MY ORANGES, which usually ripen in spring, are already brighter than Caltrans vests, and the peach trees will have to be pruned in December rather than in January. A neighbor who's a physician taught me the orchardist's art of editing the shoots a few years ago. I call him "prune picker" since he claims to have harvested them as a boy, back when people did that sort of thing around here. I ask the doctor if he thinks it's global warming. I'd read the New York Times article about how New England's storied fall foliage no longer screamed with the fiery reds, yellows and oranges of the past. If climate change could dull the intense colors of a Connecticut autumn, could it not be wreaking similar havoc on the tattered vestiges of Santa Clara Valley's orchards? My two trees hatched only a few ratty specimens last summer, rather than the voluptuous bushels of oversized, juicy, sweet peaches that hung from their branches in previous seasons.

Like me, the doctor's a sucker for a conspiracy theory. He agrees that something's strange. He says he plans to prune as well this week.

Leland Lester, whose apricot-, peach- and prune-growing family landed in the valley in the 1880s, knows when to unholster the clippers. "The best time to prune is when the leaves are off," he advises when I interrupt him during a televised Sharks game. Because there hasn't been much cold this fall, "we're going to have an early year," Lester says. "It's not unusual. I don't think it's global warming. Personally, I don't believe in that."The 82-year valley native says he's seen mild winters before. "I've been here a long time."Camel breeder and winemaker Jon Anderson, who moves trees and landscapes corporate campuses in his day job, doesn't think the sky is falling either. "There have always been cycles, even when we didn't have cars warming things."

We're having an early spring, Anderson explains, because "it never really got cold."

"When the temperature gets below 54 degrees, the plants slow down like a bear in hibernation. But if it doesn't get cold, they tend to want to bloom earlier."

Carl Cilker, a fifth-generation grower and Lester cousin whose valley agricultural roots go back about 140 years, tracks cumulative cold hours on UC-Davis' website to project his crop yields, and he's concerned about two consecutive warm, rainless winters. "There are always seasonal variations, but with all the news and all the discussion about global warming, it makes you more aware of those things."

At his San Jose home, his citrus, like mine, is maturing precociously. "They don't normally get ripe until January, and they've got color on them now," Cilker observes. "The orange tree this year is definitely early."

Last year's walnut crop at his family's Central Valley groves was off, and he blames the decline in the number of cold hours during wintertime. "Trees that go dormant need to have that," Cilker says.

Though hardly an Al Gore–style redwood hugger, Cilker worries about climate change's effect on California agriculture. "It's not like it's all going to fall apart all at once. But the economics are based on some anticipated yields. And if the trees don't produce what you expect them to—especially orchard crops that take a lot of capital investment and time for them to produce what you want them to—the economics can come apart in a hurry."Another region that shares our "Mediterranean climate" is Southern France, where researchers from the French National Institute for Agricultural Research, who are studying the effects of climate change on apple, peach and apricot trees in the Rhone Valley, have identified earlier flowering patterns. "The trend to earliness observed in the last decade" leads them to believe that the "evolution has already started, pointing out the need for breeders and farmers to reconsider the geographical adaptation of genotypes."

The Agricultural Institute of Canada, studying fruit production in eastern Canada, developed climate models that project winter temperatures rising by 2 to 6 degrees Celsius by 2050, according to the Canadian Journal of Plant Science.

In India's Kashmir Valley, academics have spotted rises of 3 to 8 degrees that have caused formerly fruitless orange trees to sprout fruit. In the Southeastern United States, one study measured the loss of as many as 11 chilling days. Georgia might have to take the peach off its license plate someday.

Closer to home, at the California Climate Change Center, Dennis Baldocchi and Simon Wong wrote in 2006: "Global warming seems to be in motion, as all sites studied are experiencing a negative trend in winter chill accumulation. Calculations of trends in future chill ... indicate that by 2100, the occurrence of adequate winter chill may be lost for many fruit species."

"California produces over 95 percent of the United States' apricots, almonds, artichokes, figs, kiwis, raisin grapes, olives, cling peaches, dried plums, persimmons, pistachios, olives and walnuts," Baldocchi and Wong add. "In principle, a reduction in chill degree hours will result in a reduction in crop yield and quality. If true, this effect could have major economic and social consequences on fruit products in California. And if critical thresholds are reached with further warming, sustained production of high-value fruit crops like almonds, cherries, apricots and others will be in jeopardy."

In Brentwood, just a half-hour north of Livermore, the two scientists documented between 1986 and 2005 "a significant trend indicating a reduction in accumulated chill degree hours," an observation they called "alarming." If trends continue, there will be no chill hours at all before long, they say.

That's why even conservative orchardists like Cilker are thinking greener these days. "When you see the glaciers melting, you say, 'Damn, what's happening here?' Human or not, it's happening, and we do have a food supply issue. As a result, it makes me nervous."

Send a letter to the editor about this story.