Roger C. Bohmrich, Master of Wine
February 2019

To state the obvious, alcohol is a key constituent of wine, yet one largely taken for granted by casual imbibers and oenophiles alike. It is unlikely that many consumers have noticed the gradually rising alcohol levels in recent years. According to tests conducted by the Liquor Control Board of Ontario, Canada, the average alcohol content of wines from all countries increased approximately one percentage point from 1992 to 2009 (Alston, 2011, 2016). Even a seemingly insignificant change in alcoholic strength can alter the overall sensory profile – for better or worse. Lofty ratings suggest many critics prefer richer and more generous wines, especially reds. The rise in alcohol has, however, provoked debate among academics and journalists.

It is important to remember that, until recently, complete ripeness was achieved only intermittently in the famous regions of Northern Europe where cool, wet weather has been prevalent. In these marginal vineyards, weak, green vintages have been all too frequent. Atypically hot, dry growing seasons – those producing grapes with more generous sugar levels – created the reputation of Bordeaux and Burgundy. To compensate for the deficiencies of nature, Jean-Antoine Chaptal, a Minister under Napoleon, promoted a simple solution in a treatise published in 1801: sweeten the must (unfermented juice). While the idea of enrichment had earlier origins, it was popularized as “chaptalization” and eventually enshrined in French wine legislation of the 1930s. In turn, the process was adopted by the European Union, which set rules for each of its three main geographic zones. The latest revision (EC, 2008) permits a boost of 1.5% to 3.0% in alcohol by volume (ABV). The appellation rules stipulate both minimum natural sugar and alcohol levels (Fig. 1) as well as maximum alcohol content after chaptalization in those northerly vineyards where enrichment is permitted. By raising alcohol in small increments, wines gain in substance and the sweetening effect of ethanol offsets high acidity, all without compromising innate flavor – at least in theory.

Figure 1. Requirements for selected French appellations of origin governing minimum natural sugar and alcohol content before enrichment.

In warm, sun-filled Mediterranean regions with comparatively low rainfall when the vine is in leaf – and elsewhere such favorable conditions prevail – sugar ripeness is easily attained and chaptalization is not allowed. In the celebrated regions of northern Europe, counterintuitively, under-ripe fruit was a persistent concern for much of history. In just a few years, circumstances have changed dramatically. Explaining this about-face is not as straightforward as one might hope, in part because a single explanation does not fit all regions. A changing climate – a warming trend – has been linked to enhanced ripeness. The dictum that elevated temperatures stimulate sugar accumulation is fundamentally valid but does not tell the full story. Some scholars argue that lower precipitation correlates more closely with riper grapes (and vintage quality) in rainy regions such as Bordeaux (Van Leeuwen, 2016). Others point to human intervention, saying inflated fruit sugars may also reflect how vineyards are set up and managed and the choices made by winemakers.

There is certainly convincing evidence that global temperatures have been rising at an accelerated rate over the past five decades in tandem with increasing CO2 levels (Fig. 2).

Figure 2. Top graph: Annual mean temperature of the earth from 1880 to 2016 (°C). Bottom graph: Direct measurement of atmospheric carbon dioxide from 1958 to 2017 (parts per million). Sources: Statista, NASA, NOAA.

While the aggregate global pattern has been conclusively documented (IPCC, 2007, 2014), it is distributed unevenly across the globe. Jones compiled data for Bordeaux, showing an upward trend of 1.8°C from 1950 to 1999 (Fig. 3).

Figure 3. Graph of growing season average temperatures (April-October) for Bordeaux over a 50-year period. The dots plot each year and the straight solid line represents the linear trend (Jones et al., 2005).

He found somewhat similar trends of +1.5° C in the Rhine Valley and +1.2° C in Napa. The rate was lower in Champagne, +0.54°C, and in the Hunter Valley of Australia, +0.77°C (Jones et al., 2005). Jones observed that average temperatures in the growing season in the world’s “high-quality wine producing regions” increased by 1.26°C from 1950 to 1999. Based on vintage ratings, warmer growing seasons have been largely positive for many European regions (up to now) whereas very few New World areas show the same benefits.

Scientists have determined that rising temperatures are brought about by increases in CO2 and other greenhouse gases (IPCC, 2007, 2014). Temperature is the parameter with the greatest impact on the grapevine and is the driver of phenology; that is, the timing of bud break, flowering and onset of ripening. The production of carbohydrates may be enhanced by rising temperatures, but the vine has its own shut-off valve: when heat stress reaches a certain point, leaf stomata (pores) close and photosynthesis stops.

Independently of temperature, CO2 is central to the physiology and chemistry of plants; yet species do not respond identically. The general rule is that, through photosynthesis, plants consume carbon dioxide. With greater availability of CO2, growth is stimulated and the plant produces more carbohydrates. This “fertilizer” effect has been found to affect a large number of species (Taub, 2010). Could this account for higher grape sugars in those regions where temperatures have not changed meaningfully? Lisa Perrotti-Brown MW, Editor-in-Chief of The Wine Advocate, suggests this is an avenue to investigate (Perrotti-Brown, 2017).

Researchers are able to assess the direct impacts of CO2 with free- air carbon dioxide enrichment – FACE for short – an experiment in which CO2 is blown onto plants growing in a real-world setting such as a field in the open air. This technique has been employed for many species, but studies of grapevines have been few in number. An initial investigation focusing on Sangiovese was carried out in Italy in 1996-1997 (Bindi et al., 2001). In this inquiry, increased CO2 stimulated grape sugar content, but only up to the middle of the ripening period.

Another FACE study of considerable interest was conducted over three years (20014-2016) at Geisenheim in the Rheingau (Germany). The unusual equipment installed in the vineyard can be seen in the photographs below (Fig. 4). Riesling and Cabernet Sauvignon vines were fumigated with CO2 continuously throughout the year to gauge the impact at all stages of growth.

Figure 4. Left: MONI measuring head in the Geisenheim FACE facility. Right: Ring of CO2 outlets. Picture courtesy of S. Tittmann and Y. Wohlfahrt, Hochschule Geisenheim.

Elevated CO2 in the Geisenheim trial promoted higher yields but did not alter the sugar content of the berries at harvest (Wohlfarht et al., 2018). Other inquiries in Portugal and Australia similarly do not support the theory that elevated CO2 results in grapes with higher sugars (Goode, 2017).

California illustrates the complexities of the ripeness question. A widely read paper in the Journal of Wine Economics (Alston et al., 2011) reported that average California grape sugars increased from 21.4 degrees Brix in 1980 to 23.3 degrees Brix in 2008. Converting Brix into alcohol (using a multiplier of 0.55), the strength of all California wines grew from 11.8% to 12.8% over this period. The authors’ attention-grabbing conclusion was that “warming average temperatures in the growing season did not contribute substantially or significantly to the increase in sugar content.” Rather, they place the emphasis on human actions: higher- density plantings, rootstocks, and, especially, later harvesting. They suggest “sugar content is being managed in the vineyard”: vintners are deliberately opting for longer “hang times.” High scores from wine writers such as Robert Parker and James Laube, the authors suggest, motivated winemakers to produce “wines with strong, intense, riper fruit flavors.” This may well be a plausible theory with regard to the ultra-premium segment. But why then would California wineries rely on technologies such as the spinning cone and reverse osmosis to strip alcohol from finished wines, or dilute sugar-rich musts with water to the same end? It might be to insure lower-priced wines avoid the higher federal excise tax rate at 14% ABV and above. It is tempting to say that carbon dioxide is the missing element in this analysis since sugar levels seem to be rising in conjunction with atmospheric concentrations (Fig. 5). As we have seen, however, science has not yet substantiated the direct impact of CO2 on higher berry sugars.

Figure 5. Napa Valley Brix at harvest for Cabernet Sauvignon, corresponding alcohol levels and CO2. Degrees Brix measure the dissolved solids in grape juice to approximate sugar content.

While many factors influence grape ripening, hotter regions or localities tend to produce wines with higher alcohol relative to cooler ones, regardless of continent and country. The fundamental challenge, especially in already warm-to-hot and dry growing seasons, is that higher temperatures are accelerating sugar accumulation without doing the same for phenolic maturities. To obtain ripe, “sweet” tannins, winemakers may be obliged to delay harvesting; in so doing, they end up with more sugar, possibly as an unintended consequence. Clones selected to boost yields and sugars, combinations of grape varieties and rootstocks, and vine- training practices all might play a part – and can be manipulated as part of a solution. Logically, balanced ripening can only be achieved in the vineyard. The larger question is this: Are the most suitable varieties planted in areas most affected by climate change?

There is persuasive evidence that increasing ripeness as a global phenomenon seems to be driven mainly by rising temperatures, albeit with local variations. Higher concentrations of greenhouse gases are pushing up temperatures; but studies do not indicate CO2 independently boosts berry sugar. Viticultural practices are a contributing factor depending on locale. In the short term, more heat and less rain in Europe’s cool, damp regions have resulted in an unprecedented number of critically acclaimed vintages and the elimination of green, acidic ones. Wines with more richness, fruit and, yes, another degree or so of alcohol can be delicious. Contrarily, a Cabernet Sauvignon, Syrah or Châteauneuf-du-Pape flirting with 16% ABV may be just too hot and potent for many palates. Whether enhanced ripening is seen as a bonus or liability thus depends on the region, wine style and taster.