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UC Berkeley Press Release

New climate change study predicts hotter summers, water shortage in California

– Using the latest, most sensitive climate models to date, a team of 19 scientists, including a researcher from the University of California, Berkeley, predicts that California will experience significantly hotter summers by 2100, with resulting impacts on human health and the availability of water that could upend the state's current water rights system.

"These new predictions illustrate more than ever the urgent need to control greenhouse gas emissions now," said study co-author W. Michael Hanemann, professor of agricultural and resource economics and director of the California Climate Change Center at UC Berkeley. "Because of lags in the natural system, what we do today will affect climate thirty years from now."

The findings are released today (Monday, Aug. 16) in the journal Proceedings of the National Academy of Sciences. The lead author is Katharine Hayhoe of ATMOS Research and Consulting. Other co-authors include Norman Miller and Larry Dale, both of Lawrence Berkeley National Laboratory.

The researchers studied two scenarios presented by the Intergovernmental Panel on Climate Change, an international organization formed by members of the United Nations and the World Meteorological Association. One scenario assumes a business-as-usual approach to the use of fossil fuels, while the other factors in lower emissions when switching to alternative energy and more fuel-efficient technology.

The researchers chose to focus on California because of its diverse climate and limited water supply.

Under the study's lower emissions scenario, summer temperatures in California will rise 4 to 5 degrees Fahrenheit by the end of the century. If nothing is done to curb our use of fossil fuels, summer temperatures rise a dramatic 7.5 to 15 degrees Fahrenheit, according to the study.

Those figures are several degrees higher than previous models had predicted, particularly in the summer months. Statewide, the length of the heat wave season extends from an average of 115 days in a year to 178 to 204 days by the end of the century if fossil fuel use isn't curbed, or 149 to 162 days in the relatively lower emission scenario, the study says.

This rise in temperature corresponds to a projected increase in heat-related mortality in Los Angeles, according to the study. The region now averages 165 heat-related deaths per year, but that would increase two- to three-fold if emissions are controlled, or a stunning five- to seven-fold if emissions are left unchecked.

The researchers also find that hotter weather triggers reductions in the Sierra Nevada Mountains snowpack, which feeds into California's streams and reservoirs. By mid-century, the snowpack decline translates into a loss of 2.6 to 4 million acre-feet of water storage. By the end of the century, the snowpack could decline by as much as 30 to 90 percent, depending upon whether emissions are controlled, the study finds.

"The models show that even if we take action now to reduce emissions, we will still face serious stresses to water supply in California," said Hanemann. "Increases in temperature both decreases water availability while increasing demand. It will no longer just be a battle among the farming industry, the environmental groups and the cities, but those within each interest group will be competing with each other for water."

Hanemann said the change in water availability sets up a conflict between those who retain rights to summer streamflow, projected to decrease by 40 to 50 percent, and those who obtain water from storage in reservoirs operated by water projects.

"In many parts of California, water rights have not been formally quantified," said Hanemann. "We need to start now to clarify water rights in preparation for the coming shortage."

The climate models used for this study improve upon previous ones because they factor in the effects of land surface and air temperature interaction, said Hanemann.

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