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Global warming and wine quality: are we close to the tipping point?

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Wine grapes are one of the most lucrative crops in the world and this value is founded heavily on traditional winegrowing regions established over hundreds of years. These regions are now experiencing marked changes in climate. People speculate that global warming could reshape the distribution of premium winegrowing regions, pushing regions to higher latitudes and elevations with cooler temperatures. A major redistribution of this kind would be catastrophic for numerous regional economies. Here we examine relationships between warming, fruit ripening, and wine quality in two renowned red wine regions; Napa Valley California, U.S.A. and Bordeaux, France. We show that both regions have warmed substantially over the past 60+ years and that until now this warming has contributed to increases in the average wine quality. However, ripening relationships revealed that we are reaching a plateau and raise concerns that we may be approaching a tipping point in traditional winegrowing regions.
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OENO One - Global warming and wine quality: are we
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close to the tipping point?
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S. Kaan Kurtural 1 and Gregory A. Gambetta 2*
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1 Department of Viticulture and Enology, University of California Davis 1 Shields Avenue Davis,
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CA 95616, USA
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2 EGFV, Bordeaux Sciences Agro, INRAE, Univ. Bordeaux, ISVV, Villenave d'Ornon, France
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* corresponding author: gregory.gambetta@agro-bordeau.fr
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Abstract
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Wine grapes are one of the most lucrative crops in the world and this value is founded heavily on
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traditional winegrowing regions established over hundreds of years. These regions are now
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experiencing marked changes in climate. People speculate that global warming could reshape the
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distribution of premium winegrowing regions, pushing regions to higher latitudes and elevations with
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cooler temperatures. A major redistribution of this kind would be catastrophic for numerous regional
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economies. Here we examine relationships between warming, fruit ripening, and wine quality in two
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renowned red wine regions; Napa Valley California, U.S.A. and Bordeaux, France. We show that
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both regions have warmed substantially over the past 60+ years and that until now this warming has
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contributed to increases in the average wine quality. However, ripening relationships revealed that
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we are reaching a plateau and raise concerns that we may be approaching a tipping point in traditional
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winegrowing regions.
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climate change, ripening, Bordeaux, Napa Valley
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Introduction
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Wine grape growing is one of the most lucrative and culturally important cropping systems in the
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world (Alston & Sambucci, 2019). The industry was founded upon specific region-climate-cultivar
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rapports that have a strong historical context, and there is now growing concern that global warming
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could reshape these regions, pushing them to higher latitudes and elevations in search of cooler
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temperatures (Hannah et al., 2013; Schultz & Jones, 2010). The logic underlying these predicted
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regional shifts relies on the observation that specific wine grape cultivars each have an optimum
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temperature range within which they can reliably produce high quality wines that have commercial
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acceptance (Keller, 2010). Thus, as regional climates warm outside of these optimum ranges wine
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quality would decrease. For a region to survive it would have to adapt, presumably by changing
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management strategies to maintain fruit and wine quality and/or changing cultivars to those better
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suited to the new, warmer climate norm (van Leeuwen et al., 2019). A major redistribution of
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winegrowing regions would be catastrophic for numerous regional economies. But even changing
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cultivars could be extremely disruptive since they bring about the distinctiveness of the wines that
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define a region’s identity.
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Optimum temperature ranges are delimited by a lower threshold necessary to ripen the fruit and an
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upper threshold that would lead to over-ripe (or even damaged) fruit (B.G. Coombe, 1987). What
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constitutes optimum ripeness and quality is somewhat subjective and depends on the particular wine
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style and target market. Despite the variability of style and taste, ripe fruit must include sufficient
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levels of sugar (that will be transformed into alcohol via fermentation) and secondary metabolites that
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contribute to the wine sensory profile (i.e. color, aromatics, flavor, mouthfeel, etc.). In fact, ripening
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targets are often defined by grape sugar concentration (Bryan G. Coombe et al., 1980). This is because
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the accumulation of sugar and numerous quality-related secondary metabolites are strongly correlated
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during ripening, although they may be uncoupled late in ripening (Martínez-Lüscher et al., 2017; Sun
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et al., 2017) and/or by particular environmental factors (Torres et al., 2021).
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Cultivar specific temperature ranges were suggested as primary criteria used to predict viticultural
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suitability under future climate change scenarios (Hannah et al., 2013; Jones et al., 2012; Morales-
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Castilla et al., 2020; Parker et al., 2020). Suitability studies have defined the current zeitgeist in the
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wine world: that climate change will determine regional “winners” where temperature increases
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above lower thresholds will allow for increased quality winegrape production, and “losers” where
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temperature increases above upper thresholds will make quality winegrape production impossible (or
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unsustainable). One example of a current winner is the United Kingdom which is rapidly expanding
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its winegrape production and reputability (Nesbitt et al., 2016). In well-established regions anecdotal
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evidence regarding the impact (positive or negative) of climate change on ripening and wine quality
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is mixed (Adelsheim et al., 2016), and how much warming is too much for established wine regions
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remains an open question, as the majority of the world’s vineyards are planted in warm to hot climate
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regions.
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Here we examined the relationships between warming, ripening, and wine quality in two of the
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world’s top red wine regions, Napa Valley in California, U.S.A. and Bordeaux, France. We showed
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that both regions have warmed substantially over the past 60+ years and that until now this warming
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has resulted in increases in the average wine quality. However, ripening relationships reveal that we
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are reaching a plateau and raise concerns that we may be approaching a tipping point.
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Materials and methods
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1. Historical climate and phenology data for Napa and Bordeaux
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Historical climate data was curated from the United States National Oceanic and Atmospheric
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Administration data base for Napa, CA U.S.A. and from the Meteo France data base for Bordeaux,
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France. The growing degree days were calculated using 10°C as base temperature in each year with
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no upper limit. The growing season was defined as the period from the 1st of April to through October
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30th. Heat spike days were defined as those days with a maximum temperature greater than 34°C.
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Phenology intervals between veraison and harvest were calculated for the years available using data
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from the Napa Valley Vintners (https://napavintners.com) Harvest Reports and from Chevet et al
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(2011) (Chevet et al., 2011) for Bordeaux, France.
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2. Sugar concentrations at harvest and wine quality ratings
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The harvest sugar concentrations were acquired from the California Grape Crush Report data base
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(www.cdfa.com) for Napa, CA U.S.A. and from Soyer and Chevet (2007) (Soyer & Chevet, 2007)
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for Bordeaux, France. The Napa, CA U.S.A. wine quality ratings were acquired from compiling
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Wine Spectator (www.winespectator.com) ratings for “Cabernet Sauvignon” and “Merlot” wines
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from California from over 1,900 wines and taking the mean rating per year. For Bordeaux, France
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wine quality ratings were acquired by taking the mean of three different vintage quality sources:
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Tastet and Lawton (presented in Soyer and Chevet, 2007), Le Guide Hachette des Vins 2020
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(“Bordeaux rogue” category), and Cellar Insider (www.thewinecellarinsider.com).
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3. Sugar and anthocyanin analysis in Napa and Sonoma
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Must sugar and skin anthocyanin concentrations in two vineyards in Napa and Sonoma County, CA
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U.S.A. were measured at four times in each year. Briefly, 75 berries from 48 plants at each vineyard,
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spaced 30 m × 30 m equidistantly were sampled at ~ 12, 16, 21 and 25% total soluble solids and
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processed immediately. Fifty berries were crushed by hand and filtered to obtain must. A digital
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refractometer (Palette PR-32, Atago Tokyo, Japan) was then used to measure total soluble solids
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asdescribed elsewhere (Yu et al., 2020). Twenty randomly chosen berries from the 75 berry set was
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used for anthocyanin measurement. The berry skins were peeled by hand using a scalpel and freeze-
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dried (Centrivap, Labconco, Kansas City, MO, USA). Dried skins were pulverized in a ball mill
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(MM400, Retsch, Mammelzen, Germany). A solution of MeOH:H2O:7 M HCl (70:29:1) was added
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to 50 mg of freeze dried, pulverized skin to quantify anthocyanins and allowed to extract overnight
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at 4 °C. Following extraction, samples were centrifuged at 14,000 rpm for 10 minutes and
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supernatants filtered (0.45 μm; VWR, Seattle, WA, USA) into HPLC vials and analyzed. An HPLC-
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DAD (1260 series, Agilent, Santa Clara, CA) equipped with a degasser, quarternary pump,
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thermostatted column compartment and an auto-injector connected to a diode array detector was used
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to analyze the anthocyanins. Mobile phase elution gradient, anthocyanin quantification followed
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previously established procedures (Martínez-Lüscher et al., 2019) with a reversed phase C18 column
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LiChrosphere® 100, 250 × 4 mm with a 5 μm particle size and a 4 mm guard column of the same
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material (Agilent Technologies, Santa Clara, CA, USA). The mobile phase flow rate was 0.5 mL min-
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1, and two mobile phases were used, which included solvent A = 5.5% aqueous formic acid; solvent
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B = 5.5% formic acid in acetonitrile. The HPLC flow gradient started with 91.5% A with 8.5% B,
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87% A with 13% B at 25 min, 82% A with 18% B at 35 min, 62% A with 38% B at 70 mins, 50% A
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with 50% B at 70.01 min, 30% A with 70% B at 75 min, 91.5% A with 8.5% B from 75.01 min to 91
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min. The column temperature was maintained at 25° C. Detection of anthocyanins was carried out by
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the diode array detector at 520 nm, respectively. A computer workstation with Agilent OpenLAB
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(Chemstation edition, version A.02.10) was used for chromatographic analysis.
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4. Data analysis, visualization, and statistics
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All data analysis, visualization, and statistics were performed using R software (http://www.R-
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project.org) and associated packages. Regression analyses for all variables were carried out either a
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linear or a second order polynomial model as specified in the presentation of the results.
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Results and discussion
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1. Napa, Bordeaux, and the 1980s global regime shift
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Several different temperature metrics from 1950-2020 in Napa and Bordeaux all support the same
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conclusion; temperatures have increased significantly (Figure 1, Supplemental Figure S1). Annual
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growing degree day (GDD) increases are strikingly similar between the two regions, and increased
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markedly starting in the early 1980s (Figure 1A). This coincides with the 1980s global regime shift
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and suggests global viticulture similarly reflected the major perturbations to earth systems observed
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during this period (Reid et al., 2016). Both regions have moved through several “Winkler Regions”
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(i.e. climatic zones delimited by GDD that define optimal cultivar/climate rapport (Amerine &
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Winkler, 1944)) since the late 1970s, moving from Region II, through Region III, and into Region IV
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in just under 30 years (Figure 1A). The same increases in temperature are observed in average
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growing season temperature (Supplemental Figure S1A). Observations of the average growing season
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maximum temperature, as well as the frequency of “heat spikes” (defined as a day with a max.
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temperature > 34°C), both showed significant increases as well (Figure 1B & C). Although there have
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been no apparent long-term changes in precipitation in the regions, in Bordeaux increasing maximum
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growing season temperature is weakly correlated with decreasing precipitation (Supplemental Figure
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S1). The speed of these temperature changes are alarming and raises a serious question; if similar
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regime shifts occur in the future can viticultural cropping systems adapt fast enough?
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Figure 1. Long-term evolution of different temperature metrics in Napa and Bordeaux.
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A) Increases in the cumulated annual growing degree days (GDD) over time in Napa and Bordeaux
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(P<0.0001). The local average and 95% confidence intervals are shown with the blue line and
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shading. Winkler regions are delimited with dashed horizontal lines and are labeled by Roman
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numerals from I to V corresponding from the coolest to warmest region designations. B) Historical
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changes in the average growing season (from April 1st until October 31st) maximum temperature.
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Linear regressions and corresponding 95% confidence intervals are shown (Napa; r2=0.32, P<0.0001,
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Bordeaux; r2=0.54, P<0.0001). C) Historical changes in the number of heat spike days, defined as a
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day with a max. temperature > 34°C, during the growing season. Linear regressions and
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corresponding 95% confidence intervals are shown (Napa; r2=0.07, P<0.05, Bordeaux; r2=0.33,
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P<0.0001).
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2. Increased temperature and riper fruit
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One of the most immediate concerns for the wine industry is that higher temperatures could negatively
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impact fruit composition and wine quality (Torres et al., 2020). Coinciding with the abrupt
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temperature increases in the 1980s sugar concentrations in Napa and Bordeaux began to increase
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significantly and these increases have continued (Figure 2A). Interpretations of these changes need
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to be made with caution since harvest is a human decision, and thus changes do not necessarily reflect
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a climate driven cause. If growers simply decided to harvest fruit later (and thus at higher sugar
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concentrations) then we would expect the interval between the onset of ripening (i.e. veraison) and
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harvest to increase. This has not been the case in Bordeaux, although in Napa there is a (non-
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significant) suggestion that harvest decisions may have played some role (Supplemental Figure S2).
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A longer growing season may afford growers the opportunity to decide exactly when to harvest which
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could be an advantage. The relationship between annual GDD and sugar concentration is highly
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significant (Figure 2B). This is consistent with the published literature supporting temperature’s
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causative role in the observed sugar increases (Pastore et al., 2017) resulting in part from accelerated
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ripening (Torres et al., 2020) and/or increased fruit dehydration (Martínez-Lüscher et al., 2020).
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Higher sugar concentrations at harvest mean riper fruit (Brillante et al., 2017) and higher alcohol
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wines of a different style and sensory profile. Thus far these different sensory profiles have resulted
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in greater consumer acceptance in the marketplace (Bindon et al., 2013).
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Figure 2. Evolution of average sugar concentrations at harvest in Napa and Bordeaux over
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time, and their relationships with temperature.
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A) Historical changes in average sugar concentrations at harvest in Napa (based on Cabernet
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Sauvignon) and Bordeaux (based on Medoc red blend alcohol levels from Chevet and Soyer, 2009).
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Polynomial regressions and corresponding 95% confidence intervals are shown (Napa; r2=0.81,
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P<0.0001, Bordeaux; r2=0.41, P<0.0001). B) The relationship between cumulated annual growing
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degree days (GDD) and average sugar concentrations at harvest. The horizontal black dashed lines
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are presented as a reference for the sugar levels above which there is a marked loss of berry color (i.e.
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anthocyanins) from Figure 4. Linear regressions and corresponding 95% confidence intervals are
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shown (Napa; r2=0.16, P<0.009, Bordeaux; r2=0.36, P<0.0001).
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3. Wine quality and the tipping point
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The observed increases in temperature in the two regions are positively correlated with the average
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vintage quality rating (Figure3) although the correlation is much weaker for Napa, as it was
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comparatively warmer than Bordeaux prior to 1980s global shift. High vintage ratings occurred across
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almost the full range of GDD (except for the coldest years), and higher temperatures significantly
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decreased the likelihood of a poorer vintage. Importantly, even the warmest vintages do not show any
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significant decrease in quality. A similar positive relationship exists between fruit sugar
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concentrations at harvest and quality (Figure 3C & D). The idea of a lower temperature threshold is
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clearly evidenced in Bordeaux where the coolest vintages mostly diverge for the relationship between
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sugar and quality (Figure 3D). It is important to stress that quality ratings are subjective and likely
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differ between the two regions, thus we are not concluding that there is strict causality between these
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factors. In addition, both regions have had some high quality vintages at lower sugar concentrations
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suggesting that high sugar tantamount to high quality. What is true is that to date increased warming
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and riper fruit have not been associated with any loss of wine quality. Instead, higher temperatures
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have made wine quality more consistently good, perhaps due in part to evolving consumers preference
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for more ripe flavors with subdued, vegetative aromas and tannin profiles (Bindon et al., 2014). But
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if we assume that warming will continue, where is the tipping point?
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Figure 3. The relationships between average vintage quality ratings, temperature, and fruit
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sugar concentrations at harvest in Napa (1961-2016) and Bordeaux (1950-2018).
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Polynomial regressions and corresponding 95% confidence intervals are shown for all relationships.
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A & B) The relationship between cumulated annual GDD and vintage quality in Napa and Bordeaux
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(Napa; r2=0.27, P<0.0001, Bordeaux; r2=0.48, P<0.0001). C & D) The relationship between average
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sugar concentrations at harvest and vintage quality in Bordeaux and Napa (Napa; r2=0.24, P<0.005,
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Bordeaux; r2=0.32, P<0.0001). Points are colored according to the accumulated GDD for the giving
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vintage (legend shown in panel D).
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It is well-established that high temperature can have deleterious effects on wine grape composition
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that include decreases in anthocyanins (the pigmented molecules that give red wine its color) and
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other quality related molecules (e.g. other phenolics, volatile aromas, etc.) (Martínez-Lüscher et al.,
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2020; Torres et al., 2020, 2021). Examining the relationship between sugar and anthocyanin
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accumulation in Cabernet Sauvignon across 5 vintages in California we see that anthocyanin showed
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a clear maximum beyond which riper fruit (as defined by sugar concentration) were associated with
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a loss of color (Figure 4, Supplemental Figure S3). At sugar levels above 200-225 g/l (~21-22 °Brix)
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anthocyanins no longer accumulated and became decoupled from sugar accumulation (Martínez-
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Lüscher et al., 2017, 2020; Torres et al., 2020, 2021). Above 250 g/l (~25 °Brix) of sugar, color was
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lost to some extent (Torres et al., 2020). Anthocyanins are just one of many important quality related
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compounds for red wines, but skin flavonols also undergo a marked degradation above similar
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thresholds (Martínez-Lüscher et al., 2019). The degradation of these quality related compounds and
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the observed plateaus of wine quality ratings suggested there can be too much of a good thing.
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Figure 4. Relationships berry sugar and anthocyanin concentration in Cabernet Sauvignon
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grown in Napa and Sonoma.
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The data were taken across 5 vintages (see Supplemental Fig. S3), and all data are presented as the
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grey points. The colored points represent averages of berries sampled on the same day (3-5 days per
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vintage, error bars represent ± standard error). Polynomial regressions and corresponding 95%
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confidence intervals are shown (r2=0.86, P<0.0001).
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Conclusions
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Here we show that two of the world’s top wine regions, Napa and Bordeaux, are warming at a
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remarkable rate and their pattern of warming reflects global regime shifts observed across other earth
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systems during the same period. Growers, scientists, and wine professional all speculate that these
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drastic increases in temperature will negatively impact fruit and wine quality at some point; however
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our analyses here suggest that setting hard temperature thresholds for optimal berry composition and
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wine quality is difficult. Case in point, a past study using data through 1999 predicted an optimal
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average growing season temperature of 17.3 °C for Bordeaux (Jones et al., 2005). However,
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Bordeaux surpassed that barrier more than a decade ago (Supplementary Figure S1A). Fruit-based
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metrics such as the sugar-phenolic dynamics presented here and/or specific secondary metabolites
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(e.g. kaemferol) maybe better indicators of losses in fruit quality associated with warming (Martínez-
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Lüscher et al., 2019). In Napa and Bordeaux viticulture has successfully adapted to a drastically
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changing climate thus far, but fruit-based metrics raise concerns that we are approaching a tipping
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point.
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Acknowledgements
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The authors would like to thank Kees van Leeuwen for his critical input and perspective in crafting
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the article.
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