A Napa Valley winemaker last week 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're 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 F 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.
On Tuesday, Howell and two others will present the summary data to wine journalists at Vin Expo in Bordeaux, France, an international industry gathering.
There's no indication that changing global climate conditions have affected Napa Valley wines, emphasized Terry Hall, a spokesman for the vintners group.
"What you're tasting in your glass is not climate change, it's wine style," Hall said.
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.
Paradoxically, higher regional temperatures could make the Napa Valley cooler, as heat farther east creates a "vacuum effect" that draws ocean fog inland. The hottest years on record globally -- 1998, 2005, 2006 and 2010 -- were among the valley's coolest, Hall said.
But research also suggests that as temperatures rise, the fog belt might concentrate close to the coastline, leaving Napa Valley vulnerable to higher temperatures.
Adding to the puzzle, the 30-mile valley has many microclimates, from the cooler southern area to the warmer north. The flatlands also are warmer and drier than the mountains.
"Climate change is real," Howell said. "What's happening in our valley, we don't know."
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.
Records from the valley's two oldest weather stations show a warming trend of 0.03 F per year since 1931, but only at night and primarily between January and August. Daytime warming "has been close to zero," the authors wrote. The problem with that warming figure, which they consider inflated, is the two stations' locations. Both pick up "urban heat effects," said Hall, referring to heat from sun-warmed pavement, heated buildings and other types of development.
Cayan and his team, however, also scrutinized data from "nondeveloped" areas. Most were from the late 1970s, along with a few handwritten notes dating to the 1930s. Those data, too, showed slight night warming over the past 60 to 80 years in the valley, although "significantly less" than the two weather stations in the developed areas. The authors noted that the valley needed to improve its monitoring system.
While the analysis yields a fuzzy picture of Napa's recent climatic past, "there has been a real warming trend," the researchers wrote.
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.
Howell said wine growers can counter changing conditions with techniques such as leaf canopy pruning to allow more or less sun to hit grapes, or growing vines closer or farther from warm soil.
Many wine regions in California and worldwide face similar challenges, Hall said.
However, Napa Valley's prominent perch -- it yields 4 percent of the state's wine grapes but generates 34 percent of California wine sales -- is one reason the vintners group is stepping up its monitoring, and working to lead by example.
"We don't just want to be sitting on the sidelines," Hall said. "I don't know that any other region is putting the same energy into developing a benchmark of where they are."
Suzanne Bohan covers science. Contact her at 510-262-2789. Follow her at Twitter.com/suzbohan.