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New Paper Ponders Richard Lindzen’s Iris Hypothesis

Andrew Montford, Bisdhop Hill

Some months ago I picked up wind of a new paper that was going to provide some support for Richard Lindzen’s Iris Hypothesis – the idea that in a warming planet there would be reduced levels of cirrus cloud, which would allow the extra heat to escape from Earth.

The paper in question seems to be this one, published in Nature Geoscience. The authors are Bjorn Stevens and Thorsten Mauritzen, the former the author of a much-discussed paper on aerosols and the latter best known for his paper on the subject of GCM tuning.

Here’s the abstract:

Missing iris effect as a possible cause of muted hydrological change and high climate sensitivity in models

Equilibrium climate sensitivity to a doubling of CO2 falls between 2.0 and 4.6 K in current climate models, and they suggest a weak increase in global mean precipitation. Inferences from the observational record, however, place climate sensitivity near the lower end of this range and indicate that models underestimate some of the changes in the hydrological cycle. These discrepancies raise the possibility that important feedbacks are missing from the models. A controversial hypothesis suggests that the dry and clear regions of the tropical atmosphere expand in a warming climate and thereby allow more infrared radiation to escape to space. This so-called iris effect could constitute a negative feedback that is not included in climate models. We find that inclusion of such an effect in a climate model moves the simulated responses of both temperature and the hydrological cycle to rising atmospheric greenhouse gas concentrations closer to observations. Alternative suggestions for shortcomings of models — such as aerosol cooling, volcanic eruptions or insufficient ocean heat uptake — may explain a slow observed transient warming relative to models, but not the observed enhancement of the hydrological cycle. We propose that, if precipitating convective clouds are more likely to cluster into larger clouds as temperatures rise, this process could constitute a plausible physical mechanism for an iris effect.

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