"We're projecting dramatic decreases in total production in the continental United States and this is really due to the increase in the frequency of extremely hot days in response to elevated greenhouse gas concentrations," climate scientist Noah Diffenbaugh of Purdue University said by telephone.

A study published in the Proceedings of the National Academy of Sciences forecast an 81 percent drop in the US areas suitable to grow premium wine grapes by the year 2099.

Diffenbaugh and his co-authors built on earlier research that suggested little damage to US wine grapes from expected global warming, and even a possible boon for the most expensive wines.

The previous work was based on projections of the mean temperature -- the average, in meteorological terms -- during the growing season, and did not take into account the number of very hot days.

The current study did take daily temperatures into account, and projected a huge increase in the number of days with temperatures above 95 degrees F (35 degrees C), which could be damaging to wine grapes.

TOO MANY HOT DAYS

Even the most heat-tolerant grapes can only take this kind of extreme heat for 14 days each season, Diffenbaugh said. The new study projected as many as 50 or 60 such super-hot days in a season.

"The bottom line is it's just too hot, too many days out of the year in too many areas," he said.

Currently, seasonal weather favors wine grapes over much of the Mid-Atlantic region, stretching from Tennessee and Kentucky in the southwest to Massachusetts and New York state in the northeast. In the west, prime wine grapes grow comfortably in valleys and coastal areas of California and valleys in Oregon and Washington state.

The new study projected suitable growing temperatures for premium wine grapes by the end of this century could be confined in the East to northern Pennsylvania, New York state and coastal New England, including Cape Cod in Massachusetts.

On the West coast, wine-producing areas could be limited to a narrow area along the Pacific and in some valleys in Oregon and Washington state, the new study found.

"We see production disappearing essentially in what are the prime producing areas, which is Napa Valley, Sonoma Valley, the Santa Barbara area (in California) and the Willamette Valley (in Oregon)," Diffenbaugh said.