The Napa Valley Vintners (NVV) non-profit trade association has announced the release of the Napa Valley-specific climate study titled Climate and Phenology in Napa Valley: A Compilation and Analysis of Historical Data by Dr. Daniel R. Cayan, Dr. Kimberly Nicholas, Mary Tyree, and Dr. Michael Dettinger.

In 2006, a researcher garnered national media attention by predicting that Napa Valley would soon become too warm to grow fine wine grapes. These reports noted signs of warming in California and the Western United States in recent decades, calling attention to several changing indicators in weather, hydrological and biological systems. Evidence from other Mediterranean climate regions around the world indicated that climate warming may be taking hold in these settings.

However, the experience of Napa Valley growers has been contrary to the notion that Napa Valley has warmed substantially. A problem in applying this previous research to the Napa Valley is that it has considered just a few weather station records in Napa Valley, which has long been known for very diverse micro-climates and growing conditions.

This just-released Napa-specific study by Cayan and colleagues scrutinized weather and phenology (the growing cycle of grapevines) records based on many more stations within Napa Valley, and arrived at a number of important new conclusions. Over the four years of the study, more than 12,000 data points were collected from measurements made at geographically diverse sites in the valley, using information ranging from hand-written journals kept by long-time growers to digital data from current-day automated weather stations positioned valley-wide. Most of the observations were from records taken since the late 1970s, but some of the hand-written entries were from as early as the 1950s.

The Executive Summary of the study finds that the region has experienced some warming, approximately 1 F to 2 F over the past several decades, but considerably less warming than would be inferred from the standard cooperative observer weather stations in Napa Valley. The warming has been primarily in winter, spring and summer, and it has concentrated during nighttime rather than daytime. Over the last several decades in growing season temperatures, there has been little warming in the daytime and the available observations provide little evidence that the growing cycle of the grapevines has changed substantially.

The results, overall, provide good short-term news that consumers are not "tasting" climate change in Napa Valley wines. It reinforces the firmly held belief among growers and winemakers that the taste profile of Napa Valley's wines is driven by its place of origin, as well as by the solid direction of the in-field practices related to viticulture (clonal and rootstock selection, canopy management, irrigation, crop load and hang time, among others) along with stylistic preferences in winemaking.

The Napa Valley-specific climate study began in 2006 when the NVV board of directors created a climate study task force of vintner members who had both interest and knowledge of the issue. The task force identified Dr. Dan Cayan of Scripps Institution of Oceanography at UC San Diego, one of the most experienced climate scientists in the state, to lead a research team. A key member the team was Dr. Kimberly Nicholas, a North Coast native, who at the time was in the midst of her doctoral degree program at Stanford University, studying the potential effects of climate change on high-quality winegrowing in Napa and Sonoma. With this team in place, and working with the vintners and growers in Napa Valley, a set of criteria was designed to collect and investigate as much historical, in-field data as possible to determine climate patterns and trends specific to the Napa Valley appellation.