WALNUT CREEK, Calif. -- A Napa Valley winemaker recently traversed a steep hillside to reach a 12-foot white weather station that towers above rows of cabernet sauvignon vines absorbing the midmorning sun.
Curiosity drove him to install the relatively inexpensive device in 1995, said Christopher Howell, general manager and winemaker at Cain Vineyard and Winery in the valley's Spring Mountain region in St. Helena.
But in the years since, its wirelessly relayed data -- along with those of 100 like it now operating in the valley -- has become crucial as Napa Valley vintners uneasily brace for a changing climate that they are sure will come.
The region's wine growers had long heard of melting glaciers and Arctic ice sheets breaking apart in rising global temperatures.
During the 20th century, global temperature increased by about 1.33 degrees Fahrenheit, and a U.N. climate panel estimates that, depending upon carbon dioxide emissions, temperatures will rise an additional 2 to 11.5 degrees by 2100.
"But we shockingly hadn't connected the dots and said, 'Oh my, our world is going to change too,' " Howell said. "We are as anxious about this as we are about the arrival of any new pest or disease."
They're playing catch-up in monitoring for climate change, and the Napa Valley Vintners, an industry group of 400 wineries, is wrapping up a four-year study of climate conditions in Napa Valley.
There's no indication that changing global climate conditions have affected Napa Valley wines, emphasized Terry Hall, a spokesman for the vintners.
Yet Hall said it's "when" more than "if" climate change will ultimately affect the valley, where cool summer nights and warm days temperatures yield premium grapes.
Higher regional temperatures could make the Napa Valley cooler, as heat farther east creates a "vacuum effect" that draws ocean fog inland.
But research also suggests that as temperatures rise, the fog belt might concentrate close to the coastline, leaving Napa Valley vulnerable to higher temperatures.
To get a better handle on the future, Napa Valley Vintners asked Daniel Cayan, a leading climate scientist with the Scripps Institution of Oceanography, to head its study. Cayan and three other researchers analyzed more than 12,000 data points on weather and the annual life cycle of vines, such as when leaves first emerge.
Kimberly Nicholas, a study co-author, said research shows that there's been a 15 percent increase in Napa Valley's growing season since the 1970s. In addition, a 2001 study Cayan co-authored reported a drop in frost days from 27 days per year in 1950 to fewer than 10 per year in 2000.
As for how this could affect grapes, fewer frosts mean less risk for emerging buds. A longer growing season lets winemakers harvest at optimal times, when sugars, acid and pH are in balance. The acid content develops during the cool of night.