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For Immediate Release: March 9, 2007
Media Contact: Claudia Chandler - 916-654-4989

New Climate Change Research Shows
Air Pollution Reduces Rainfall in Mountains

California's Water Supply May Be Impacted

SACRAMENTO - New research published in Science magazine's current issue finds that air pollution particles, responsible for decreased rainfall in China, may also affect California's water supply.

"California is leading the nation and working with the world to fight the impacts of global climate change," said Secretary for Resources Mike Chrisman. "Using a science-based approach to establish significant benchmarks and identify critical data, like this important fine particulate research, is essential to reduce green house gas that may affect rain and snowfall in the Sierra Nevada and all California's mountain ranges."

Similar to scientific research funded by the California Energy Commission, a new study in the journal Science by Professor Daniel Rosenfeld of the Hebrew University validates preliminary findings reported about similar conditions in the Golden State. Because California's physical conditions, climate, and topography matches the research area, Rosenfeld's study offers compelling evidence that man-made air pollution may affect the state's fragile water supply.

"The Governor's recent actions to fight global climate change show the seriousness and importance of the state's commitment," noted Energy Commission Chair Jackalyne Pfannenstiel. "The Energy Commission's research demonstrates the interconnectedness of energy use, the associated emissions and the impact on our environment."

The California research effort, funded in part by the California Energy Commission's Public Interest Energy Research (PIER) program and in partnership with the Department of Water Resources, identified similar effects in the Sierra Nevada mountain range. The research showed California's precipitation losses over the mountains are projected at 10-25 percent, presumably because of pollution aerosols from urban and industrial areas. These losses have not noticeably affected the state's water supply because they may have been masked by an increased trend of statewide precipitation.

The impact of this research suggests that decreased mountain rainfall translates to a net loss of snowpack, loss of water for agricultural use, and reduced hydroelectric power production in the state. Additionally, the California research quantified the losses in stream flows at 15-35 percent of annual flows in major Sierra Nevada rivers. Without man-made air pollution particles, precipitation levels in the Sierra Nevada in the past would have been higher.

Using observations from specially instrumented airplanes and satellite imagery, the research team measured fine air pollution particles and raindrops over the Sierra Nevada.In the California study, decreased mountain rainfall translates to a net loss of precipitation and snowpack. His hypothesis is that fine air pollution particles, known as atmospheric aerosols, serve as "seeds" where water vapor condenses forming water droplets. Increased concentrations of these aerosols result in more water droplets but of smaller sizes which do not tend to precipitate as easily as larger water droplets.

This phenomenon affects clouds that form as air masses rise in the presence of mountains. These short-lived clouds (known as orographic clouds) are mainly found during winter months in California.

"Water is the lifeblood for California's farmers and its abundant agricultural production; water powers 15 percent of the state's in state hydroelectric generation; and it is necessary for daily life, continued Secretary Chrisman. "We must continue to use the state's scientific efforts to identify, understand, and mitigate how this phenomenon affects the quality of life in California."

Rosenfeld and his team of researchers discovered that the average rainfall in the mountains of China decreased up to 20 percent at the same time air pollution levels increased. The loss was doubled on days with the poorest visibility. Using precipitation and visibility data amassed since 1954, the Chinese study examined trends in decreased mountain rainfall relative to rainfall in nearby densely populated lowlands. Rosenfeld's new findings give more credence to his hypothesis that man-made air pollution threatens the water resources in regions where orographic precipitation makes a significant contribution to total rainfall.

By using detailed precipitation and visibility records, Rosenfeld identified a direct causal link between atmospheric aerosols and reduction in rainfall at higher elevations in China. While climate change mostly focuses on greenhouse gas emissions, this research demonstrates the need to continue to monitor orographic precipitation and explore ways to mitigate the problem.

The Energy Commission's research and development program is pursuing additional scientific studies to further understand the role of aerosols on precipitation levels in the Sierra Nevada where these particles seem to be affecting precipitation levels.

The complete report by Dr. Daniel Rosenfeld and Dr. William Woodley, Physical / Statistical and Modeling Documentation of the Effects of Urban and Industrial Air Pollution in California on Precipitation and Stream Flows, may be downloaded at the Energy Commission website at:

Pollution shown as cutting rainfall in hilly areas
News Release from Hebrew University

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