Bacchus Revels in a Warmer World!

Roman agricultural writer Plinius once wrote, "Vitis amat colles" – the vine loves the hills. Most grape growers and winemakers know this to be a fact and have searched out perfect places in the world where topography, soil, and climate come together to produce high quality grapes and wine. Today, however, many grape growers and winemakers might change their tune to "Vitis amat global warming."

Global warming – popularly characterized by temperature increases, rising sea level, and melting ice caps accompanied by increased incidence of drought, flood, hurricanes and disease – is generally anticipated to have negative consequences for earth’s plant and animal systems. Such climate-related events are mostly blamed on increases in fossil fuel use and the consequent increase in greenhouse gas concentrations in the atmosphere. While such threats can and may be real under certain circumstances, many of the relationships and interactions in the earth/atmosphere system are still unknown or are not well defined.

Current research from the Napa and Sonoma regions of California – two of the world’s highest quality wine regions – indicates that the grapevine loves a warmer world. California produces 90% of all US wine and dominates the $16 billion a year domestic retail wine industry. Since the 1950s, California’s wine grape growers have seen dramatic increases in grape yield, crop value, and premium wine quality (along with decreased year-to-year variability in quality). Advances in viticulture practices such as irrigation, nutrition, trellising, and pest/disease control accompanied by greater experience in winemaking certainly have contributed to their success. However, despite such advances, grape growers generally believe climate plays the most significant role in determining the overall quality and style of wine from a given region. Climate variability also controls year-to-year variations in the quantity and quality of vintages, they believe.

Depending on the magnitude and seasonality of climate changes, their agricultural impact can be either positive or negative. For example, warmer winter and spring temperatures would reduce frost damage and increase the length of the growing season in northern latitudes. Research by Nemani et al. revealed this effect for the Napa and Sonoma grape growing areas where climatic changes were documented to be diurnally and seasonally asymmetric, meaning the greatest warming was at night and during spring.

As a consequence of this asymmetric warming, the range in diurnal temperature (the difference between daily maximum and minimum temperatures) declined by 1.9°C over 47 years. Although the average annual warming trend was a modest 1.1°C over the 47 years, there was a 71% decline in frost frequency (a 20-day reduction) and a 25% increase in the length of the frost-free growing season (a 65-day increase). If the current trend in frost frequency continues, Napa/Sonoma will become a frost-free climate in another decade or two. For grape growers, longer growing seasons allow for greater flexibility in scheduling various viticulture operations such as pruning and harvest.

The Napa and Sonoma regions have enjoyed a 0.22 point per year increase in their wine quality ratings between 1963 and 1996, as compiled by Sotheby’s (Figure 1). This is given on a 100-point scale, averaged for each vintage. While wine quality ratings are a subjective measure of the vintages that are compared, they can have a major impact upon wine value. For example, analysis of Wine Spectator data showed that for the 1995 vintage, Napa wines had an average rating increase of 10 points over the previous year. That translated into a 220 percent price increase per bottle.

Among the Napa and Sonoma climate variables that were studied (daily and monthly temperature, precipitation, vapor pressure deficits, and growing degree days, among others), the decline in frosts was significantly correlated with the increase in wine ratings. This is depicted in Figure 2 where R² = 0.41.

A possible explanation for this relationship could be that frost damages buds on the vine, delaying subsequent plant physiology and ultimately leading to uneven maturity and poor wine quality. It should also be noted that grape yields grew 34 percent (from 7.3 tons per hectare to 9.8 tons per hectare) during the same time period (Figure 1) and found to be significantly influenced by minimum temperatures in spring and decreases in summer vapor pressure deficits (R² = 0.56, not shown). Consequently, increasing quality and quantity raised the value of the grape crop from $640 per hectare in 1963 to $19,600/hectare in the Napa Valley in 1996.

Findings similar to those of the Napa and Sonoma regions have been documented in Bordeaux, France – a region long considered to be one of the (if not the) finest wine regions of the world. Research by Jones and Davis reveals that the highly-prize, high-priced Bordeaux Grand Cru Classés reds have experienced increased vintage ratings (with decreased year-to-year variability) over the last three decades. These increases in vintage quality have been related to better grape composition levels (mainly sugar and acid levels which largely determine wine quality) coupled with earlier budding, flowering, and harvest and, ultimately, warmer and longer growing seasons in the Bordeaux region.

Other wine-producing regions in Australia, South Africa, and Chile may be reaping benefits from minimum temperature warming trends of similar or higher magnitude. Easterling et al., in two separate analyses on climatic extremes, have documented that the most significant temperature changes in many of these grape-growing regions are a rise in minimum temperatures and fewer frost days.

Unfortunately, along with the positive effects from recent climatic changes, there could be future negative impacts for the wine industry. Although Napa and Sonoma humidity levels currently are optimal, a trend toward increasing humidity and air temperature suggests that, in the future, the risk of fungal and vector-borne disease outbreaks may increase. Pierce's disease, a fatal bacterial (Xylella fastidiosa) disease transmitted by sharpshooter beetles (of the Cicadellidae family) and that apparently is limited by frost occurrence, is increasing in Napa and Sonoma. Climatic change may therefore require increased investment in application of pesticide, disease-resistant rootstocks, or more advanced bio-control mechanisms.

Beyond these mostly beneficial changes in temperature and frost occurrence, Big Media presumes rising greenhouse gas concentrations to be detrimental. As a greenhouse gas by-product of fossil fuel combustion, carbon dioxide is tagged as the biggest culprit. However, if one steps back and examines the issue with an eye toward plant systems, CO₂ generally is found to be the limiting factor to plant growth. The question then becomes how might increased levels of CO₂ affect grapevine growth and wine quality?

An article in Climate Research reported on the relationship of climate and grapevine yield. Bindi et al. studied the effects of increased CO₂ levels and associated changes in climate on Cabernet Sauvignon and Sangiovese grapes in Northern Italy. Using field data from 1992-1994 and a model of grapevine growth and yield, they found that doubled concentrations of CO₂ in the atmosphere result in a 36 percent increase in yield. While increased temperature reduced yield under some future climate scenarios, CO₂ increases clearly outweighed the temperature effects. Additionally, with land prices skyrocketing in the best grape growing regions and with land availability declining, new viticulturists are seeking other regions where they can produce high quality grapes and wine. Today’s site selection procedures increasingly include an assessment of the ability of the site to accumulate CO₂. This is accomplished by finding optimum slopes that facilitate CO₂ accumulation (pooling), thereby potentially increasing water-use efficiency and plant growth.

Myriad other studies have documented this CO₂ fertilization effect for many other plant species. While we can not be certain about the precise magnitude of the impact that climate change may have on future yields, it appears that more CO₂ in the atmosphere is not all bad for grapevines. Salute!

References:

Nemani, R. R., White, M. A., Cayan, D. R., Jones, G. V., Running, S. W., and J. C. Coughlan, "Asymmetric climatic warming improves California vintages." Proceedings of the International Conference of Biometeorology, Sydney, Australia, November 1999. (Also submitted to Climate Research).

Jones, G. V. and Davis, R. E., "Climate Influences on Grapevine Phenology, Grape Composition, and Wine Production and Quality for Bordeaux, France," American Journal of Viticulture and Enology, (in press 1/2000).

Bindi, M., et al. 1996, "Modeling the Impact of Future Climate Scenarios on Yield and Variability of Grapevine." Climate Research, 7, 213-224.

Easterling, D.R., et al., 1997. "Maximum and minimum temperature trends for the globe." Science, 277, 364-367.

Easterling, D.R., et al., 2000. "Observed variability and trends in extreme climate events: a brief review." Bulletin of the American Meteorological Society, 81(3), 417-425.

Figure 1: Trends in Napa and Sonoma Valley wine quality and Napa Valley yields.

Figure 2: Relationship between spring frosts and vintage ratings for Napa and Sonoma Valleys. Frequency of spring frosts was the only significant climate variable describing wine quality. The major exception to the relationship represents the 1964 vintage, which had a high number of frosts, but was followed by a miraculous climatic turn-around in the late stages of growth.

Gregory V. Jones, Ph.D., is an Assistant Professor of Geography at Southern Oregon University. His BA in Environmental Sciences (1993) and his Ph.D. (1997) with a focus in Atmospheric Sciences were awarded by the University of Virginia. Dr. Jones’ dissertation was "A Synoptic Climatological Assessment of Viticultural Phenology" (Bordeaux, France). His research interests include the effects of climate variability and change on agriculture (grapes, pears, and other orchard fruits), climatic controls on wildland fires and pollution events in the Inter-Mountain West, the economics of grape growing and wine making, and atmospheric circulation variability and control on regional climates.