Terroir and the Importance of Climate to Winegrape Production

Wine is the result of myriad influences that are often embodied in the concept of terroir, a term which attempts to capture all of the environmental and cultural influences in growing grapes and making wine. Terroir is derived from the Latin “terra” or “territorium” and its first modern definition appears as “a stretch of land limited by its agricultural capacity.” Historically, the use of terroir as defining aspects of landscapes grew out of the wine production traditions of the Cistercian monks in Burgundy, but the term was also broadly embraced by the French as an agricultural production concept tied to specific regions (i.e., wine, cheese, pâté, and other specialty crops). Burgundians also used the concept to market their wine, promote tourism, affirm regional traditions and obtain a comparative advantage over other regions, leading some to see it as an economic protection mechanism, even back then. Over time the French approach and ideals embodied in terroir eventually led to the Appellation d’Origine Contrôlée (AOC) system in 1935—a French certification system that is used to legally delineate geographical regions and regulate agricultural products (produits du terroir).

While the natural components of terroir encompass weather/climate, geology, soil and their interactions, general perceptions of terroir often point to “land” or “soil,” a form of “geographic identity,” “a sense of place,” or as Matt Kramer of the Wine Spectator put it so eloquently: “somewhereness.” As one might expect, there have been controversies and debate in wine circles between Europe and the New World, whereby terroir is discussed in “traditional” versus “industrial” production terms, as being “naturally endowed” versus “marketed,” and as an outcome of “protectionism” versus “experimentation” approaches to growing grapes and making wine. While examination, scrutiny, and debate over terroir's role in wine production is rightfully ongoing, what is clear is that climate is arguably the most critical environmental aspect in viticulture and wine production.

For all agricultural enterprises climate plays a dominant role in influencing whether a crop is suitable to a given region, largely controlling crop production and quality, and ultimately driving economic sustainability. In viticulture and wine production, climate is arguably the most critical aspect in ripening fruit to achieve optimum characteristics to produce a given wine style. For wine drinkers the most easily identifiable differences in wine styles come from climate: the general characteristics of wines from a cool climate versus those from a hot climate. Varieties that are best suited to a cool climate tend to produce wines that are more subtle, with lower alcohol, crisp acidity, a lighter body, and typically bright fruit flavors, while those from hot climates tend to be bigger, bolder wines with higher alcohol, soft acidity, a fuller body, and more dark or lush fruit flavors (Table 1). Climate is at control here. Geology or soil does not produce these general differences; their impact is found in the subtle differences and/or expression of fruit characteristics and wine styles within the same climate or region.

When assessing climate’s role in growing winegrapes for wine production, one must consider a multitude of factors that operate over various time periods and over many spatial scales. Namely, climate influences must be considered at the macroscale (hemispheric to global climates), the mesoscale (regional climates), the toposcale (site climates), and the microscale (vine row and canopy climate).

Global to Regional Climate Comparisons

Grapevines are grown in regions across the globe where temperatures during the growing season average 55-70°F (13-21°C; see below). These temperature limits are found mostly in the mid-latitude regions of the continents. However, using latitude as a comparison for climate suitability for viticulture and wine production is often misused, or simply misunderstood, by many. A common statement made by growers/producers in the United States is that “we are on the same latitude as Bordeaux; therefore we can grow the same varieties and make the same quality and style of wine as Bordeaux.” But unless one has been to Bordeaux, it is hard to imagine that the climate experienced there is substantially more humid with much greater rainfall during the growing season than the commonly compared Napa Valley. While both are known for their Cabernet Sauvignon wines, they do it in quite different climates. Bordeaux has relatively low daytime temperatures and high nighttime temperatures due to higher humidity, while the Napa Valley has much higher daytime temperatures and much lower nighttime temperatures due to lower humidity. This mostly has to do with the ocean temperatures offshore and the orientation of the coastlines (see below).

Throughout history, geographers have attempted to define or classify climates. This has resulted in numerous terms that we use today to compare wine region climates. The most common is the Köppen climate classification system. In its original form it was broadly based on the expression of climate in native vegetation, and the system combined average annual, seasonal, and monthly temperatures and precipitation to define homogeneous regions. The modified version of the system in use today divides all climates into seven broad groups: tropical climates, dry climates, mild mid-latitude climates, severe mid-latitude climates, polar climates, and highland climates. Each of these seven groups are further divided according to various temperature and precipitation relationships. Due to the geography of the old world wine regions of Europe and the use of the Köppen system, wine production has become synonymous with Mediterranean climates. However, true Mediterranean climates are confined to a relatively narrow zone around the Mediterranean Sea and even narrower zones in other similar regions worldwide. Furthermore, it is clear that today’s wine regions span a wide range of these classified climate types. While the mid-latitude climate types make the majority of the world’s defined wine growing regions (55%), Mediterranean climate types only make up roughly 15 percent of the surface area of wine regions globally. Other important mid-latitude climate types include humid subtropical (eastern US, eastern Australia, Uruguay), maritime temperate (Bordeaux), and maritime subarctic (British Columbia). Other prominent Köppen climate types that wine regions fall within include mid-latitude dry-cold (Colorado, Iran, etc.), subtropical dry climates (Priorat, Mendoza), and humid continental severe mid-latitude climates (Finger Lakes Region, New York and Niagara, Canada).

General comparisons with Mediterranean climates have also been misinterpreted. One of the reasons is that the Mediterranean region has the influence of two large bodies of water, the Atlantic and the Mediterranean, while most other regions that have comparable climates have a more linear coastline, cooler ocean temperatures, and typically one body of water (the exception is South Africa). Due to the complex arrangement of land and water in Europe, climate types can change over very short distances such as in northwest Portugal and Spain (Vinho Verde and Galicia) where the climate is more Atlantic maritime temperate, while move a short distance south and inland (Douro, Rueda and Toro) and the climate is more Mediterranean. Another interesting example of climate differences is with Italy, where the dry Mediterranean climate structure in the south gives way to more humid subtropical climates in the north. In the United States similar differences are found only when one goes from California to the eastern coast of the US. Furthermore, even the limitation of wine production to Mediterranean-like climates has shifted; viticulture has spread throughout much of the world with vineyards found as far north as in Scandinavia, where winegrowing is helped by a warming climate, and near the equator where two crops per year are produced, as in Brazil. In these regions additional weather/climate risks of winter freezes, untimely rainfall, tropical cyclones, or increased disease pressures pose challenges (see below), but innovation and the pioneering spirit of growers has helped local and regional wine identities thrive.

So while we commonly use general climate classifications like the Köppen system—and they do provide a broad regional to global generalization of climates—they lack specific detail in weather/climate parameters, and their use in viticulture and wine production suitability is limited. Individual weather/climate factors affecting grape growth, production, and wine quality include: solar radiation, average temperatures, temperature extremes, heat accumulation, diurnal temperatures during ripening, wind, and precipitation, humidity, and soil-water balance characteristics. Furthermore, climate influences also come from both broad structural conditions and individual weather events that result from many different and interactive aspects of these factors.

To understand climate’s role in growing winegrapes and wine production one must consider:

• The weather and climate structure of the region
• The climate's suitability to different winegrape cultivars
• The climate's variability, both seasonally and long-term
• The influence of climate change on the structure, suitability, and variability of climate in the region

Individual Weather/Climate Factors

Solar Radiation
Incoming solar radiation (insolation) provides the energy necessary for grape growth and maturation. Throughout the growth stages of the grapevine, the amount of insolation is critical in maintaining the proper levels of photosynthesis. The most critical stages come during the development of the berries (yes, grapes are called berries) starting at bloom and continuing through the harvest. During bloom, high amounts of insolation result in effective plant tissue differentiation into flowers. Low absolute insolation during the bloom stage can influence coulure or the failure to fully flower and set berries. The relationship between low amounts of insolation and coulure is not linear, nor predictable, but is more tied to cultivar characteristics. During the ripening of the berries, insolation mainly acts to control the amount of sugar in the grapes, and therefore, the wine’s potential alcohol content. Controls on the amount of insolation include: 1) those that are inherent with Earth/Sun relationships, such as overall amount received by any point on the surface of the Earth, seasonal variations in the angle of incidence of the sun’s rays, and the day length, and 2) those that are controlled by variations at or near the Earth’s surface, such as cloud cover, the reflective nature of the surface of the soil, and the role topographic variations (slope, aspect, and obstructions) have on the relative amount of insolation received.

Average Temperatures
Growing season length and temperatures are a critical aspect because of their major influence on grape ripening and fruit quality, and therefore cultivar adaptation to specific regions or sites. It is in their ideal climates that a given cultivar can achieve optimum ripening profiles of sugar and acid that can be naturally timed with flavor component development to maximize a given style of wine and the vintage quality. The growing season necessary for the cultivation of winegrapes varies from region to region but averages approximately 170-190 days. Broad suitability for grape production can be found where the mean temperature of the warmest month is more than 66°F (18.9°C) and that of the coldest month exceeds 30°F (-1.1°C). The general thermal environment for grapevines has numerous influences, which can be positive or detrimental depending on the timing with plant growth. Negative influences typically come from extremes (see below) but can also come from prolonged periods with average temperatures below normal during growth events such as bloom. Positive influences include how temperatures above 50°F (10°C) can initiate plant growth in the spring and how temperatures influence heat accumulation, which in turn drive ripening potential (see below).

In addition, generally the less variability in the temperature leading up to harvest on a day-to-day basis the better the wine quality. This is evident in that the majority of the most renowned and established vineyards in the world are in regions with the most equitable day-to-day climates. However, differences do occur where some cultivars ripen better in higher diurnal temperature ranges (moderate days, cool nights for cool climate cultivars) while others do better in lower diurnal temperature ranges (moderate to warm days, warm nights for warmer climate cultivars). Overall, the relative amount of insolation, the composition and color of the soil, the local topography and slope aspect, and drainage capabilities can all be major factors in the temperature structure of a vineyard, especially at night. In addition to the air temperature, both within and outside the canopy, the temperature of the soil can have a strong influence on vine growth and fruitfulness. This is especially important during the spring where warmer soils initiate root growth sooner and, when combined with warm air temperatures, hastens bud break. During later growth stages warm soil surfaces, enhanced by heat retention from rocky material, aid in ripening by warming the vine canopy during the day and into the night. Furthermore, during the dormant stage (from after leaf fall through budbreak the next year), an average temperature minimum or effective chilling unit (hours below a certain temperature) is generally needed to effectively set the latent primary buds for the following year.

Temperature Extremes
In contrast to average temperature influences on potential vine growth and wine style, some of the most important individual temperature aspects include the potential of mid-winter low temperature injury, late spring frosts, and the influence of excessive summer heat on grape quality. In regions that have a more continental climate, low temperature injury to grapevines during the winter is often a limiting factor to production viability. Research has also shown that there is a minimum winter temperature that the grapevines can withstand. This minimum ranges from 23°F to -4°F (-5°C to -20°C), with some cultivars and hybrids more cold hardy than others, and is chiefly influenced by microscale climate variations controlled by location and topography. Temperatures below these thresholds will damage plant tissue by the rupturing of cells, enzyme reductions by dehydration, and the disruption of membrane function.

In the spring prolonged temperatures above 50°F (10°C) initiate vegetative growth. However, during this stage temperatures below 32°F (0°C) can adversely affect the growth of the vegetative parts of the plant, and hard freezes (< 28°F; <-2.2°C) can reduce the yield significantly. Nearing maturation, early frost or freezes can lead to the rupture of the grapes, which influences disease development and can result in a significant loss of weight in the fruit. Frost and/or freeze occurrence during the spring and fall generally comes in two forms: 1) advection frosts and 2) radiation frosts. An advection frost occurs as cold air masses are brought into a region with the passage of a cold front. Frosts and freezes associated with cold air masses can occur sporadically during the spring and fall and can cause problems over the majority of a region. Radiation frosts, on the other hand, occur throughout most of the fall, winter, and spring and are a much more common problem in wine regions. Radiation (or ground) frosts occur as the ground and the air in the lower layers of the atmosphere (within and just above a grapevine canopy) gives off heat, warming the air in successive layers upward. If the dew point temperature is low enough, the result is that the air near the ground is cooled to the frost point. As the ground and lower layers of the atmosphere cool down, the heat energy lost is conveyed upward to form what is called a radiation inversion (a situation in which temperature increases with height from the surface). On nights when inversions form, a warmer thermal zone or belt develops upslope that provides a measure of protection from the coolest valley bottom sites. The thermal zone varies from region to region, but is generally found from 100-1000 feet (30-300 meters) off the valley floor. (In narrow valleys, vineyards need to be situated higher up in elevation than in broad valleys.) Inversions are common in many grape-growing regions globally and occur most frequently on long, calm, cloud-free nights.

At the other end of the spectrum of temperature influences, extreme heat (temperatures greater than 95°F; 35°C) in either the growing season or the ripening period negatively impacts winegrape production by “shutting down the vines,” through inhibition of photosynthesis and reduction of color development and anthocyanin production. While a few days of temperatures greater than 86°F (30°C) can be beneficial in the ripening potential, prolonged periods can induce heat stress in the plant and lead to premature véraison, a possible abscising of the berries, and partial or total failure of flavor ripening.

Heat Accumulation or Bioclimatic Indices
One of the most common ways to characterize and compare mesoclimates in viticultural areas is to use expressions of temperature that permit the calculation of heat accumulation or bioclimatic indices. These indices are typically summed over a period of time important to the vine’s growth and production (usually the 6 or 7 months of the vine’s growth and development cycle). The relationships between heat accumulation, vine growth, and maturation potential was first suggested by a French scientist (A.P. de Candolle) in the 19th century—he observed that vine growth started when the mean daily temperature in the spring reached approximately 50°F (10°C). Numerous indices using variations on the heat accumulation approach have been created and were typically related to the typicity or style of the wine that can be produced, resulting in classes associated with cool-climate cultivar wines to warm-climate cultivar wines to fortified wines and table grapes. However, each of these indices has been criticized in one way or another (i.e., too simplistic or appropriate for one region only), and no single, universally validated index has been developed.

Various forms of these bioclimatic indices have been created and include the degree-day formulation used in the Winkler Index, different forms of a heliothermal index (Huglin Index), climate-quality indices, a latitude-temperature index, and a growing season average temperature index (see below) with each helping to define the suitability of a region to the planting of certain winegrape cultivars. Others have suggested indices that combine numerous factors such as five dimensions of mean temperatures, continentality (defined as the difference between the average mean temperature of the warmest and coolest months), sunlight hours, aridity (based upon the difference between rainfall and evaporation), and relative humidity. Still others modified the standard growing degree-days (the Winkler Index) by imposing upper limits on mean temperatures, a correction factor for latitude, and a correction for each month’s temperature range. However, many of these climate indices are more complex to calculate and compare; they require parameters not readily available everywhere and are less usable by growers/producers. Arguably the most commonly applied index is the Winkler Index which uses standard growing degree-days that are classed into “Regions” of fruit and/or wine style types (Table 2). The Winkler Index has recently been updated to establish a lower limit to Region I, dividing Region I into Ia and Ib for hybrid and cool climate vinifera, respectively, and adding an upper limit to Region V.

Another measure of climate suitability, which is often compared to growing degree-days, is the maturity groupings based upon simple average growing season temperatures (Figure 1). Average growing season temperatures (April through October in the Northern Hemisphere) are functionally identical to growing degree-days, but are generally easier to calculate and have been related to the potential of varieties to mature in climates worldwide. Using average growing season temperatures shows that quality wine production is limited to 55-70°F (13-21°C) worldwide. While many of the cultivars shown in Figure 1 are grown and produce wines outside of their individual bounds depicted, such examples are usually bulk wines (high-yielding) for the lower-end market, and they do not typically attain the typicity or quality expected for the same cultivars in their ideal climates. Furthermore, growing season average temperatures below 55°F (13°C) are typically limited to hybrids or very early ripening cultivars that do not necessarily have large-scale commercial appeal. At the upper limits of climate, some production can also be found with growing season average temperatures greater than 70°F (21°C), although it is mostly limited to bulk wines, fortified wines, table grapes and raisins.

Good examples of how average growing season temperatures define suitability are found with three common cultivars: Pinot Noir, Chardonnay, and Cabernet Sauvignon. Pinot Noir is typically grown in regions that span from cool to lower intermediate climates with growing seasons that range from roughly 57-61°F (14.0-16.0°C), such as in Burgundy or the Willamette Valley. One of the coolest of these is the Tamar Valley of Tasmania, while one of the warmest is the Russian River Valley of California. Across this 4°F (2°C) climate niche, Pinot Noir produces the broad style for which is it known, with the cooler zones producing lighter, elegant wines and the warmer zones producing more full-bodied, fruit-driven wines (Table 1). While Pinot Noir can be grown outside the 57-61°F (14.0-16.0°C) growing season average temperature bounds, it is typically unripe or overripe and readily loses its typicity. In contrast with Pinot Noir, Chardonnay is one of the most flexible and forgiving cultivars, producing a range of wine styles across a wider range of climates. Chardonnay can be grown in relatively cool climates (~57-61°F; ~14-16°C), creating an elegant, crisp style that is flavored more by apple, pear, and fig; in warmer climates (~61-65°F; ~16-18°C) it produces a bolder style with more peach to honey notes often enhanced through oak barrel ageing. On the other hand, Cabernet Sauvignon is an intermediate to warm to hot climate cultivar with growing seasons that range from roughly ~61-68°F (~16-20°C) (e.g., Bordeaux or Napa). An example of a region near the lower climate limit for Cabernet Sauvignon suitability is Hawke’s Bay, New Zealand while one of the warmest at the upper climate limit is in Robertson, South Africa.

Figure 1: Climate‐maturity groupings based on growing season average temperatures. The horizontal bars represent the range of temperatures that each variety is known to ripen and produce high to premium quality wine in the world's benchmark regions. The dashed line at the end of the bars indicates that some adjustments may occur as more data become available, but changes of more than +/‐ 0.4‐0.8°F are highly unlikely. The figure and the research behind it are a work in progress and are used with permission by the author, Dr. Gregory V. Jones (Jones, 2006; Jones et al. 2012).

Ripening Period Temperatures
Another temperature-based characteristic of climate that helps define where winegrape cultivars do best is the ripening period day-night temperature pattern. The ripening period is broadly defined as the growth stage that starts with véraison (initiation of sugar accumulation and the change in berry color) to harvest. This period averages 40-60 days for most cultivars and wine regions, with research pointing to the importance of the last 30 days when the majority of the polyphenols, color, flavor and aroma compounds develop. The two most common temperature measures are the diurnal temperature range and the average nighttime temperature during the last month of the vintage. For example, the majority of the cooler climate cultivars achieve greater typicity in ripening periods with higher diurnal temperature ranges whereby daytime temperatures are high enough to facilitate ripening while nighttime temperatures slow respiration and metabolism. These conditions bring about the more delicate aromas and wine styles associated with cool to intermediate climate cultivars. Many warmer climate cultivars, on the other hand, typically require warmer nighttime temperatures to help more fully ripen and to metabolize compounds that are considered detrimental to quality. For example, Cabernet Sauvignon contains a high concentration of methoxypyrazines, which are responsible for the characteristic “green, herbaceous, or vegetative” aromas from under-ripe fruit. While many factors such as vine training, sunlight exposure, and canopy management can influence methoxypyrazine content, regions with lower diurnal temperatures or higher nighttime temperatures tend to allow for greater metabolism of the compounds, producing less “green” aromas. Another example of how ripening diurnal temperature ranges influence wine styles is found with Syrah. Rotundone is a compound that distinguishes the peppery aroma and flavors of Syrah and high concentrations typically result from a cooler climate with higher diurnal temperature ranges. Syrah grown in warmer climates with lower diurnal temperature ranges during ripening tend to have less peppery notes and more fruit-driven flavors. Not enough research has been done to adequately define the region and cultivar limits and diurnal temperature ranges for quality wine production. However, there is some evidence that average nighttime temperatures less than 54°F (12°C) during the last month of the vintage are best for those cultivars that require cooler nights, while those above 59°F (15°C) suit later ripening varieties that require warmer nights.

Wind
The role that the wind plays in the growth of the grapevine and the production of fruit is exhibited mainly through its effects on vine health and yield, but wind can also play a role on the heat budget of a vineyard. This is physically apparent through direct contact with the vines and through physiological effects of photosynthesis disruption (stomata closure) and reduced disease infestations. During the early stages of vegetative growth, high winds can break off the new shoots, delaying and even reducing the amount of flowering. As the berries proceed through véraison and into the maturation stage, high winds can be very effective at desiccating the fruit and can result in lower volume and quality. However, drying winds that occur at night and early morning can help reduce the occurrence of fungus-borne diseases by limiting the formation of dew on the leaves and berries. Nighttime winds can also be beneficial—they can help limit the occurrence of radiation frosts.

Local winds, generated from a region’s topography, are very common in viticultural areas worldwide. The most common local winds are the general land-sea breeze (affecting coastal regions or those near large bodies of water) and the mountain-valley breeze (affecting inland areas with substantial topographical relief), which provides dry-summer viticulture regions with some relief through late afternoon advection from the coast or down the mountains. While the overall occurrence of winds can have both positive and negative effects on the growth and maturation of grapevines, wind can be mitigated by location, topography, and the use of natural and man-made windbreaks. However, windbreaks in a region also might serve as an obstruction for cold air drainage and could enhance frost or freeze conditions.

Precipitation, Humidity, and Water Balance Characteristics
Given its importance to the growth and productive balance in grapevines, fruit quality, yield, and disease pressure, understanding water relationships in any wine region is very important. Factors such as ambient atmospheric moisture or humidity, local rainfall frequency and timing, soil water-holding capacity, and evapotranspiration rates are all important aspects. In addition, each of these aspects of water availability can be evaluated in terms of a water balance or budget.

Atmospheric moisture is very important in regulating the evaporative demands put on the grapevines and the occurrence of fungal diseases. During the growth stages of the grapevine, some of the climatic conditions that can most severely afflict the vines and berries are associated with moisture. Atmospheric moisture is commonly measured as relative humidity, and as such it displays a distinct diurnal and seasonal cycle. Relative humidity is normally highest early in the morning, when temperature is lowest, and at a minimum during the maximum heating of the day. The contrasts between morning and afternoon relative humidity are smallest along coastal regions and greatest inland. Since lower temperatures give higher relative humidity, and since cold air pools in valleys and in obstructed areas on slopes, humidity levels and the associated fungal problems can be enhanced in these zones. Over the course of the season, relative humidity is lowest during the summer and highest during the winter, with the greatest contrast between the seasons seen in the afternoon values. In extreme cases, water stress resulting from low relative humidity can be seen in leaf loss, severe reductions in vine metabolism, and fruit damage or loss. Even moderate periods of moisture stress can substantially reduce the relative level of photosynthesis, resulting in lower fruit yields and quality. On the other hand, controlled moisture stress through irrigation—applied at the right times during growth and ripening—can enhance quality and control yields.

While high levels of atmospheric moisture allow certain fungal problems to develop, the occurrence of rain during critical growth stages can lead to devastating effects. Ample precipitation during the early vegetative stage is beneficial to initial growth; however, during bloom it can reduce or retard flowering, and during berry growth it can enhance the likelihood of fungal diseases. During maturation rainfall can increase fungus occurrence and growth, dilute the berries (which reduces the sugar and flavor levels), and severely limit the yield and quality. There is little evidence from the world's viticulture regions that suggests that there is an upper limit on the amount of precipitation needed for optimum grapevine growth and production. On the other hand, grapevine viability seems to be limited in some hot climates by rainfall amounts less than 20 inches (500 mm), although this can be overcome by regular irrigation, if available and allowed. Extreme meteorological events like thunderstorms and hail, while generally rare in most viticultural regions, are extremely detrimental to the crop. Both events can severely damage the leaves, tendrils, and berries during growth. If they occur during maturation they can split the grapes, causing oxidation, premature fermentation, and a severe reduction in volume and quality of the yield.

As an integration of many climate parameters, a soil-water balance takes into account seasonal variations in temperature, precipitation, and available soil moisture to give an estimation of water requirements, whether natural or via irrigation. A water balance essentially defines the “water need” by plants and the atmosphere in any region. In most grape-growing regions there is a period of soil water surplus from late fall through late spring, followed by a period of draw-down of soil moisture through evaporation (by the atmosphere) and transpiration (by plants) during the summer through the early fall, when precipitation finally begins to replenish the soil. Adequate soil moisture recharge during the spring can drive vine growth and result in more effective bloom and berry set. While some soil moisture during the summer growth period can reduce heat stress, too much soil moisture can drive excess vegetative growth and lead to inadequate ripening and delayed leaf fall, putting the vines at risk of late fall frost/freeze events.

Is there an Ideal Climate for Wine Production?

Given the discussion above regarding the complex climate influences on growing grapes for wine production, is there such thing as an ideal climate? First, it is clearly best to match the cultivar to the climate where production generally occurs in the optimum zone (Figure 2). In the optimum zone a given cultivar will more consistently produce higher quality wine as it provides a more equitable growth period and tends to balance the four ripeness clocks that are running simultaneously but at different rates—sugar accumulation, acid respiration, phenolic ripeness, and fruit character. However, even if a cultivar is planted in its optimum zone vintage variations in weather and climate can drive large differences in fruit production and wine quality. Furthermore, any given cultivar has thresholds associated with climate (see Figure 1). If it is being grown in too cool of a region, or the vintage is too cool, then lower sugar levels, unripe flavors and unbalanced wines will result. Alternatively, if a given cultivar is being grown in too warm of a region, or the vintage is too warm, then lower acid retention, overripe flavors and unbalanced wines will result.

Figure 2: Relationships and thresholds between climate and wine production and quality metrics.

Even though the influences are complex and science has not yet come up with a simple, single measure to define and compare climate’s role in wine production universally, an ideal weather/climate situation looks similar to Figure 3 below. After harvest a grower would like to see a slow decline in temperatures that leads to a slow hardening of the vines. (Rapid temperature drops during this time could damage the plants before they are ready for winter.) Ideally the winter would be free of extreme temperature impacts, yet it would carry enough of a chill for the vines to break buds evenly and initiate growth in the spring. In the spring, slow changes in day length, combined with moist yet warming soil profiles and frost-free conditions, would allow for optimal and risk-free budbreak and initial plant growth. Around flowering, moderate daytime temperatures with high insolation, low cloud cover, and no rainfall would allow for an ideal bloom and fruit set. From fruit set to the start of ripening, a summer filled with optimum heat accumulation for a given cultivar, with low day-to-day temperature variability and low heat stress, would lead to a greater chance of a timely and even véraison. From véraison to harvest ideal conditions would see little to no rainfall with the vines ripening into the early fall, where shortening day lengths bring about changes in diurnal temperature ranges and a balance of the four ripeness clocks.

Figure 3: Idealized climate influences throughout one complete year from harvest to harvest.

Climate Variability

While the average climate structure in a region determines the broad suitability of winegrape cultivars, climate variability from year to year influences issues of production and quality. Depending on the region, climate variability can influence grape and wine production through cold temperature extremes during the winter, frost frequency and severity during the spring and fall, high temperature events during the summer, extreme rain or hail events, and broad spatial and temporal drought conditions. Climate variability mechanisms that influence wine regions are tied to large-scale atmospheric and oceanic interactions that operate at different spatial and temporal scales. The most prominent of these is the large-scale tropical Pacific sector El Niño-Southern Oscillation (ENSO), which has broad influences on wine region climates in North America, Australia and New Zealand, South Africa, South America, and Europe. However, the effects of ENSO on wine region climate variability differs tremendously in magnitude and has opposite effects depending on the location of the wine region, and it is often coupled with other more influential regional mechanisms.

Climate Trends

In addition to the discussion above on the climate structure, suitability, and variability, it is important to consider long-term changes in climate and their role in the sustainability of the region’s industry. What is clear is that viticultural regions worldwide are located in relatively narrow geographical and climatic ranges. In addition, winegrapes have relatively large cultivar differences in climate suitability further limiting some winegrapes to even smaller areas that are appropriate for their cultivation. These narrow “niches” for optimum quality and production put the cultivation of winegrapes at greater risk from both short-term climate variability and long-term climate changes than other commercial crops. In general, the overall wine style that a region produces is a result of the baseline climate, while climate variability determines vintage yield and quality differences. Climatic changes, which influence both variability and average conditions, therefore have the potential to bring about changes in wine production and wine styles. Our understanding of climate change and the potential impacts on viticulture and wine production has become increasingly important as the earth system undergoes natural cycles and fluctuations, and changing levels of greenhouse gases and alterations in Earth surface characteristics bring about changes in the Earth’s radiation budget, atmospheric circulation, and hydrologic cycle. Observed warming trends over the last hundred years have been found to be asymmetric with respect to seasonal and diurnal cycles, with the greatest warming occurring during the winter and spring and at night. The observed trends in temperatures have been related to agricultural production viability by impacting winter hardening potential, frost occurrence, and growing season lengths. For grapevines, changes in growth events have resulted with earlier budbreak, bloom, véraison, and harvest dates occurring in the majority of regions worldwide. Many regions are seeing much-earlier ripening, in a hotter period of the summer, with greater disconnect between the four ripeness clocks. Trends of higher sugar levels and lower acidity in grapes and higher alcohol levels in wines have been observed. As such challenges to future wine production will require the industry to consider the changing suitability to cultivars regionally, the need for adaptation in vineyard management and winemaking, and better knowledge of and use of the genetic diversity of winegrapes globally.

What does all this mean for the Sommelier?

Terroir is complex. There are numerous influences derived from the complex interactions between climate, geology, soils and landscapes—further complicated by human management of the plant system—that ultimately produce the joy that is wine. While we know that climate is the baseline factor in the continuum of terroir influences, by ultimately controlling what can be grown where and how, the complexity of these interactions means that simple descriptions of wine’s geographic origin are hard to come by, especially as one goes from larger scales to more local scales.

As we discuss wine region climates with the public, simple statements of climate types such as Mediterranean, maritime, or continental are common, but often misunderstood. Not all of Europe’s wine regions are Mediterranean! Furthermore, while latitude is often used to compare regions it is only partially correct in that one must also know whether the region is on the east or west coast and how far inland it is. So knowing something about the geography of winegrowing regions allows one to better characterize the climate and therefore the wines that come from them. One approach would be to examine how climates are classified for wine regions worldwide and then to develop analog comparisons with other regions, especially lesser-known ones.

After general geographic location details for wines, some of the most common and easily understood descriptors for wine are the broad climate-style driven comparisons that come from cool vs. warm growing conditions. For example Pinot Noir styles can be quite different when coming from a cool region (lighter, elegant style) as compared to a warm region (bolder, fuller-bodied style). The same cool vs. warm comparisons can be made within a given region due to vintage climate variability, where quite large differences in wine styles and quality can be seen from year to year. Understanding these vintage wine style and quality variations provides sommeliers with a wealth of comparative information for the consumer.

While one could specify an almost ideal climate for wine production (see above), it is seldom experienced due to numerous factors such as untimely rain, cold periods, heat stress, etc., which make each vintage unique. Adding to the puzzle of climate’s role in wine production is the fact that one must also remember that geology, landscape and soil are very important factors that mediate the interaction between climate and the vine, especially soil water supply and nutrition. These local-scale influences produce the subtle style differences that give rise to much of the lexicon of wine.

Ultimately, to better understand the differences that climate can have on wine styles one must visit as many wine regions as possible, taste as many wines as possible, and talk to the people that craft them… ah, the hard work of a sommelier!

Gregory V. Jones is the Director of the Division of Business, Communication and the Environment and a professor and research climatologist in the Department of Environmental Studies at Southern Oregon University. He specializes in the study of climate structure and suitability for viticulture, and how climate variability and change influence grapevine growth, wine production and quality. He conducts applied research for the grape and wine industry in the western US and many regions worldwide. He has given hundreds of international, national, and regional presentations on climate and wine-related research. He is the author of numerous book chapters and other reports and articles on wine economics, grapevine phenology, site assessment methods for viticulture, climatological assessments of viticultural potential, and climate change. He was named to Decanter Magazine’s 2009 Power List representing the top 50 most influential people in the world of wine, named the Oregon Wine Press’s 2009 Wine Person of the Year, and has been in the top 100 most influential people in the US wine industry in 2012 and 2013 (intowine.com).

Further Reading

Anderson, J.D., Jones, G.V., Tait, A., Hall, A. and M.T.C. Trought (2012). Analysis of viticulture region climate structure and suitability in New Zealand. International Journal of Vine and Wine Sciences, 46(3):149-165.

Gladstones, J., (1992). Viticulture and Environment. WineTitles: Adelaide, Australia, 310 p.

Hall, A. and G.V. Jones (2010). Spatial analysis of climate in winegrape growing regions in Australia. Australian Journal of Grape and Wine Research, 16:389-404.

IPCC (2007). Alley R. et al. Climate Change 2007: The Physical Science Basis. Summary for Policymakers. Contribution of the Working Group I to the Fourth Assessment of the Intergovernmental Panel on Climate Change. IPCC Secretariat (http://www.ipcc.ch/).

Jones, G.V. (2006). Climate and Terroir: Impacts of Climate Variability and Change on Wine. In Fine Wine and Terroir - The Geoscience Perspective. Macqueen, R. W., and L. D. Meinert, (eds.), Geoscience Canada Reprint Series Number 9, Geological Association of Canada, St. John's, Newfoundland, 247 pages.

Jones, G.V. and Goodrich, G.B., (2008). Influence of Climate Variability on Wine Region in the Western USA and on Wine Quality in the Napa Valley. Climate Research, 35: 241-254.

Jones, G.V., M. A. White, O. R. Cooper, and K. Storchmann. (2005). Climate Change and Global Wine Quality. Climatic Change, 73(3): 319-343.

Jones, G.V., Duff, A.A., Hall, A. and Myers, J.W., (2010). Spatial analysis of climate in winegrape growing regions in the western United States. American Journal of Enology and Viticulture, 61(3), 313-326.

Jones, G.V., Reid, R., and A. Vilks (2012). Climate, Grapes, and Wine: Structure and Suitability in a Variable and Changing Climate pp 109-133 in The Geography of Wine: Regions, Terrior, and Techniques, edited by P. Dougherty. Springer Press, 255 pp.

Mullins, M. G., A. Bouquet, and L. E. Williams. (1992). Biology of the Grapevine, Cambridge University Press, Great Britain. 239 pp.

Smart, R. E. and P. R. Dry. (1980). A climatic classification for Australian viticultural regions: Australian Grapegrower and Winemaker, 17:8-16.

Winkler, A. J., Cook, J. A., Kliewer, W. M. and Lider, L. A., (1974). General Viticulture, (4th ed.), University of California Press, Berkeley. 740 pp.