By any measure, California wines rank among the best in the world. But a 2-degree rise in temperature could make Napa Valley chardonnay a thing of the past.

A couple more degrees and Napa would no longer be prime territory for wine of any kind. And warmer grape growing regions such as the Livermore valley could be knocked out of the premium wine game entirely.

"It's clear that there's the potential for really substantial problems, and almost certainly going to be some change," said John Williams, owner and winemaker at Frog's Leap Winery in Napa Valley.

Among the issues Williams could face are warmer winters that hinder bud development, changes in rainfall patterns and increasing pressure from pests that thrive in hotter weather. But uncertainty about the timing and severity of those challenges makes it hard to plan.

"You can't prepare for it," he said.

Although grapes may feel the heat first, they won't be alone. Many of the state's signature crops -- avocados, oranges, almonds -- will face serious declines in yield by midcentury, according to computer models that project climate changes

Agriculture is the industry whose fate is most closely linked to climate, and California is by far the biggest agricultural producer in the country. In a warming world, California's agricultural riches are among the most vulnerable in the country, so farmers and economists are starting to pay attention to the prospect of climate change.

The state grows more than half of the nation's fruits, nuts and vegetables and is virtually the sole source of more than a dozen crops, including nectarines, raisins, artichokes and olives.

No other state comes close. According to the most recent agricultural census, in 2002, the No. 2 state, Texas, did not bring in even half of the $26 billion grossed by California farmers and ranchers.

Other states may escape relatively unscathed, and some studies show that the uptick in temperature and longer growing season predicted by climate models could actually be a boon to agriculture in the northernmost states.

But California's climate is already close to ideal for many of the fruits and vegetables it is famous for, and even the most optimistic predictions show California on the losing end of the warming stick.

"At the current crop mix that we have, we're pretty much at the optimum, so changing that would push us over the peak of that curve," said economist Olivier Deschênes of UC Santa Barbara.

In a study forthcoming in the American Economic Review, Deschênes and Michael Greenstone of the Massachusetts Institute of Technology estimated the economic impact of global warming on U.S. agriculture by analyzing how random year-to-year variations in temperature and precipitation affected profits in the past. They then used a climate model to project those effects into the future.

They calculated that global warming will result in a 4 percent, or $1.3 billion, increase in agricultural profits for the country during the next century. But those gains were not evenly spread, and California may see an annual loss of 15 percent, or $750 million, by the end of the century.

One of the biggest reasons for this is the state's precarious water situation.

Adios avocados

Unlike the eastern half of the United States, California's agriculture is largely dependent on irrigation. About 90 percent of California's crops are produced on irrigated land.

Some climate models project the state will get more rain during the growing season, but this increase will be far outweighed by a decrease in winter snow.

California doesn't have sufficient reservoir capacity to sustain agricultural water needs through the dry season, so storing water as Sierra snowpack is critical. A good snowpack ensures water will be available in the summer and fall when irrigation demand is still high and reserves from rainwater are low.

Currently, 80 percent of water used in the state goes to agriculture. With reductions in snowpack estimated from 30 percent to 90 percent by 2100, agriculture could take a big hit depending on how dwindling water resources are allocated among cities, farmers and the environment.

"I cannot emphasize enough how critical a factor that is for California's agriculture," said ecologist Chris Field of the Carnegie Institution at Stanford University. "It doesn't matter how fast or slow a plant can potentially grow. In California, if you don't give it sufficient irrigation water, it's not viable as a crop."

To make matters worse, the bulk of the state's agricultural profits come from perennial plants that live for 30 years or more and are not easy or cheap to swap for more heat- or drought-resistant crops, or to move to cooler locations.

Climate scientist David Lobell of Lawrence Livermore Laboratory led a study published in the journal Agricultural and Forest Meteorology in November that used past yields of six of the state's most lucrative perennial crops to calculate the potential impact of future warming predicted by more than 20 different climate models. Five of the six crops suffered significant yield losses by midcentury in nearly all of the models.

"It's not good news," Lobell said. "I was a little surprised at how unlikely it is for climate change to have no effect or a positive effect. Even the most conservative models show some decline."

Avocados, grown mostly in Ventura and San Diego counties, could see yields drop as much as 40 percent. The Central Valley's almonds and walnuts and the San Joaquin and Coachella valleys' oranges and table grapes could decline as much as 20 percent.

One potential mitigating factor that Lobell's study did not take into account is the positive effect that higher concentrations of atmospheric carbon dioxide can have on plant growth.

Plants use carbon dioxide during photosynthesis to convert solar energy into plant material and fuel. Early research suggested that adding carbon dioxide to the atmosphere would substantially increase plant growth. But more recent research has shown a much smaller benefit, on the order of a 10 percent to 20 percent increase in growth by the end of the century with twice as much carbon dioxide as was in the atmosphere before people began producing industrial greenhouse gases.

"If the negative impact of climate was 10 (percent) to 20 percent, it's possible that it would come out in the wash, that they would more or less even each other out," Field said. "But if the impact of climate is really substantial, such that you can't grow crop 'A' in place 'B,' then it doesn't really matter that in some places crop 'A' was growing 10 percent better."

Farmers get squeezed

Some California crops are already feeling the heat.

Farmers in the middle of the country may be able to adapt fairly quickly to rising temperatures by switching to more heat-tolerant crops. But California won't be nearly as agile.

Much of the state's agriculture involves long-lived plants such as grape vines and avocado, peach and nectarine trees. It can take as many as eight or 10 years for some of these plants to mature and begin bearing a full load of fruit. So switching to warmer-weather fruit such as oranges or lemons is no small endeavor.

"That's a huge difference between California agriculture and everywhere else," Field said. "We're overwhelmingly dependent on perennial crops. It means across all our crops, we are automatically more vulnerable to climate change."

This could prove to be a major problem for farmers such as Len Delchiaro who can't afford to uproot his 70 acres of cherry trees in Brentwood and wait six or seven years with no income before a new set of trees begins to bear fruit.

The biggest threat to Delchiaro's orchard is a decline in "chill hours," or hours below 45 degrees. Cherry trees need from 900 to 1,200 chill hours during which they go dormant, a process that enables normal bud and blossom development. Any temperature spikes above 65 or 70 degrees during the winter can also adversely affect dormancy.

Warmer winters, particularly a rise in nighttime low temperatures, have caused Delchiaro's trees to blossom several weeks late and weakened the buds so that they can't hold on to the fruit.

"When you don't have enough chill, you may have a good blossom, but the tree's not strong enough to sustain the fruit," he said. "So then you have a large drop-off where you lose immature fruit."

Delchiaro, who has been farming cherries in Brentwood for more than three decades, has seen his crop yields drop from 60 percent to 70 percent the past two years.

Years with too few chill hours are a normal part of a naturally fluctuating climate, and Delchiaro is happy with a cold winter so far this year, but he believes warmer winters have been more common in recent years

"It's been more noticeable over the last five or 10 years we're getting less and less chill," he said, noting that urban encroachment could also be a factor.

With the cost of labor increasing, crop yields declining and the price of cherries static, farmers like Delchiaro are getting squeezed.

"We're kind of in a vise," he said.

Still, he plans to hang in there with his cherries. "It's just something that we know. We're geared to it," said the 56-year-old farmer. "I'm at a point in my life where I don't want to have to wait seven, eight years to change."

A rise in average temperatures isn't the only problem farmers may face in a warmer California. Brief bouts with extreme weather such as heat waves or sustained heavy rains can cause serious problems for some crops. And climate models suggest that extreme weather could become increasingly common as temperatures climb.

Farmer Ruth Hartnett has been growing a variety of fruits and nuts and raising various livestock on nine acres of Grand Island in the Sacramento River near Rio Vista. This summer's heat wave claimed three of Hartnett's turkeys, and has her pondering a warmer future in which extremes become the norm. She says many farmers in her area have been struggling with the unusual weather, causing some of them to rethink their crop choices as they brace for more of the same.

This year at least three pear orchards in Hartnett's neighborhood were plowed under to make way for hardier crops such as citrus fruits or, in one case, a housing development.

"The weather has gotten so strange, and crops are so unreliable, especially when it comes to fruit, that these farmers just bulldozed their orchards," she said.

Some farmers, including Hartnett, are convinced that global warming is at least partly to blame for the peculiar weather. Although some are biting the bullet and switching crops, others are contemplating quitting the business altogether, and a few are thinking of relocating.

"If we're going to get this kind of heat in this area, there are farmers who are seriously considering moving north. Maybe Canada, maybe Alaska," said Hartnett.

For now, Hartnett is considering somewhat less-drastic steps such as replacing a few pear trees with citrus trees, but she counts herself among those who could be persuaded to hit the road.

"We seem to be at a tipping point. People are looking toward economic survival," she said "The undercurrent is, 'How do I not lose my shirt and everything I've ever worked for?'"

Wines on the move

Many crops will be affected, but the state's prized wine industry may be the proverbial canary in the coal mine when it comes to climate change. Grapes, particularly those used for premium wines, require a delicate balance of climatic conditions.

"Wine grapes are especially vulnerable because they have a sensitive temperature range in which they can grow," said Stanford University ecologist Kim Nicholas Cahill, who studies the effects of extreme heat on grapes.

Although wine grapes might not suffer major declines in yield, quality rather than quantity is the issue with this crop.

Too hot, and grapes may ripen too quickly and produce flabby wines with too little acid and too much alcohol. Too cold, and a wine's character will tend toward less desirable green flavors such as grass or bell pepper.

The Napa Valley region is blessed with a 64 degree average temperature that falls smack in the middle of the comfort zones of many popular varietals, including merlot, syrah and cabernet sauvignon.

But Napa just barely tags the range for chardonnay grapes, which thrive in 57 to 63 degree temperatures. A small bump up in the average growing season temperature, even just 1 degree, could push Napa into questionable territory for chardonnay.

Of course, this same small bump in temperature would nudge the valley closer to the ideal climate for zinfandel grapes. But a few more degrees could be a disaster for Napa. And some of the state's warmer wine-growing regions, such as Santa Barbara and Paso Robles, might be lost altogether.

"You add another couple of degrees onto warming in Fresno, and it will become real challenging to grow anything other than table grapes or raisins because you can't produce premium high-quality wine in that hot of a climate without technology we really don't have today," said climatologist Gregory Jones of Southern Oregon University in Ashland.

Jones is part of a team that used a computer climate model to look at the future of the U.S. wine industry in a warming world -- and it is bleak.

According to the study, published in July in the Proceedings of the National Academy of Sciences, 60 percent of the best terrain for premium grapes will be lost by the end of the century if greenhouse gas emissions continue unabated. And the picture is particularly grim for California, currently responsible for 90 percent of the country's wine grape production.

Most of the remaining top-quality acreage will shift northward to the Pacific Northwest. California will retain and possibly gain a little bit of territory along the coast, but the inland vineyards, including those in Napa Valley and Sonoma County, will be lost.

"There's a tremendous amount of culture and value that's associated with the wine industry where it is," Field said. "It would be a heavy price to pay if we had to move out of there.

"If all of a sudden, Mendocino County becomes a better place to grow wine, even if you are producing the exact same wine there, it's not a hundred percent clear that the value will transfer immediately. It may take the world's wine community anywhere from months to decades to learn that now the wines associated with a certain quality are coming from some other place."

Williams at Frog's Leap Winery knows some growers who are hedging their bets by buying property in cooler areas with an eye to growing wine grapes there in the future.

A narrow band along the northern Central coast may maintain a good climate for wine, and farther north the coast could warm up enough to become suitable. But problems with high humidity and excess precipitation will persist along the coast. And climate projections don't take into account the "terroir," or character of the earth; even if an area gains a climate appropriate for wine grapes, it may never produce premium wines.

If carbon emissions continue unabated, the statewide annual temperature could go up 10 degrees. That's roughly equivalent to the difference in average annual temperature between Oakland and Los Angeles.

But if emissions are curbed significantly, the rise could be kept to around 3 degrees. That's something that wine growers might be able to handle by changing their vine-management practices, Cahill said.

The temperatures that the grapes on the vine actually experience can be changed quite a bit by controlling how much leaf cover they have. Many growers in Napa trim the leaves back to give their grapes more direct sunlight, which in turn produces bolder wines, said Terry Hall, communications director for the Napa Valley Vintners.

Currently, growers often thin out clusters of chardonnay grapes to keep them cooler, Hall said, and this practice could be used on other varietals as well if temperatures rise.

"I've seen a lot of interesting innovations in management. People do have some capacity to adapt," said Cahill. "But at the higher end of the (possible) temperature increase, business as usual will definitely be much less possible."

Betsy Mason covers science and the national laboratories. Reach her at 925-847-2158 or bmason@cctimes.com.