Global warming could wither many premium vineyards in California and across the nation by the end of the century, according to a new computerized climate projection released Monday.

A predicted rise in the number of days hotter than 95 degrees during the growing season could sharply reduce the amount of areas suitable for vintage wine-grape production, an international team of scientists concluded in the journal Proceedings of the National Academy of Sciences.

Marginal vineyards nationwide might be eliminated and those capable of producing the most expensive premium wines may be reduced by half, the researchers reported. Although wine is produced in 48 states, California's $16.5-billion industry, with more than 500,000 acres of vineyards, accounts for almost 90% of the nation's wine grapes.

"We found that at elevated greenhouse gas concentrations, the frequency of extremely hot days increases to the point where it is impossible to grow premium wine grapes in many areas of the country," said Noah Diffenbaugh, one of the study's researchers at Purdue University who studies the impact of climate change.

"Certainly, the Napa Valley, the Sonoma Valley and the Santa Barbara area all exhibit enormous losses of production in the future climate," Diffenbaugh said.

Several wine industry experts not connected with the study were skeptical of the new climate projection and its prediction.

"If their definition of an extremely hot day is 95 degrees, I'd tell them that they don't know squat about grapes," said James T. Lapsley, an expert at UC Davis on the economics of wine and the history of California wine.

"Ultimately we will have to be looking for other places to grow grapes" if temperatures do rise high enough, Lapsley said. "What these guys are saying is a little more stark."

Gladys Horiuchi, a spokeswoman for the Wine Institute in San Francisco, said the climate projection "raises a lot more questions than it answers."

Until now, studies centered only on the impact of average temperature increases on wine production and concluded that grape growers might benefit from the temperature increases expected in coming decades.

A 2003 study by researchers at the Southern Oregon University predicted that a 3.6-degree rise in temperature over the next 50 years would have "no negative impacts." The researchers studied 27 winemaking regions around the world.

In the main wine-grape regions of California, Oregon and Washington, the growing season temperatures have risen by about 1.5 degrees since 1948 — promoting larger grape yields and higher-quality wines, records show.

The new study, funded by the National Science Foundation, NASA and Purdue University, involved five months of supercomputer calculations. It is the first study in which researchers have been able to calculate daily temperature swings from various climate-change scenarios in detail.

For the purposes of their computer simulation, the researchers assumed that levels of carbon dioxide and other gases would continue to rise to more than twice their current level by 2100, as outlined in standard global warming scenarios.

The resulting change in climate would disrupt the delicate balance of heat and cold necessary to produce quality grapes, the researchers said.

Eventually, the computer simulation showed, the number of extremely hot days during the growing and ripening season would increase by three to eight weeks in much of the South central and Southwestern United States — too hot to produce premium wine grapes.

The number of cool days would also decline by more than three weeks in many regions of the country, opening up some areas for new grape production but also increasing the possibility of more rot, mildew and fungus infections in those areas.

On average, the growing season would start 22 days earlier around the country. The West Coast would experience one of the biggest changes, resulting in a significant alteration to the quality of its grape harvest.

As growing seasons become hotter, winemaking areas would shift north and to higher elevations, the researchers said. Grape-growing areas in California would shrink to a narrow coastal band. In the Southwest and the Midwest, wine production would be almost completely eliminated.

Premium wine-grape areas would shift into New England and the Pacific Northwest.

"This is a call to arms," said Karen Ross, president of the California Assn. of Winegrape Growers, which represents 1,000 growers in the state. "We need to pay attention now. We ought to start thinking about what can be done now to impact the severity of what might happen."