Vintner and climate study task force member Christopher Howell of Cain Vineyard & Winery said, "We winemakers are farmers — as farmers, we live not by the climate, but much more by the weather, i.e.: day to day, week to week, season to season, and year to year. In order to get clear evidence of climate change, we need to be able to compare trends over decades--this is not a perspective on the usual human scale." Indeed, this study and previous research shows that Napa temperatures are correlated, to some extent, with changing ocean temperatures along the Pacific coast; for example, sea surface temperatures along the central California coast have been unusually cool in recent years, associated with relatively cool air temperatures in the Napa Valley.

Howell continued, "We love the quote attributed to Mark Twain who said, 'The coldest winter I ever spent was a summer in San Francisco.' The Pacific Ocean is our region's greatest temperature control. From living here, we know that the warmer the Central Valley becomes on a summer day, the more intensely the fog pours in from the coast. This is the 'vacuum effect' of the warmer interior valley. We have been blessed to have the perfect mix of warm days and fog/coastal cooling that allow us to grow some of the finest wines in the world.

Globally, the years 1998, 2005, 2006 and now 2010 were the warmest years on record, but they were some of the coolest for the Napa Valley. There is a suggestion by some climate scientists that, as the interior areas warm in the future, Napa temperatures may actually remain relatively moderate, or even cool as maritime air gets drawn further up the Valley. Either way, warmer or cooler, it's different than what we're experiencing today--so as prudent farmers we need to look at all of our possible scenarios and consider best practices to continue to grow the best wine grapes," Howell concluded.

The new study emphasizes the need for maintaining regular observations at high-quality weather stations around the Valley. Estimates of temperature changes in the Napa region are hampered by local changes in exposure, buildings and paved areas around the longest existing weather stations. For example, the commonly used weather station at Napa State Hospital, with a record going back 100 years, is situated over an irrigated lawn next to a black top driveway and a building with a large window air conditioning unit, and the St. Helena weather station is mounted on the roof of the city fire station, but was moved three times in recent decades.