The reverse iris
About ten years ago Richard Lindzen proposed that the climate system had an "adaptive infrared iris" which allowed more IR light to escape to space when the sea surface temperature warmed
Other researchers found a weak positive feedback in the tropical Pacific, rather than the stronger negative feedback Lindzen claimed
“We wanted to see if the amount of cirrus associated with a given unit of cumulus varied systematically with changes in sea surface temperature,” he says. “The answer we found was, yes, the amount of cirrus associated with a given unit of cumulus goes down significantly with increases in sea surface temperature in a cloudy region.”
This is the finding that led Lindzen’s team to propose that the Earth has an adaptive infrared iris—a built in “check-and-balance” mechanism that effectively counters global warming (Lindzen et al. 2001). 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.
That is, Lin found that clouds in the tropics do change in response to warmer sea surface temperatures, but that the cloud changes serve to slightly enhance warming at the surface. Specifically, whereas Lindzen’s experiment predicts that cirrus clouds change in extent to reduce warming at the surface by anywhere from 0.45 to 1.1 degrees, Lin’s experiment predicts that changes in the tropical clouds will help warm the surface by anywhere from 0.05 to 0.1 degree (Lin et al. 2001).In the 24 July issue of Science, Clement, Burgman and Norris report a positive feedback in the NE Pacific.
Feedbacks involving low-level clouds remain a primary cause of uncertainty in global climate model projections. This issue was addressed by examining changes in low-level clouds over the Northeast Pacific in observations and climate models. Decadal fluctuations were identified in multiple, independent cloud data sets, and changes in cloud cover appeared to be linked to changes in both local temperature structure and large-scale circulation. This observational analysis further indicated that clouds act as a positive feedback in this region on decadal time scales. The observed relationships between cloud cover and regional meteorological conditions provide a more complete way of testing the realism of the cloud simulation in current-generation climate models. The only model that passed this test simulated a reduction in cloud cover over much of the Pacific when greenhouse gases were increased, providing modeling evidence for a positive low-level cloud feedback.Opening the IRIS, might let more IR out, but it lets more sun in to warm the ocean surface also and this accounts for the positive feedback. A comment by Richard Kerr describing the result notes that
When the results were in, only two models showed low clouds producing a positive feedback as observed. One of them stood out from the pack. The HadGEM1 model from the U.K. Met Office's Hadley Center in Exeter produced patterns of warming and circulation changes during greenhouse warming that resembled those of all 18 models averaged together—the best guide available. The group also concluded that HadGEM1's simulation of meteorological processes in the lowermost kilometer or two of the atmosphere—where the key low-lying clouds reside—is particularly realistic. As it happens, the HadGEM1 model is among the most sensitive of the 18 models to added greenhouse gases. When carbon dioxide is doubled, the model warms the world by 4.4°C; the median of the models for a doubling is 3.1°C.Worry and comment.