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.

Winemakers and weather

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.

Artificial warm bias

Shorter records collected by Napa weather observers indicate that these long-term stations are registering an artificial warm bias which has likely increased over the last several decades. The study recommends that the Napa Valley farming community should formally assess the adequacy of its current climate observations and establish a protocol to maintain a high-quality, long-term climate monitoring network.

"I am proud of the leadership taken by the Napa Valley Vintners in this climate study. We have strong benchmarks in place that will further allow us to track what changes may occur in our unique climate--really specific to the Napa Valley. Though we are just 4 percent of California's wine grape harvest, we account for 34 percent of the value of the California wine industry on the US economy. It's in all of our best interests to ensure a long and healthy future in fine wine from the Napa Valley," said Kathleen Heitz Myers, president of the NVV board of directors.

Additional research beyond this study continues on a number of fronts, looking at what in-field practices could be employed should climate change take the form of regional warming, such as how canopies and cover crops are managed as easy, short-term solutions. Napa Valley growers and vintners are raising awareness of what can be done locally while thinking globally with programs like Napa Green Certified Land and Winery, which are the most comprehensive green initiatives in the wine industry and that have the well-earned reputation for going above and beyond when it comes to environmental best practices.

The Napa Valley Vintners is the non-profit trade association responsible for promoting and protecting the Napa Valley appellation as the premier winegrowing region. From seven founding members in 1944, today the association represents 400 Napa Valley wineries and collectively is a leader in the world-wide wine industry. To learn more about our region and its legendary American wines, visit www.napavintners.com

Climate report

Climate and Phenology in Napa Valley:
 A Compilation and Analysis of Historical Data

Daniel R. Cayan, Dr. Kimberly Nicholas, Mary Tyree, Dr. Michael Dettinger

Executive Summary

In response to, and sponsored by the Napa Valley Vintners (NVV), this study was designed to evaluate the climate of the Napa Valley (NV) and explore links between climate and wine grape phenology and composition. Specific objectives of this study are to a) describe the spatial and temporal structure of climate in NV; b) explore linkages between climate and phenology/harvest; and c) evaluate historical trends in temperature.

NV shares traits of both coastal and interior climates. The southern portion of NV is more coast-like while the central northeastern side of NV is more interior-like. Elevation also plays a strong role, e.g., heavier precipitation and lower summer daytime temperatures generally occur in higher elevations relative to the valley floor.

Approximately 30 private weather stations distributed throughout NV allow inspection of fine-scale temperature variability from north to south, and along a transect of changing elevations across the valley from east to west. These records vary in length, but mostly cover two to 15 years from the last two decades. Generally, all stations display similar variations (i.e., warm periods are relatively warm at all stations). There is marked variability by location (i.e., southerly and higher-elevation sites are cooler in summer months, containing lower daytime maximum temperatures and higher nighttime minimum temperatures than more inland and valley floor sites. The daily and seasonal temperature ranges and temperature extremes observed in NV are greater than cool coastal climates, but smaller than warmer inland climates. During summer, cloud cover in NV shares patterns more similar to the coast than to the interior Central Valley. Cloud cover in the southern part of the NV is greater in July and August than it is in April, May, and June. Days with higher morning cloud cover in NV generally do not attain afternoon temperatures as high as those with lower morning cloud cover, even though cloudiness may dissipate by early afternoon.

In addition to local influences, climate in NV has a very clear, significant association with large-scale variations and trends. Large-scale atmospheric circulation plays a strong role in setting up anomalously warm and anomalously cool days in NV. Anomalous Pacific Ocean temperature patterns are linked quite strongly to NV temperatures during winter and early spring months, but not so much during summer months.

COOP stations and stable temps

Temperature records from the longer-term stations in the region indicate that the NV has experienced warming over the last several decades. At several of the stations, the warming that is detected is stronger during the nighttime than in the daytime, and it has occurred preferentially during the year--primarily during January through August. Relatively high rates of warming in NV are found in six to nine decades of temperature records within the NV from the Napa State Hospital and St. Helena cooperative observer (COOP) stations. Similar warming trends are found at other cooperative stations surrounding NV.

The trends of minimum and maximum temperature at the COOP (COOP) stations, which amount to a warming of mean temperature that is approximately .03 F/yr since 1931, are essentially the same trends that have been reported in previous studies of regional temperature in the Napa region, including the study by Jones and Goodrich (2008). However, COOP stations have undergone several location changes and have had instrument changes. The present installations and locations do not give confidence for stable, un-altered temperature records. The Napa State Hospital installation is very close to a building and an air conditioner outlet. St Helena is mounted on the roof of a building. That there may be an excessively high warming in the Napa and St Helena COOP records is suggested by comparisons between the temperatures from the COOP stations with those from other stations in the region. These include the temperature recorded at and above the earth’s surface by the Oakland radiosonde (upper air sounding) record. California Irrigation Management Information System (CIMIS) weather records and several sets of vineyard temperature records mimic the variability shown by the longer records.

The U.S. Historical Climate Network (HCN) cooperative station temperature record from the National Climatic Data Center provides a record that is adjusted, in attempt to eliminate spurious trends. The adjusted trends in maximum and minimum temperature are lower than the trends from the COOP data, however this change is problematic because of the lack of a long stable record in the immediate region that could be used as a reference series. Petaluma, which appears to be the record having minimal amounts of adjustment, has itself gone through moves and is in a dubious site as it is also close to a structure. Thus it is possible that even adjusted Napa State Hospital COOP trend may still be affected by some, unknown amount of artificial trend, although the amount of that error could be by way of either too little or too much warming trend.

Nighttime warming trend

While the records that have several decades of record are not of sufficiently high quality to precisely determine the temperature trend in the non-developed portions of NV, the evidence suggests that the warming in most non-urban parts of NV over the last six to eight decades has been significantly less than the approximately +0.03 F/year trend in mean temperature that is contained in the unadjusted COOP stations from Napa State Hospital and St. Helena. Comparisons between the COOP stations and other temperature records from sites that are less affected by human alterations suggests that the amount of nighttime warming has been significantly less than the .05 F/yr that is derived from the raw COOP records, and that daytime temperature warming has been close to zero.

It is important to emphasize that while the trends from the COOP stations appear to be artificially affected and too much warming, there nonetheless has been a real warming trend. Overall, it appears the warming that has occurred in the Napa region has mostly occurred during nighttime hours, as exhibited by daily minimum temperatures. Warming of nighttime temperature exceeds that of daytime temperature, as indicated by daily maximum temperatures.

NV grape phenological stage timing and harvest characteristics are significantly influenced by antecedent weather and climate, with these antecedent influences in some cases being detected as early as early winter. A rule of thumb is that earlier phenological stages are linked to prior warm conditions. Correlations between phenological stages are relatively robust amongst three stages (bloom, véraison, and harvest), but weak for any of these stages in association with budburst.

Phenological dates are not very well-correlated with total annual growing degree-days (January-December), in the sense that higher degree days associates with earlier phenological timing. However, each phenological stage is strongly correlated with the accumulation of specific thresholds of degree-days (which may vary by variety). These could be used to predict the timing of development by variety.

Analysis of comprehensive county-wide crush reports from 1990 onward demonstrated that there is a strong trend of increasing Brix at harvest over time. The most dramatic increase was for Zinfandel, from close to 20 to above 26 over the 18-year record. Sauvignon blanc has seen the least change, an increase of about 1 degree Brix. Cabernet has increased from about 23 to 26 degrees.

There is also a trend across all varieties except Sauvignon blanc for decreased yields over time, most dramatically for Zinfandel (from close to 6 to about 2.5 tons/acre) and Merlot. Chardonnay yields have declined only slightly, holding steady near 4 tons/acre. Sauvignon blanc has the highest yields of all varieties studied, and has been increasing over time. There is some synchronicity in yields across varieties within a given vintage, presumably due to climatic conditions (for example, 1997 was a high yielding year for all varieties).

Vineyard management practices, used to achieve desired wine styles, have changed over the last 20 years, in ways that may affect phenological dates. For example, later pruning can lead to later budburst dates. Practices such as leaf thinning and cluster thinning may act to speed up ripening, while vineyard practices such as hedging may delay ripening. However, we lack access to management records that would allow examination of the statistical importance of these practices.

Nonetheless, climate plays a dominant role in setting phenological dates (for example, in initiating bud growth in the spring, and affecting fruitfulness and berry set, among other factors). As a general pattern, Brix increases with later harvest dates. However, recent harvests (last 8 years) have been very high in Brix, and not anomalously late. Generally, the trend has been toward later harvest dates, though it varies by variety (Pinot earlier; Cab Franc, Cab Sauvignon and Merlot about two to three weeks later over 30+ year period). Recent Brix increase is believed to be due to stylistic and winemaking preferences, but needs further investigation.

This data collection and synthesis was a substantial effort on the part of both contributors and analysts. Now that it has been completed, there is a valuable template for moving forward. The effort highlights the need to continue to, and improve monitoring, which will provide the means for an ongoing assessment of both climate and vine development in NV.