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Insides of clouds may be key to climate change

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As climate change scientists develop ever more sophisticated models to project an expected path of temperature change, it is becoming increasingly important to include the effects of aerosols on clouds, says Joyce Penner, professor of atmospheric, oceanic and space sciences.

That's because aerosols, fine particles such as smoke and dust that form droplets in clouds and change their thickness, affect how much sun is able to pass through clouds to Earth, as well as the amount of moisture that is returned to Earth. Both moisture and sunlight play significant roles in climate change.

"Think of it as having two clouds—one made of cotton and the other of Styrofoam," Penner says. "More sunlight and moisture will pass through a cloud of cotton as opposed to the denser cloud of Styrofoam. This difference is becoming more critical in terms of modeling future changes in the climate as we continue to produce more and more aerosols that form thicker and thicker clouds."

Penner was scheduled to present a talk Feb. 17 on "Aerosol-Cloud Interactions and Climate Projections" during a panel at a meeting of the American Association for the Advancement of Science in San Francisco.

By comparing the observed temperature change record since 1850 with two different climate models, one that has low sensitivity and small amounts of aerosols and one that has high climate sensitivity and high amounts of aerosols, Penner's group showed that both models follow almost identical predictive paths in the past but diverge significantly when predicting the temperature in the future.

Penner's presentation also looks at the predictive capability of three climate models: U.S. NCAR-Oslo, French and Japanese. She shows that differences are large, especially when the models predict both aerosols and their cloud effects in the assumed level of aerosols at the time, as these factors significantly change the results. The differences are large partly because these models do not have high enough resolution to reproduce observations.

"We know that aerosol effects on clouds need to be included in climate models," Penner says, "but we need more research to reach optimum predictive properties for climate models."

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