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Rising temperatures could transform Wine Country's mild climate into one as sweltering as Tijuana, Mexico, eliminating Sonoma and Napa counties' competitive edge in producing world-class wines.

That's the conclusion of a study released Monday and published in the Proceedings of the National Academy of Sciences.

Areas in California suitable for growing premium wine grapes could be reduced by 50 percent -- and possibly as much as 81 percent -- by the end of this century because of global warming.

The study, the second report in two years on global warming's impact on premium wine, indicates increasing weather woes for wine grapes in California if fossil fuel consumption continues unabated.

In 2004, the Union of Concerned Scientists released a report warning that within 80 years most of Sonoma and Napa counties could become too hot to grow premium wine grapes. If so, it would severely affect an industry that contributes more than $5 billion to the regional economy, defines the Wine Country lifestyle and anchors more than 100,000 acres of vineyards.

While growers and winemakers are concerned about global warming, they don't believe rising temperatures would necessarily destroy the state's $45 billion wine industry. They said adjustments in vineyard management, irrigation practices and varietal selection could keep grapes the leading crop in the North Coast and in California.

Nick Frey, executive director of the Sonoma County Grape Growers Association, said the Pacific Ocean and its cooling maritime influence are the most critical factors in producing Sonoma County's premium-quality grapes. He said even if there are more hot days in the future, the Pacific Ocean will still be there to cool things down at night, creating the sugar-acid balance essential for fine wine.

"It's the cooling nighttime temperatures and fog from the Pacific Ocean that make Sonoma County such a great wine region," said Frey.

The primary change in the weather will be an increase in the frequency of extremely hot days, said Noah Diffenbaugh, a co-author of Monday's report and a scientist in the Department of Earth and Atmospheric Sciences at Purdue University.

Grapes used in premium wines need a consistent climate, he said.

When temperatures top 95 degrees, the vines have problems maintaining photosynthesis and the sugars in the grapes can break down, Diffenbaugh said.

"We have very long-term studies of how this biological system (of vineyards) responds to climate," said Diffenbaugh. He said that information gives the researchers confidence in their projections.

The issue of global warming and its impact on wine grape growing is one that certainly needs to be explored further, said Gladys Horiuchi of the Wine Institute, a trade association representing more than 800 of the state's wineries.

She said there are plans to discuss global warming at a major wine industry conference early next year.

Meanwhile, growers and winemakers, whose fortunes depend on the weather, wonder what the future will bring.

"It's on everyone's mind. A shift in temperatures could tilt everything," said David Cooper, winemaker at Yoakim Bridge Vineyards & Winery in Healdsburg. "I keep my fingers crossed."

Scientists and environmental experts have become increasingly alarmed in recent years by accumulating gasses such as carbon dioxide in the atmosphere as a result of the burning of fossil fuels.

A panel of climate scientists convened by the National Academy of Sciences reported last month that the Earth is heating up and "human activities are responsible for much of the recent warming."

The scientists said average global surface temperatures rose by about 1 degree in the 20th century.

That may not sound like much, but many blame it for melting glaciers, weather changes -- perhaps even more hurricanes -- and threats of spreading diseases.

While problems are foreseen for California's Wine Country, the researchers suggest grape-growing conditions might improve in parts of the Northwest and Northeast.

However, the Northeastern and Northwestern states have higher humidity levels than the current top wine regions.

High humidity is associated with fungus outbreaks and other potential growing problems, Diffenbaugh said, "so it could be very expensive to produce premium wines in those areas."

"Our simulations suggest that the area suitable for the production of premium wine grapes will both contract and shift over the next century," the researchers concluded.

The new study, funded by the National Science Foundation, the National Aeronautics and Space Administration and Purdue University, involved five months of supercomputer calculations.

It is the first study in which researchers have been able to calculate the daily temperature swings from various climate change scenarios in such detail.

The Los Angeles Times and the Associated Press contributed to this story.