The impact of weather on wine quality has been well documented for thousands of years. However, not until recently have economists approached this phenomenon quantitatively. As documented by Orley Ashenfelter (see our article about Judging Bordeaux Vintages: Intuition and Super Crunching), warm and dry weather in the Bordeaux region can produce a wine that is substantially more expensive than a wine from a wet and cold vintage. In regions that are already at the temperature optimum the relationship may be the other way 'round.

At least on paper, the step from weather to climate is a small one. For the last 6-8 years, there has been is a growing body of literature on the relationship between viticulture and global warming. Some of these papers assess the economic impact on wine and vineyard prices. (see, for instance, "Go North Young Grapes" in SLATE) others evaluate the impact of global warming on wine quality (e.g., this article in the journal 'Climatic Change').

Last year, Greg Jones, a climatologist at Southern Oregon University, and collaborators published a widely noticed paper in the Proceedings of the National Academy of Science PNAS (click here for the paper). They predict that increases in the frequency of extreme hot days (>35°C) during the growing season will eliminate winegrape production in many areas of the United States, including parts of the Napa Valley. Grape and wine production will likely be restricted to a narrow West Coast region and the Northwest and Northeast, areas currently facing challenges related to excess moisture.

At the Annual Conference of the America Association of Wine Economists in Portland (Aug 14-16) Greg Jones (together with Gregory Goodrich of Western Kentucky University) will present a paper that evaluates the causes of longer growing seasons, less year-to-year variability and fewer frost days on the U.S. West Coast. Is really global warming the sole driving force or are these effects caused by periodical phenomena such as El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO)?

The result: The currently experienced longer growing seasons and small year-to-year variability is the result of a combined effect of both a general temperature increases and periodical effects. When the PDO returns to a multi-decadal cold phase, wine growers across the western USA will likely experience greater variability in wine quality.

Here is their abstract:

Trends in climate variables important to winegrape production in the western United States include fewer frost days, longer growing seasons, and higher spring and growing season temperatures. These trends have been related to a steady increase in wine quality and a decrease in year-to-year variability. While the trends in climate have been linked to increasing sea surface temperatures in the eastern Pacific, it is unknown whether this is caused by climate change or may be part of natural oscillations in the Pacific. In this study, 15 climate variables important to winegrape production were analyzed for 10 wine regions over the western USA. The variables were stratified by phases of the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), both separately and then in combination (modulation effect) to determine if there are any significant differences between teleconnections. ‘Wine Spectator’ vintage ratings for Cabernet Sauvignon wines from the Napa Valley were also stratified by the same method, and multivariate statistics were used to determine which variables are most important to wine quality.

ENSO phase by itself was not found to be important to either climate variability in wine regions in the western USA or wine quality in Napa Valley, but the cold phase of the PDO was found to be associated with increased spring frosts and a shorter growing season that results in lower ratings relative to warm PDO. The combination of neutral ENSO conditions during the cold phase of the PDO was nearly always associated with low quality wine in the Napa Valley, which is a function of cold springs with increased frost risk, cool growing seasons, and ripening period rainfall (cold PDO), and above-average bloom and summer rainfall (neutral ENSO). Although climate trends toward generally warmer growing seasons with less frost risk have occurred, this research highlights the impact of climate variability on wine quality where, should the PDO return to a multi-decadal cold phase, wine growers in the Napa Valley and across the western USA will likely experience greater variability in wine quality.

Evidence of these conditions have occurred during the 2007-08 winter and into spring 2008 where a much colder and wetter than normal winter in the PNW and northern California has been seen. A lingering moderate to strong La Niña event in the tropical Pacific has been boosted by the influence of the larger cold phase of the PDO in the North Pacific. The result has been higher snow packs throughout most of Washington, Oregon, and northern California and anywhere from a one to four week delayed bud break or bloom (grapevines and orchard fruit, respectively) followed by an increased frequency of frost. During the third week of April 2008, some of coldest conditions since the mid 1970s caused widespread damage to everything from cherries, peaches, pears, apples, to winegrapes. As of April 24^th winegrape growers in Sonoma and Napa had already estimated a 10-25% yield loss due to frost and are still waiting for spring to come.