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, according to a study by team of 19 scientists.

“These new predictions illustrate more than ever the urgent need to control greenhouse gas emissions now,” says study coauthor 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 published in the August Proceedings of the National Academy of Sciences; the lead author is Katharine Hayhoe of ATMOS Research and Consulting. Using the most sensitive climate models to date, the researchers studied two scenarios: one 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. Under the lower emissions scenario, summer temperatures in California would rise 4°F to 5°F by the end of the century; if nothing is done to curb the use of fossil fuel, summer temperatures would rise a dramatic 7.5°F to 15°F. Those figures are several degrees higher than previous models had predicted, particularly in the summer months.

Statewide, the length of the heat-wave season could be dramatically extended from an average of 115 days per year to 178 to 204 days by 2100, while the Sierra snowpack could decline by as much as 90% if fossil fuel use isn't curbed, the study finds.

“Increases in temperature decrease water availability while increasing demand,” Hanemann says. “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.”