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Pollution May Be Reducing Calif. Rainfall
By DON THOMPSON, Associated Press Writer
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.
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On the Net:
California
Energy Commission: www.energy.ca.gov
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