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Climate model simulations will be closer to actual observations if they include a representation of the so-called Iris Effect, according to new research.
The Iris hypothesis (Lindzen et al. 2001) suggests that increasing sea surface temperatures in the tropics would lead to a decrease in high cirrus clouds, which in turn allows more infrared radiation to be released back into space and causes a cooling influence at the surface (negative feedback).
Dr. Thorsten Mauritsen, a senior scientist and Professor Bjorn Stevens, director of the department "The Atmosphere in the Earth System" at the Max Planck Institute for Meteorology suggest in a new study in the journal Nature Geoscience that precipitating convective (showers/thunderstorms) clouds are more likely to cluster into larger clouds as temperatures rise. This feedback would be a plausible, physical mechanism for an Iris-like response of tropical convection to warming.
The inclusion of this Iris-like Effect into the MPI Earth System Model actually brings it's simulated temperature and hydrological cycle response to rising atmospheric CO2 more in line with actual observations.
However, other missing mechanisms in the model such as aerosol cooling, volcanic eruptions and reduced ocean heat uptake can be a reason for the reduced rate of warming, according to the Max-Planck Institute news report.
Expert response via RealClimate.org.
Dr. Andrew Dessler, an expert on climate change and water vapor from the Texas A&M University recently wrote an opinion piece about this study in a guest column for RealCimate.org. Below are key excerpts from that piece........
What they (Mauritsen & Stevens) find is that, even though cloud cover is reduced as the climate warms, it does not generate a strong negative cloud feedback. While reducing cloud cover does indeed let more infrared energy out, it also lets more sunlight in. These two effects, while independently large, act in opposite directions. The net effect is the small residual of their difference. For runs with the strongest “iris”, the model’s climate sensitivity is reduced from 2.8°C for doubled carbon dioxide to 2.2°C — still well within the IPCC’s canonical range.
It’s also worth pointing out what this study doesn’t prove. It doesn’t validate Lindzen et al.’s original hypothesis — in fact, it does the opposite – even with an iris effect, the sensitivity does not become negligible. Additionally, there is little evidence that the rate of conversion of cloud water to rain actually changes with temperature, although Mauritsen and Stevens show that incorporating the iris into the model does improve the model’s simulations of some aspects of the climate system (even though it doesn’t change climate sensitivity much).
Actual study abstract from Nature Geoscience.
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