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New climate-change study predicts hotter summers, water shortage
Projected impacts range from a shorter ski season to huge jumps in heat-related mortality

| 26 August 2004

Using the most sensitive climate models to date, a team of 19 scientists predicts that California will experience significantly hotter summers by 2100, with resulting impacts on human health and the availability of water. The scenarios examined are dramatic enough that the future they envision could well upend the state’s current water-rights system.

“These new predictions illustrate more than ever the urgent need to control greenhouse-gas emissions now,” says 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 30 years from now.”

The findings were released last week in the Proceedings of the National Academy of Sciences. Among its co-authors, in addition to Hanemann, are Norman Miller and Larry Dale, both of Lawrence Berkeley National Laboratory. The lead author is Katharine Hayhoe of ATMOS Research and Consulting.

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 resulting from a switch to alternative energy and more fuel-efficient technology.

The researchers chose to focus on California because of its diverse climate and limited water supply. The climate models they used for this study improve upon previous ones, Hanemann says, because they factor in the effects of the interatction between air temperature and land surface.

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 will rise a dramatic 7.5 to 15 degrees Fahrenheit.

Those figures are several degrees higher than previous models had predicted, particularly in the summer months. Statewide, the length of the heat-wave season, which currently averages 115 days a year, will increase to between 178 and 204 days by the end of the century if fossil-fuel use isn’t curbed; under the relatively lower-emissions scenario, the season will still increase to between 149 and 162 days, 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, a number 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 identify another ramification of increasing average temperatures: significant reductions in the Sierra Nevada snowpack, runoff from which feeds into California’s streams and reservoirs. By midcentury, under the two examined scenarios, the snowpack decline translates into a loss of between 2.6 and 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 have the effect of decreasing water avail-ability while increasing demand. It will no longer just be a battle among the farming industry, the environmental groups, and the cities; those within each interest group will be competing with each other for water.”

Hanemann said the projected change in water availability will set 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.”