry regions cool radiatively, resulting in local subsidence and further suppression of convection, ultimately leading to an aggregated state with localized convective clusters. The cooling of the dry and clear regions is expected to increase with warmer temperatures and hence promote aggregation1. In addition, in a warmer climate convective clouds may further be invigorated by enhanced latent heat release.

As larger convective clouds dilute less by lateral mixing they precipitate more of their water during ascent, and fewer large clusters can provide the necessary latent heating to sustain atmospheric radiative cooling (Fig. 1). Both cloud-resolving simulations and observations confirm that outgoing longwave radiation does increase as a consequence of a drying environment in more aggregated states. Shortwave absorption also increases, which tends to cancel some of the effect. All in all, however, we conclude that it is plausible that convective aggregation constitutes a negative longwave feedback on climate change — and to our understanding, the underlying processes are not explicitly represented in climate models.

Mauritsen and Stevens cites numerous papers supporting the above statements. There are several additional papers of relevance, that were not cited by MS:

Horvath and Soden (2008) Lagrangian Diagnostics of Tropical Deep Convection and its Effect upon Upper-Tropospheric Humidity [Horvath-Soden-i1520-0442-21-5-1013]

. . . anvil area per unit cumulus area, that is, cirrus detrainment efficiency, decreases as SST increases

Trenberth and Fasullo (2009) Global warming due to increasing absorbed solar radiation [TrenberthFasullo2009GL037527]

There is an increase in net radiation absorbed, but not in ways commonly assumed. While there is a large increase in the greenhouse effect from increasing greenhouse bases and water vapor (as a feedback), this is offset to a large degree by a decreasing greenhouse effect from reducing cloud cover and increasing radiative emissions from higher temperatures. Instead the main warming from an energy budget standpoint comes from increases in absorbed solar radiation that stem directly from the decreasing cloud amounts.

JC reflections

When I first encountered the iris hypothesis, I bought into the hype that this had been ‘discredited’, without giving the whole thing much thought. The idea seemed counter intuitive to me.

I did take a close look at the follow on paper by Lindzen and Choi (2010), which I reviewed at Climate Audit [link] – an interesting piece of blogospheric history. While I found a number of methodological problems with the paper, none of these struck at the core of the iris hypothesis itself.

So this ‘discredited’ theory essentially languished until Mauritsen and Stevens. Andrew Dessler makes a remarkable statement in a post at RealClimate on this topic:

By 2006, when I submitted an analysis of tropospheric water vapor that investigated whether there was an iris in that, one of the reviewers pointedly questioned why anyone was still working on this issue. I subsequently withdrew the paper. Nevertheless, just because Lindzen et al. did not convincingly demonstrate their case does not mean the iris hypothesis is wrong.

So the ‘consensus enforcers’ found it necessary to ‘discredit’ the iris hypothesis, and by extension Lindzen himself, since the reduced sensitivity threatened the ‘consensus’. You can see how this pernicious behavior discouraged scientists from investigating the iris hypothesis (I can only imagine how a grant proposal to investigate the iris hypothesis would have fared in peer review).

I would love to find out how Mauritsen and Stevens came to have the idea for this paper, and why they bothered to investigate this ‘discredited’ hypothesis. The end result is a very good paper with new insights that has opened the door again to seriously consider negative cloud and water vapor feedbacks. I also wonder how this paper would have fared if it had been submitted to Nature Climate Change (consensus enforcing), rather than Nature Geoscience (which seems to do a good job editorially).

Moderation note: This post is a companion post with Rud Istvan’s Modeling Lindzen’s adaptive iris.