Something to wine about: Climate change and viticulture

In the United States, the labeling of wine is overseen by the Department of the Treasury, Alcohol, and Tobacco Tax and Trade Bureau (TTB), which ensures that bottle labels accurately relay to consumers where their wine is coming from. The TTB also regulates the establishment of AVAs; viticulture areas are created based on the geographical and climatic distinctiveness of the proposed region (TTB.gov). While AVAs and stringent regulation of wine labels ensures that winemakers cannot falsely mislabel where their grapes are grown or wines are processed, appellation has had a major impact on the wine industry at all levels. Wineries are able to market their wines as more desirable based on specific AVA designation or tout the uniqueness of the particular terroir of their grapes (Elliott-Fisk 2012). Likewise, consumers are able to recognize and reference the regions of their favorite wines and utilize labels as an educational tool for future consumption (Hoemmen et al., 2013)

Different aspects of climate change will have profound impacts on different parts of the grape growing and wine production process. For this project, I will examine the potential threat of two different aspects of climate change - increasing temperatures and changing precipitation patterns - in order to determine the areas within Napa Valley at greatest risk under current models of climate change. I will conclude by introducing some of the practices viticulturalists may incorporate in order to mitigate these negative impacts.

The Napa Valley region is considered to have a Mediterranean climate, found in just 2% of the world. This unique climate comes courtesy of the two mountain ranges that border the region - the Vaca Range in the east and the Mayacamas Mountains in the west - that help funnel in cooler winds from the Pacific Ocean.

By 2050, however, global warming is likely to greatly impact the average temperature of this region.

From this, we can see a distinct pattern in areas that are predicted to warm the most and those that may be better protected from the impacts of climate change. The sub-AVAs in the darkest blue - a predicted change of under .6°C - are within the base of the valley between the two mountain ranges, at around 100 ft - 200 ft elevation. However, sub-AVAs with notable hotspots of greatly increased predicted average temperature - up to nearly 1.9°C increase - are all much higher elevations, ranging from 1900 ft - 2500 ft above sea level. Some studies have shown that more pronounced impacts of climate change on grapevine phenology are seen at higher elevation (Alikadic et al., 2019), indicating these sub-AVAs may be at greatest long-term risk for the quality of their wine production. Warmer annual temperatures will put added stress on the grape berries as they grow, due to increased evapotranspiration, resulting in smaller or worse-quality mature fruits (van Leeuwen & Darriet 2016).

When looking at maximum temperatures experienced in the region, predicted change is even greater within Napa Valley, though following a different geographic pattern.

Maximum temperatures are predicted to rise by a minimum of 1.6°C within this region, with much of the area predicted to experience 5.9°C - 6.8°C of warming during the hottest time of the year by 2050. Unlike the average temperature predictions, the valley doesn't provide much thermal protection for maximum temperature changes, exempting the most southern regions. While this data doesn't account for the amount of time that may be spent under this temperature increase, with so much of the region set to experience temperatures more than 5°C warmer than current, even a week under these conditions could have profound impacts; optimal growing temperatures for most grape cultivars are between 25°C - 28°C (Keller 2010), and prolonged exposure to heat adds stress to grapevines that can decrease future yields and leave berries exposed to warmer temperatures that accelerate the ripening process (Monteverde & De Sales 2020).

Minimum yearly temperatures are also predicted to rise in Napa Valley by 2050.

With minimum predicted temperature increases of 2.7°C across the county, the region is likely to be pushed out of the below-freezing range. Extended warm temperatures in the spring push the grapevines out of dormancy and initiate premature bud break (Ashenfelter & Storchmann 2014). However, this leaves the yield susceptible to major losses if cold temperatures and frost return and can compromise the establishment of cold tolerance in the grapevines during autumn, winter, and spring (Keller 2010). Accelerating the growing season may not be beneficial in the long run, impacting the quality of grapes grown at different temperatures than desired for specific cultivars. Shifting the grape harvest by even a month earlier in the year - an estimate predicted under models of climate change that is likely supported here - could push the quality of grapes from the 'optimal' timing to 'marginal' and 'impaired', depending on location (Cahill et al., 2007).

Not only will overall growth of grapevines be impacted by warming temperatures, but the actual quality of grapes as well.

Current precipitation patterns are susceptible to change under the projected conditions of climate change.

And by 2050, average rainfall in Napa Valley will see both increases and decreases across sub-AVAs.

While the northern regions of Napa will see an increase in rainfall up to 20.2 mm in a year, southern regions will only see increases of closer to 2 mm of rainfall. The high elevation sub-AVA of Coombsville, with the dark blue coloration in the east, is the only area predicted to decrease in rainfall, seeing anywhere from 2.2 mm - 4.1 mm less rainfall annually than present. It is important to note that some of these regions with predicted increases in rainfall over 10 mm are also some of the areas currently experiencing the least amount of rainfall; while the northern mountain regions already experience high levels of rainfall, the valley contains some of the lowest current levels of rainfall. An increase in rainfall by up to one-third current values could have vast impacts on wine production within these areas. As mentioned earlier, depending on when in the season this additional rainfall occurs, there is a risk of increased rot and loss of yield (Ashenfelter & Storchmann 2014). Additionally, in red wines accostumed to growing in drier climates, dry year vintages are routinely rated as higher quality than wet year vintages, due to factors like more developed grape skin phenols (van Leeuwen & Darriet 2016). While it may seem like overall projections of greater annual rainfall would be beneficial, in the careful balance of viticulture, conditions outside of the norm can lead to decreased production value and quality.

The amount of precipitation that will be seen during the rainiest months are also predicted to increase, which may have a serious impact on potential flooding.

Again, we see a similar pattern as was demonstrated with the overall rainfall; the northern regions of Napa Valley are likely to experience increases in rainfall in 2050. Increases in rainfall are likely to exacerbate flooding during at-risk seasons. With over 80% of Napa's grapevines grown on hillsides - a practice that already degrades topsoil stability and infiltration ability - prolonged or more intense wet seasons bring the serious risk for flooding and washout via erosion around the base of plants (Battany & Grismer 2000). Additionally, these increases in rainfall demonstrated here do not account for the predicted increases in periods of drought for the region (Pathak et al., 2018). Fluctuations of extreme weather - in this instance, between flooding and droughts - would have major impacts on the ability to produce high quality wines.

Based on projected climate change and the above data, we can better understand which sub-AVAs - and therefore what types of wine - are most threatened in the face of climate change. Below are the top four at-risk sub-AVAs in each category, including the predicted changes in variables by 2050, the number of wineries within the region, and the most common grape cultivars grown in the region.

While some global wine growing regions, particularly colder ones, may actually fare better under projected changing climate, others are at serious risk, including Napa Valley. Progressive warming will lead to shifts in optimal weather for wine grape growing; in cooler climates, this increase in temperature will actually expand viable growing range, particularly in European regions (Moriondo et al., 2013). However, warmer climates, specifically Mediterranean climates, are predicted to face the greatest shift in suitability, often completely out of the current region, greatly decreasing optimal growing conditions and viable acreage (Moriondo et al., 2013).

Above average year-round temperatures are the greatest threat to the viticultural industry in Napa. Wine grape phenology is closely determined by seasonal temperature shifts, and oftentimes subtle differences in the temperatures and conditions surrounding grape berry lifestage has a large impact on the final flavor, appearance, and ultimately value of that vintage. While warmer temperatures earlier may help extend the growing season in the short term, cooler weather is essential to the growth of grapes; cooler temperatures ensure that the vines enter dormancy, which allows for more uniform, regular budburst and maturation later in the growing stages (Ashenfelter & Storchmann 2014). Longer exposure to higher temperatures year-round leads to chemical changes in the individual grape berry, as evapotranspiration from the fruit raises the sugar and alcohol levels of the final product, which can result in poor or unpredictable fermentation (de Orduna 2010). Overall, the risk of experiencing greatly increased temperatures in the coming decades is likely to indicate a decreased suitability for growing certain wine grape cultivars and lower quality final products.

Cabernet sauvignon makes up the largest percentage of wine coming out of Napa Valley, and it’s grapes produce some of the most highly-coveted wine; unfortunately, cab wines don’t fare well under predicted climate conditions (Lopez 2020). The higher temperatures many vineyards are likely to experience could result in decreased thiol groups (de Orduna 2010), large shifts in pH and titratable acidity (Mozell & Thach 2014), and negative impacts to berry color (Bergqvist et al., 2001). Wineries that rely heavily on the production of cabernet sauvignon may face greater threats to their future production ability than other wineries relying on more tolerant grape cultivars.

However, the future of the wine industry in Napa Valley is far from being hopeless. With increased climate pressures come increased innovations for improving the practices of viticulture and mitigating for the negative impacts of climate change.

While climate change threatens viticulture in Napa Valley in more than one way, the employment of different mitigation techniques now may help vineyards sustain their production levels and keep one of the most valuable global wine regions from running dry.