SACRAMENTO -- The succulent cherries and juicy ripe peaches that Californians enjoy every summer could disappear in some regions as climate change warms the cold winters that trees need to bear fruit in the spring.

In the past century, the state's winter lows have warmed by 2 degrees Fahrenheit, "a significant increase," one expert told a forum last week.

Although that warming trend hasn't yet disrupted crops, it is accelerating, Dan Sumner, director of the Agricultural Issues Center at UC Davis, told the Sacramento gathering.

"There's potential for complete crop failure, especially cherries, apricots and other stone fruit," said Louise Jackson, a UC Davis researcher. As an example, Sumner said that as winters warm heat up, peach growers in the warmer southern San Joaquin Valley may have to move northward, where it's cooler.

The forum drew experts in agricultural and climate science, and water and pest management with concerns over extreme weather's threat to California's rich agricultural heartland.

California farmers grow more produce than any other state, supplying half of domestic fruits and vegetables, and almost all the almonds, apricots, raisins, grapes, olives and pistachios. California ranchers are among the leading livestock producers.

But California's fast-changing climate will challenge farmers' resiliency. Climate models predict global temperature increases from 2 to 11.5 degrees by 2100, depending on heat-trapping greenhouse gas emissions.

Among the threats discussed:


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  • A loss of up to half of the Sierra Nevada snowpack, which yields one-third of the state's freshwater,

  • Hotter summers threatening numerous crops with sunburn, leaf wilt and other maladies, and harming livestock sensitive to high temperatures,

  • Invasive insects and weeds from southern regions moving north,

  • A drying trend around the state, increasing competition for water,

  • Extreme weather, particularly heat waves and prolonged, severe droughts.

    Excessive heat harms most crops in California, Jackson said, and the major heat wave of July 2006, when temperatures in some areas rose above 110 degrees may become a yearly event, according to the state's Climate Adaptation Strategy.

    Although additional carbon dioxide in greenhouse gas emissions is expected to spur plant growth, Jackson said evidence shows that vegetative growth, like leaves, may increase, but crop yields may not. The rising temperatures also could lengthen growing season for some crops.

    The next few decades may make more obvious the link between human-caused greenhouse gases and climate, said Glen MacDonald, director of the UCLA Institute of Environment and Sustainability.

    "It hasn't gotten bad enough that we can definitely attribute it" to emissions from cars, power plants and human sources, MacDonald said.

    "But by 2040 or 2050, we'll really start feeling definite results of greenhouse gas emissions," he said.

    MacDonald focused on the odds of severe and prolonged drought in California, "since that's the most important one in terms of agricultural productivity."

    The state's climate adaptation report says that "drought conditions are likely to become more frequent and persistent over the 21st century because of climate change." Of the suggestion that farmers could adapt their crops to slowly changing conditions, MacDonald warned that an epic drought, lasting a decade or longer, could develop suddenly.

    "I'm worried that some of these events can be very rapid and then persist for a long time," he said.

    Higher temperatures and the threat of long droughts dominated the forum, but other climate change risks include flooding of fields in or near floodplains when winter and spring rains combine with earlier snowmelt, and salt water intrusion into farmlands as melting polar ice raises the sea level.

    Sumner and others emphasized that with the diversity of crops and different microclimates throughout the state, the agricultural system has a natural resiliency, although he said now is the time to begin researching ways to best cope with anticipated changes.

    "I think the risk is we don't take it seriously enough," Sumner said. "The lesson here is you have to have enough research and development to adapt and be flexible. And I don't think we're doing enough."