By mid-century, annual precipitation in Southern California could decline by 10%, and by 5% farther north, in a band near the state's midpoint, according to the climate report. Little change is projected in the most northern reaches of California.
Cayan cautioned that "our tools to get at this are still pretty crude. These are only rough numbers."
The Sierra Nevada snowpack, which stores water and then slowly releases it to the river systems that feed the state's major reservoirs, will shrink by at least a quarter over the next four decades, previous studies have concluded.
Combined with drier and hotter conditions, that will create water shortages expected to fall most heavily on the state's agricultural sector in the Central Valley.
Urban areas should be able to make up for shortfalls by buying water from farms, which use most of the supplies in the state.
But that will drive up the cost of water, prompting farmers to fallow cropland and abandon irrigated pasture and less profitable crops such as cotton, alfalfa and rice.
UC Davis agricultural economics professor Richard Howitt estimates that the Central Valley's farm acreage will shrink by roughly 1.5 million acres, or 20%, by 2050.
Revenue losses will be less steep -- about 10% or $3 billion a year -- thanks to shifts to more profitable fruits, vegetables and nuts.
The three water studies included in the climate report "describe relatively modest impacts of climate change on the water sector," according to the document.
But in a news conference, Adams and UC Berkeley economics professor Michael Hanemann said those findings were based on a "rosy scenario" of average water conditions as well as the legal and physical ability to move massive amounts of water from one user to another.
Global warming is expected to increase weather swings, from years of flooding to severe drought.
"There are more bad years than before and the bad years are worse in terms of shortages," Hanemann said.
In other areas, hydropower production is expected to decline along with the snowpack, while statewide electricity use could shoot up 55% by the end of the century and ozone pollution will increase.
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bettina.boxall@latimes.com
