Their prediction punctuated a recent report published by the U.S. Department of Agriculture, the National Center for Atmospheric Research, other agencies and scientists at several universities. The researchers synthesized more than 1,000 studies in what was billed as the most extensive assessment to date of how climate change will affect the country's natural resources.

The findings come at a time of turmoil for worldwide food supplies caused by natural disasters; the use of corn for producing ethanol; the skyrocketing cost of oil, which has created a surge in prices for basic grocery items; and growing global competition for wheat, milk and other staples.

The report's authors said climate change offers a few potential positives. Those include longer growing seasons, which can help boost the productivity of pastureland and some crops. But overall, the forecast was grim.

The researchers projected an increase in crop failures, the spread of weeds, diminished effectiveness of the herbicide glyphosate, more insect damage to crops and a rise in livestock deaths. They also cited more signs of drought severity in the West. Extended water shortages could drive up costs, push more farmers out of the business and send food prices ever higher.

"Slight changes in things like temperature and precipitation can potentially have dramatic effects," said Steven Archer, a lead author of the new report and a professor of natural resources at the University of Arizona.

Archer and others highlighted the limits of the study, including the lack of comprehensive monitoring systems to detect the effects of climate change. For that reason and others, some skeptics question whether humans play a significant role in any large-scale global warming. The new report from the Bush administration was peer-reviewed. It did not make policy recommendations, although scientists and politicians nationwide are looking for strategies to cut levels of carbon dioxide and other greenhouse gases. "We have based our entire economy on the concept of a stationary climate ... so clearly, adaptation is going to be difficult. We don't really know how to do it," said Jay Gulledge, a senior scientist at the Pew Center on Global Climate Change in Arlington, Va. He did not contribute to the latest report.

The search for answers has prompted geneticists to try to make plants more drought-tolerant. Their efforts remain at the transistor-radio stage of developing electronic technology, said Julian Schroeder, a biologist at the University of California San Diego.

"When the weather affects the farmers' crops, it trickles down to the hay broker and then that all plays into the price that we end up having to pay," said Cheryl Lange, who runs a livestock ranch in Fallbrook, Calif. "It's just like a vicious circle, and we end up at the bottom end of it."

Besides focusing on farming, the report addressed the likely future of the nation's natural resources. Among other things, it predicted that forest and range fires will become more common, soil erosion will increase and signature plants in the Southwest could disappear. Unlike many climate reports that make projections for 2100, this research team limited its time frame to the next 25 to 50 years. Scientists' climate models show much more certainty in the shorter time period than they do for a century or more. They said the world is locked into some amount of warming in the near term because of past greenhouse-gas emissions, but that the long-term future could be different if those emissions are lowered.

"There are advances that allow you to incrementally improve drought resistance and the ability of plants to retain water," biologist Schroeder said. "It's hard to be optimistic right now, but one has to work on solutions."

The report focused on major crops such as corn and soy, which generally are less vulnerable to temperature changes than other produce. Plants have different optimal ranges for temperatures, frost-free days, carbon dioxide in the atmosphere, water and other factors.

For corn, the USDA researchers predict that heat will drive down productivity in coming years - an unwelcome forecast for the price of everything from tortillas to gasoline. Rice and beans could follow suit, while plants such as peanuts and cotton could benefit from having more carbon dioxide in the air.

Whatever happens to specific crops, the overarching issue will be water - a commodity that's in increasingly short supply in the West. The USDA report said a trend toward earlier snowmelt challenges irrigation systems that rely on the mountains releasing water slowly over several months.

In San Diego County, grower Bob Polito has cut down many of his avocado trees to reduce his water demand. Polito figures that water-intensive crops such as avocados and citrus will have a hard time hanging on in Southern California without a more reliable water supply.

"I hear a lot of chain saws going around these days," he said. "I think the people are starting to panic now. It's starting to get hot and reality is starting to set in."

At the San Diego County Farm Bureau, director Eric Larson has an additional concern: The prospect that global warming would generate more insects, which in turn would harm more crops.

FINDINGS

Climate change is affecting, and will continue to affect, the nation's water supplies, agriculture, land resources and biodiversity.

The growing season has increased by 10 to 14 days over the past 19 years in many areas of the country. Higher temperatures will increase the risk of crop failures for certain grains, fruits and vegetables. Warmer winters will reduce livestock deaths, but that will be more than offset by greater mortality during hotter summers. More heat also will lessen the productivity of livestock and dairy animals.

Weeds will spread north and become more resistant to herbicides. More carbon dioxide in the atmosphere will help young trees in good soils grow faster, but it also will cause more forest fires.

SOURCES: U.S. Department of Agriculture and U.S. Climate Change Science Program

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