Claim: Cloud feedbacks from enhanced warming are negative (i.e. cause a net cooling), so that climate is essentially stable.

Why this claim is wrong: This is Richard Lindzen's "Iris Effect" hypothesis, proposed in 1990. His claimed is that higher water vapor in the warmed atmosphere rains out lower in the atmosphere, leading to fewer cirrus clouds (stratospheric cirrus clouds have a net heating effect by blocking outgoing IR radiation). The evidence and theory both point to cloud feedbacks actually trending towards increased warming, i.e. a positive feedback (Fasulo and Trenberth 2012 and earlier reference therein). Observations show that increasing sea surface temperatures correlate with a decrease in "anvil" clouds and deep convective clouds. The question is, do these clouds heat, or cool the atmosphere? Brian Soden, a Princeton climate scientist, examined the tropical greenhouse effect in and out of El Nino conditions and finds that clouds behavior is opposite to that hypothesized by Lindzen (Soden 1997). And Lin (2002) finds "The observations show that the clouds have much higher albedo 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, opposite to Lindzen's hypothesis." This recent study again shows cloud feedback is positive (more CO2 warming leads to more cloud-cover induced warming), not negative. See more studies backing up this conclusion here and here, and Dessler (2011). The next incarnation of Lindzen's idea is in Lindzen and Choi (2009), claiming that the ERBE satellite data show increased outgoing radiation from Earth to space when sea surface temperatures rise. But this work has a number of major flaws which are fatal to the hypothesis, pointed out by Trenberth et al. here. (The journal paper is here). Lindzen is reportedly reworking his claims yet again. Given his original incarnation was published now over 20 years ago, it seems likely that variants are going to be keep coming, regardless of how embarrassingly wrong they continue to be. The specific problems are not easily put into bullet points for the non-science major, so I would recommend the serious student follow the links to fully digest this. Climate models clearly agree well with the rising global temperatures and Lindzen's ideas are quite outside what both theory and observations demonstrate. More disturbing, these studies above show Lindzen cherry-picking his time interval, cherry-picking his location (Indonesian ocean water), cherry-picking his model heat flow parameters, and a deeply flawed portrayal of the relation of the tropical system to global climate (see in particular the Dessler (2011) study of tropical air/ocean/clouds study). On a larger time frame, this "Iris" hypothesis is obviously wrong, as relatively small changes in insolation at the Arctic Circle (Milankovitch cycles) are amplified by feedback via CO2 release to/from the ocean to produce the great Ice Ages. And now, today, the Arctic is melting rapidly, at roughly twice the rate of the globe as a whole, thanks to direct CO2 additions by us to the atmosphere..

Update: A 2012 New York Times newspaper article on Lindzen and clouds is a good read for the layman (refreshingly so! Imagine - a newspaper!). In it, Lindzen finally acknowledges "embarrassing" and "stupid mistakes" he made in Lindzen and Choi 2009. But the bad science continues - Lindzen's re-worked Lindzen and Choi (2011), is just as deeply flawed as the earlier incarnations. Submitted to an obscure Korean journal after being rejected by the Proceedings of the National Academy of Sciences, reviewers noted that the paper is based on "fundamentally flawed assumptions about global climate sensitivity". A detailing of the flaws is here. This link is a good place to keep up to date on the latest research on cloud feedbacks.

More evidence that climate change is experiencing a positive feedback from clouds comes from the latest research of Sherwood et al. (2014 in Nature ). As climate warms, the height over the ground in which convective mixing occurs rises, drying the air and reducing low clouds. Low clouds cool climate by reflecting sunlight and radiating from their warm tops more IR to space. The reduction in low clouds thus provides a another amplifying feedback for climate change.

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