As California warms in coming decades, farmers will have less water, the state could lose more than a million acres of cropland and forest fire rates will soar, according to a broad-ranging state report released Wednesday.
The document, which officials called the "the ultimate picture to date" of global warming's likely effect on California, consists of 37 research papers that examine an array of issues including water supply, air pollution and property losses.
Without actions to limit greenhouse gas emissions, "severe and costly climate impacts are possible and likely across California," warned state environmental protection secretary Linda Adams.
The draft Climate Action Team Report, an update of a 2006 assessment, concludes that some climate change effects could be more serious than previously thought.
By the final decades of the century, acreage burned across much of the state's northern forests could easily double and under some scenarios quadruple, said Anthony Westerling, an assistant professor of geography and environmental engineering at UC Merced.
The reason is simple: As the temperature rises, the fire season lengthens and woodlands get drier, burning more readily.
Moreover, if growth continues at the wild-land edge, more fire will mean mounting home losses -- as high as $14 billion a year by century's end.
"If you spread development all through the Sierra foothills over the next 50 years," Westerling said, "you will have a situation like the hills of Southern California or Oakland" -- where wildfire has destroyed thousands of homes at a time.
One bit of good news is that in Southern California, the Santa Ana winds that have driven the region's worst conflagrations will blow less frequently, because temperature and pressure differences between sea and land will become less pronounced.
Researchers believe they have already seen a 30% drop in Santa Ana days between the 1960s and 1990s.
One of the less certain areas of climate change modeling involves precipitation patterns. Some studies have suggested that California could get wetter with global warming.
But Dan Cayan, a climate researcher at Scripps Institution of Oceanography at UC San Diego, said "the latest generation of simulations from global models are producing a drying over the Mediterranean and lower continental latitudes -- and California sits right in that regime."
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.