Pollution cuts California rain, snow, study warns
SACRAMENTO, Calif. – Urban air pollution may be reducing rainfall in the Central Valley and along the heavily populated southern California coast, while trimming mountain snowfall that supplies much of the state’s drinking and irrigation water and hydroelectric power, a Stanford University professor’s study released Thursday shows.It’s the first study to use a new computer program to examine airborne pollutants’ effect on a regional climate. Coupled with possible reduced precipitation from global warming, the effect could be a more limited supply of water for the state’s growing population, the California Energy Commission warned Thursday.Mark Z. Jacobson, a professor in Stanford’s civil and engineering department, conducted his study for the commission’s Public Interest Energy Research Program. It is one of the preliminary studies PIER is conducting before it attempts to project California’s future climate and how the state can prepare for changing conditions.Scientists have only recently begun reaching conclusions about the role of airborne particles in climate change.Carbon specks on snow, for instance, speeds the melting of snow each spring and cuts ice fields’ ability to reflect sunlight away from the earth. Carbon flecks suspended in the air may absorb sunlight, heating the atmosphere but blocking sunlight from reaching the earth. Other particles reflect sunlight away from the earth’s atmosphere entirely.Researchers at University of Nevada’s Desert Research Institute reported findings similar to Jacobson’s conclusions earlier this year by measuring snow from actual winter storms in the Rocky Mountains. Pollution-contaminated clouds produced half as much snow, and what fell contained 25 percent less water and had as little as half the mass of its pristine counterpart, the study found.A scientist at Israel’s Hebrew University used satellite photos to show decreased snow and rain in pollution-contaminated clouds around the world in 2000, and a study this year found a similar pattern in California.Jacobson’s computer modeling shows suspended pollutants can cut Sierra snowfall by disturbing air pressure systems and thus disrupting cloud and wind patterns. In addition, more moisture-attracting particles means fewer of the resulting cloud droplets may grow heavy enough to fall to the ground as rain.”It gets accentuated in the mountains because that’s where you get the most precipitation,” he said in an interview. “Even if you get less pollution there, you have bigger effects.”He checked the model’s results against extensive actual measurements from weather in February and August of 1999.The model showed reduced precipitation and ground-level air temperatures in February in the Sierra Nevada mountains, Central Valley, and Southern California. Air temperatures increased slightly in August along the Southern California coast, while sunlight reaching the ground decreased, meaning a possible reduction in crop yields, Jacobson found.Clouds were as much as twice as visually dense and were longer-lasting. They contained more liquid water concentrated around the suspended particles – but less of it turned to ice because the sooty clouds absorbed more heat, the model showed.Higher concentrations of airborne pollutants means more contamination of ground and surface water from the resulting rainfall, the study found.Ultraviolet radiation reaching the earth was diminished, but the health benefits from less damaging radiation were offset by higher levels of harmful pollution.”The breathing of these pollutants hurts you a lot more than a few percent reduction in the UV,” Jacobson said.—On the Net:California Energy Commission: http://www.energy.ca.gov
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