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Researchers Predict Southern California to Face Ongoing Freshwater Challenges

July 1, 2013

Southern California faces a perennial freshwater availability crisis, according to a newly published study by researchers at California State University, Fullerton and the University of Southern California.
 
“Climate models predict a drying of the American Southwest, including Southern California, over the next century,” said Matthew Kirby, associate professor of geological sciences and lead author of the paper "Latest Pleistocene to Holocene hydroclimates from Lake Elsinore, California," in the Sept. 15 edition of the journal Quaternary Science Reviews.
 
To place the region’s modern and future water challenges into context, the researchers and their students looked to the bottom of Lake Elsinore, which contains a thick sequence of sediments deposited over hundreds of thousands of years.
 
Kirby and his student team extracted sediment cores dating back 33,000 years. “This core represents the highest resolution, deglacial record — 19,000 to 9,000 years — from Southern California,” he noted.
 
The authors used a variety of sediment-based analyses, such as hydrogen isotopes from leaf waxes and the size of the sediment particles, to determine past water availability.
 
"The waxy molecules from plant leaves record the isotopic composition of rainfall and shifting storm tracks through time," said Sarah J. Feakins, assistant professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences who collaborated with Kirby on the study. "We found that Southern California was wetter during the last glacial; it probably would have seemed more like Seattle today."
 
The researchers are interested particularly in this period in Earth’s climate history because of its “well-documented large amplitude and abrupt climatic changes,” noted Kirby.
 
For Kirby and Feakins, the questions were:
• How did these changes impact water availability in Southern California?
• What caused these changes?
• What will these results tell us about present and future water availability in Southern California?
 
According to the researchers, Southern California was wetter during the last glacial period, experiencing more frequent, North Pacific-sourced winter storms. As the Earth’s climate transitioned from the glacial into the present interglacial period, the frequency of winter storms decreased, along with a change in their dominant source to more sub-tropical.
 
As the frequency and source of winter storms changed, so, too, did San Jacinto River runoff into Lake Elsinore, the researchers found. Grain size data suggest that Lake Elsinore experienced two abrupt decreases in water availability 14,600 and 12,900 years ago. These abrupt changes in water availability occurred in less than 70 years each, according to their findings.
 
“Perhaps most striking is that the pattern of hydrologic change across the deglacial is not the same between Southern California and the interior southwest United States,” Kirby said.
 
For Southern California, the pattern follows changes in the concentration of greenhouse gases and the size of North America’s ice sheets.
 
For the interior southwest United States, which includes Arizona and New Mexico, the pattern of deglaciation is related more to changes in oceanic circulation in the North Atlantic.
 
By comparison, the authors conclude that climate models that consider Southern California geographically a part of the interior Southwest, as most do, may not predict accurately how future water availability in Southern California will change over the next century.
 
Moreover, the researchers say, the amplitude and abruptness of changes in past water availability in Southern California argue for a closer examination of how future changes may occur “in the already water-stressed, highly populated region.”
 
Funding for their research was provided by the U.S. National Science Foundation and the American Chemical Society – Petroleum Research Fund.

Media Contacts:
Matthew Kirby, CSUF Geological Sciences, 657-278-2158
Sarah Feakins, USC Earth Sciences, 213-740-7168
Paula Selleck, Cal State Fullerton, 657-278-4856
Robert Perkins, USC, 213-740-9226

Tags:  Academics & Research