What is in this article?:
- 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.
- In 2006, a researcher garnered national media attention by predicting that Napa Valley would soon become too warm to grow fine wine grapes.
- However, the experience of wine grape growers has been contrary to the notion that Napa Valley has warmed substantially.
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.