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Time for an eye exam

25 Jan 2023 | Science Notes

Or at least an Iris exam. The “Iris Effect” was proposed in 2001 by MIT Atmospheric Scientist Richard Lindzen as a way of explaining how behaviour of high clouds in the tropics might act as a negative feedback in the climate system. The mechanism was as follows. Clouds form in various layers up into the atmosphere, with the highest “cirrus” clouds, thin and composed mostly of ice particles, covering vast areas of the Earth every day, especially in the western tropical pacific. Cirrus clouds have a net warming effect because they are opaque to outgoing heat radiation. But what happens to them when the surface warms? Lindzen and his coauthors proposed that for every degree C that the sea surface warms, high cirrus cloud cover reduces by about 20 percent, allowing heat radiation to escape more easily to space, a phenomenon they dubbed the Iris Effect and which they found was large enough to completely offset warming due to greenhouse gases. Now in a new review published just over 20 years later, Lindzen and his coauthor Yong-Sang Choi describe the resulting controversy and summarize where the debate stands.

Lindzen referred to the episode in his recent interview with Jordan Peterson, where he explained that just after his paper on the Iris Effect was published the journal editor was fired. Because science. Then a series of papers came into print criticizing his work and arguing there was no evidence for it. Lindzen and his coauthors put years of work into responding to the criticisms. Now after two decades he is summarizing the research and proposing what lies ahead for the hypothesis.

His review essay goes into considerable detail about the various theories people have put forward regarding how the layer of high cirrus cloud forms and what causes the clouds to dissipate. There is clearly more going on than just surface warming, so trying to isolate the effect of warming requires having a lot of data. In 2001 they only had one data set from a Japanese satellite system. Now there are more data sets and numerous other teams have gone looking for the effect. When people fail to find it, Lindzen and his coauthors have usually found that they didn’t look at the data correctly. In one case the authors claimed to have disproven the theory but Lindzen showed they were going about the measurement incorrectly and, once corrected, they had even stronger results than Lindzen’s own.

Other authors have found evidence to support the Iris Effect. But a counterargument is that it happens only very locally and doesn’t have a global strong climate effect. Or the reduction in cirrus is offset by formation of other clouds elsewhere that generate warming.

One study was done a few years ago putting Lindzen’s Iris Effect into a climate model. As expected, it provided an overall negative feedback and, in combination with the other positive feedbacks in the model dragged the climate sensitivity down to the bottom of the IPCC range.

Lindzen acknowledges that the available data are very noisy and it is not yet possible to provide a definitive measurement of the effect. But he believes the accumulated evidence since 2001 strongly supports the hypothesis. And after a few years of constant attacks, more evidence has come out, and more scientists are taking the idea seriously. He describes the kinds of data sets that would be needed ideally to measure it, but they will require new satellite systems which are not likely to be developed in the foreseeable future. And, as Lindzen lamented to Peterson, he has not been able to get funding for his research for a long time. So if the necessary work ever happens it will require others to decide it’s finally time to figure out the mechanism once and for all.

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