February 1, 2001

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Serious water problems are projected for California and other western states by the year 2049 because of an increase in atmospheric levels of carbon dioxide, say scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). Warmer overall temperatures may spell more rain and less snow in the winter. This in turn will mean more flooding in the spring and a reduced water supply for summers that will grow increasingly dry.


Two papers presented at the 81st meeting of the American Meteorological Society in Albuquerque, New Mexico, discussed the importance of accurately assessing and projecting regional climate changes that can result from global warming. One focused on the problems facing the western United States at the regional level, the other looked at the potential impact of rising temperatures on a representative set of California river systems. Both of the scientists presenting the two papers work out of the hydroclimate and impacts research group with Berkeley Lab’s Earth Sciences Division. The research group is funded in part by NASA.

"Regional climate significantly affects water resources, frequency of natural disasters such as flood and drought, and the health of ecosystems," said Jinwon Kim, lead author of the paper on regional impacts. "For example, increasing populations and industrial activities in the western U.S. expand urban areas into steep slopes and flood planes. As a result, increasing populations are being exposed to natural hazards."

Norman Miller, leader of the hydroclimate and impacts research group and a member of the California Energy Commission’s California Climate Change Panel, was the principal author of the paper on California river systems. Though the projections of both studies showed the same results, Miller warns that climate projections are not guarantees.

"Caution should be exercised in reporting any climate projection as the degree of uncertainty remains significant," he says.


Nonetheless, Miller and Kim agree that accurate assessments and projections of the potential impacts of climate changes on regional and state levels are crucial for managing water resources, reducing the damage caused by natural hazards, and planning for sustainable development.

"These concerns will become even more prominent in California and elsewhere throughout the western United States as population and industrial growth continues to strain the current water resources supply," Miller says.

To obtain climate predictions for the western U.S., Kim and his coauthors downscaled the global climate change scenarios predicted by the United Kingdom’s Hadley Centre Global Climate Model (HadCM2). They did this by coupling HadCM2 to a pair of regional climate models called the Mesoscale Atmospheric Simulation (MAS) and Soil-Plant-Snow (SPS) which Kim, working with others, developed.

Projections were based on the doubled carbon dioxide (2xCO2) condition that is widely used as a standard by climate forecasters. The researchers assumed that CO2 concentrations in the atmosphere will continue to rise by a rate of one percent a year and focused their projections on the climate changes that should take place between the years 2040 and 2049.

"Preliminary analyses of the results suggest that total precipitation in the western U.S. may increase significantly, especially in the high elevation areas where heavy precipitation occurs," Kim said.

The most significant precipitation increases were projected for the Sierra Nevada and the northern California Coastal Range. However, Kim said, "Most of this precipitation increase will be due to increased rainfall. Significant increases of snowfall may occur only in very high elevation areas."

On a regional scale, the western U.S., like California, is characterized by mountainous terrain, interior deserts and coastal areas which see extreme contrasts in seasonal precipitation -- very little rainfall in the spring and summer months. Therefore, the western region, like California, depends heavily on high elevation snow packs to feed its rivers and other fresh water resources.

The paper presented by Miller also downscaled the HadCM2 global model by coupling it to regional climate models. In addition to the MAS model, Miller and his coauthors also worked with the National Center for Atmospheric Research’s RegCM, and the Danish Meteorology Office’s HIRHAM. They, too, projected significant increases in precipitation and temperatures for the Sierra Nevada.

"These increases may require California water resources managers to release reservoir storage water to reduce the risks of flooding during the wet season," Miller says. "This would decrease the supply available for the dry season."

The paper Kim presented was titled: "Downscaled Climate Change Scenario for the Western U.S. Using MAS-SPS Model Nested within the HadCM2 Scenario." His coauthors included Miller, Tae-Kook Kim of Berkeley Lab’s Earth Sciences Division, and Raymond Aritt, William Gutowski, Jr., Zaitao Pan, and Eugene S. Takle, all with Iowa State University at Ames. Miller’s paper was titled: "Assessing California Streamflow Under Present Day and a Climate Change Condition." His coauthors included Jinwon Kim and Gutowski, Pan, and Takle, plus Eric Strem at the California-Nevada River Forecast Center of the National Oceanic and Atmospheric Administration, and Jens Christensen of the Danish Meteorological Institute in Copenhagen.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

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