As a natural skeptic, I have been slow to get on board with the concerns about climate change (a.k.a. "global warming"). Recent experiences have caused me to take a closer look at the issue. My first experience was a talk by Dr. Gregory Jones at the last Unified Wine and Grape Symposium in Sacramento, California, where he discussed trends in the climate of the western U.S. He elaborated that, by extrapolating the current trends forward, grape growing regions will be moving dramatically over the next 50 years (more on that a little later in this article). My second recent experience was in the Okanagan Valley of British Columbia, Canada, where I was discussing climatic conditions with a client. I found out that they are already feeling the effects of climate change very strongly in their part of the world, and those changes are actually working to their benefit at the moment. They are finding that an increase in heat availability is allowing them to grow warmer climate varieties, including less veggie Cabernet Sauvignon. Indeed, when I took a look at seasonal heat summation units (degree-days) for their region, there had been an upward trend in recent years. And the trend was not small: 60 degree-days Fahrenheit per decade from 1950 through 2000 in Penticton (south end of Lake Okanagan). And that does not include more recent years during which the locals say that they have been feeling the warming trend even more.

Dry-farmed "bush vine" vineyard in the Cariñena D.O.C. of Spain

I am thrilled to have had the opportunity to attend the Congress on Climate and Viticulture, held in Zaragoza, Spain from April 10-14, 2007. The conference brought together the leading climate and viticulture experts from around the world (though primarily from Europe) to discuss not only climate change, but also to discuss general aspects of climatic importance to wine growing, grape ripening and terroir. I usually refrain from using the term terroir because it can be so emotional to many people, but I have long contended that climate was a major component of that concept, and many presenters at this conference clearly supported that contention.

A session concerning the mapping of climate regions was highly stimulating about how geographic tools can be used to interpolate climatic parameters between fixed weather stations. Climate change necessitates changes in viticultural practices, and the conference had discussions on how irrigation may be needed to a greater and greater extent in countries where it traditionally has not been practiced. It appeared as though the European wine growing community is now getting excited about the use of irrigation as a tool to manipulate fruit and wine qualities.

Trends of a Changing Climate

Gregory Jones of Southern Oregon State University and Hans Schultz of Geisenheim University in Germany tag-teamed a presentation on the changing climate and how it may affect wine growing and where wine grapes may be grown. The evidence was shocking, and their forecasts were foreboding. Polar ice cores are tools that enable climatologists to track levels of CO₂ in the atmosphere over hundreds of millennia. Over the last 420,000 years, CO₂, a major greenhouse gas, has fluctuated between 200 and 300 parts per million (ppm). But the largest change in CO₂ has occurred during the most recent 150 years, where it has increased to about 380 ppm. Add to that the other greenhouse gases (e.g., methane, chlorinated fluorocarbons and nitrous oxide) and the levels are at a CO₂ equivalent of 450 ppm! It is difficult to separate out the man-made effects of climate change from the natural effects, but the dramatic increases in greenhouse gases in the last century and a half can only be attributed to industrialization and population increase.

Dr. Hans Schultz of Geisenheim University explores soil pits in Somontano.

What kinds of climatic changes have been observed in recent decades and years? There has been a general trend of rising temperatures. But not only are temperatures rising on average, the variability in temperatures has been increasing. That means that while mean temperatures are increasing, there are also more frequent episodes of both cool and hot weather. I find both of these trends to be disturbing. Grapevine and fruit development is so tightly linked to temperature that it will be impossible to avoid seeing changes in the phenology of our vineyards but also the ripening characteristics of our fruit. More on that to follow.

But what disturbs me even more is the increasing variability in temperatures. Variability may be witnessed as year-to-year variability, where we experience more characteristically "hot" years and "cool" years, or it may be shorter-term variability, where we see more heat waves followed by cool spells. Generally speaking, the highest quality wine grapes are grown where temperature fluctuations are minimal. Grapevines and fruit do not respond well to extreme heat waves, as they tend to acclimate to the average conditions. Coastal influence, or influence from other large bodies of water, generally mitigates fluctuations in temperature, which is a strong reason why vineyards near oceans, seas and large lakes are of higher quality potential. We just may see a gradual decline in overall wine quality if the trend of increasing temperature variability continues.

Precipitation is strongly linked to temperature, as air temperature affects both evaporation, air movement and changes in temperature, and temperature variability will create changes in precipitation patterns. That means that distribution of rainfall within a year may change in some regions, and there may be a greater incidence of very wet years and very dry years. For viticulture, that means that regions traditionally relying on rainfed vineyards will need to begin to store water and rely on irrigation.

How About Some Numbers?

It's easy to talk about general trends, but without some numbers to go with the words, it is difficult to appreciate the magnitude or the importance that the changes may have on our industry. Jones and Schultz stated that, during the period from 1950 to 2000, there was an increase in average growing season temperature of 3.1°F in Europe, driven mostly by an increase in minimum temperatures. Over a similar time period (1948-2004), the Western U.S. has experienced an increase in average growing season temperature of 2.7°F, also driven mostly by increases in minimum temperatures. Napa has seen an increase of 2.9°F over that time in average temperature, but average minimum temperatures have increased by 5.2°F! Over that 56-year time period, the seasonal heat summation units have increased a whopping 625 degree-days Fahrenheit in Napa, more than enough to move it up to a new Winkler region. Interestingly, it has been the nighttime (minimum) temperatures that have increased while daytime temperatures have remained relatively stable. What does that mean to us? Plenty, but let me get to that in a while.

Jones showed global maps indicating the isotherms of 12-22°C (54-72°F) of average annual temperature, which are, roughly, the limits for viticulture. From 1950 to 2000, those limits shifted roughly 50 to 150 miles poleward. That means that some regions are becoming unsuitable for wine grape growing while other regions are becoming suitable that were previously unsuitable. Jones also showed a map of the Western U.S. showing modeled predictions of the changing distribution of climatic zones. The general trend suggests that some of the current inland regions will someday be too hot for wine grape growing, and regions will shift northward. The models also predicted that the coastal regions will be less affected than the inland regions, due to the moderating effect of the Pacific Ocean. Higher elevations will become more suitable for grape cultivation. There was a wealth of information and data presented by Jones and Schultz. I understand that Jones will be publishing an article on the topic very soon, and I don't want to steal any more of his thunder here.

Climate Changes and Wine Grape Growing

The vineyard system is complex, and simple answers to this question may be eventually disproved. However, some of the effects are currently being measured, and others can be put through "thought experiments." The warming climate will reduce the incidence of frost events and may even reduce the severity of winterkill. Fruit composition is likely to change due to alterations in ripening patterns. Phenological events will generally be advanced in time, and the time periods between events will be shortened. Pest and disease pressure may be increased as the activity of those organisms increases with warmer temperatures. And, periods of water scarcity and surplus will be exacerbated, necessitating the use of irrigation in previously non-irrigated regions and also increasing the need for erosion protection.

Generally speaking, for every 1°F increase in average seasonal temperature, phenological events will be advanced by three to six days. Francis Fabre (Syndicat des Vignerons of Côtes du Rhône) explained that the growers in that region of southern France have been having difficulty with early sugar "maturation" of the fruit. Fruit is reaching desired Brix before the phenolic maturity of the fruit is reached. Even in the last five years, seasonal average temperatures have increased almost 2°F, and harvest has been trending earlier since 1951. During that five-plus decade time period, harvest dates have shifted from late September to early September, meaning that fruit is engaging in ripening during the warmer summer months. The growers in the Côtes du Rhône are actually considering changing the varietal mix, which has been largely dominated by Grenache and Syrah, towards other varieties such as Carignane, Mourvedre and possibly even hybrids.

Cornelis (Kees) van Leeuwen of University of Bordeaux spoke about how critical it is (in most regions of France and Europe) to obtain proper ripeness between September 10 and October 10 of each season. This is to obtain maximum varietal expression from the fruit. He said that, while vine water status management is crucial in maintaining a proper ripening process, changes in climate are making some varieties unsuitable. For instance, he cited Sauvignon Blanc as problematic in Bordeaux, as it has been ripening too soon. So, the varietal mix may change even in Bordeaux. In order to extend the ripening period into the cooler months and within the ideal time window, he suggested that growers may need to replant on later-ripening rootstocks, such as 420A or 41B.

From the Loire Valley, Gerard Barbeau (INRA) said that phenology has advanced about two weeks in recent years due to the warmer climate. About 30 percent of the grower community is currently in favor of varietal changes. He mentioned that there is stronger agreement for varietal changes in Italy and in Germany. Growers in the Loire are currently more amenable to changes in clones and in the adoption of irrigation to lengthen the growing season.

Over and over again, researchers reported increases in temperatures, changes in rainfall distributions, gradual changes from one Winkler region to the next higher region and a significant problem with premature sugar attainment.

The Premature Ripening Question

The general consensus was that harvests are occurring earlier, and sugar attainment is being reached before flavor and phenolic ripeness have reached completion. It was suggested that climate change is at least partially responsible for the "hang time" issues that we are facing in the California North Coast region. Indeed, we have had issues with ripening occurring at high Brix levels while growers seemed to be getting fruit ripe just fine at 24 Brix in 1976. But I can't agree that climate change is a major component of the so-called "extended maturation" conundrum. There are countless other reasons why later maturity might be occurring in the North Coast, for instance, new trellis types, cleaner plant materials, yield "control" practices, leaf removal practices, rootstock changes, etc., not to mention stylistic changes that are both critic- and consumer-driven.

I have found, from experience and from talking to winemakers, that a given variety grown in "warmer" climates will ripen at higher Brix levels than the same variety in a "cooler" climate (all else being equal). But in analyzing the characteristics of each type of climate, I found that the so-called cool climates (at least in California) were cooler during the day but warmer during the night than the warm climates. Warm or hot climates tend to have larger diurnal temperature swings from day to night.

The diurnal temperature cycle seems to have been forgotten in these discussions about ripening. If there was one thing that disappointed me about this conference, it was that there was no detailed discussion about day/night temperatures during fruit development and ripening, or about the temperature dependence of the grape's primary and secondary metabolism. I am surprised at the apparent lack of knowledge on this subject even though it was stated over and over again that temperature was a critical aspect of fruit ripening and of terroir!

The nighttime temperature is a key driver of what I call "night ripening." All biological processes are temperature-sensitive, especially in plants which have no internal temperature control. If night temperatures are warm enough (not really warm but somewhere above the 50°F threshold), the flavor and phenolic ripening processes can proceed during the nighttime without the accompanying sugar loading that occurs during the daytime. That is generally the reason, I believe, that fruit ripens at lower Brix in cooler climates than in warmer climates that have warm days and cold nights.

So, it is not as simple as a warming climate during ripening, but what seems to be the common thread is the advancement of all of the phenological stages--not just a shorter ripening process. When veraison occurs earlier, the fruit ripening process begins when the weather is still at its warmest--during August. And it is clear that the Europeans are having a difficult time with the premature sugar maturity and asynchrony between sugar and phenolic maturities. It is unlikely that a similar trend will not occur here in the U.S. if it is not already occurring.

What Can We Do About It?

Obviously, in the big picture, anything that mankind can do to mitigate the process of global warming is necessary for more reasons than wine growing. But that is well beyond the scope of this article. And while varietal changes will probably be necessary, although a painful change after centuries of tradition, other solutions may be able to lessen the effects of climate change on the current wine growing regions of the world.

When the flexibility is available, planting on different slope aspects can modify the meso-climate of a vineyard and may be a way to mitigate the increasing macro-climatic temperatures. For instance, R. Di Lorenzo of University of Palermo in Italy, found that south-facing slopes were 2.7°F warmer than average while north-facing slopes were 1.8°F cooler than average.

Reducing fruit exposure to sunlight might be another way to delay premature sugar attainment when it is caused by early dehydration of berries. Carlos Lopez of Technical University of Lisbon in Portugal reported that fruit in sunlight can be 18°F above ambient temperature. Therefore, leaf removal may be a practice that has come and gone, at least in all but the coolest of wine growing regions.

Hernán Ojeda (INRA, France) discussed irrigation strategies for vineyards. He used a leaf water potential (pressure chamber) basis for discussing control of vine water status through moderate deficit irrigation. He was generally against water deficits being imposed prior to veraison, something that I don't completely agree with. However, he stressed the benefits of moderate deficit irrigation after veraison which, I agree, is appropriate for most grape varieties. And for the purpose of enhancing the phenolic ripening processes, I think his recommendations are on the mark. Moderate water deficits applied at and shortly after veraison have the effect of stimulating the phenolic ripening of the fruit, which will be a benefit to anyone who is facing premature sugar attainment. The risk, however, is that water deficits must not be so strongly applied that fruit begin to shrivel.

Yet, as Dr. Ojeda mentioned, human interaction plays a relatively minor role in the process of grape growing and in the terroir, and climate greatly overshadows anything we can do to manipulate the system.

Why This is So Important

Over and over again, speakers stressed how the temperature climate is the principle factor in determining the ripening profile of the fruit. On a higher level, it was stressed that the temperature climate is an integral, if not primary, component of a region's or a vineyard's terroir. Climate change has been measurable and is causing a shift in the terroir of very traditional European viticultural regions. And there is no reason why it won't happen in your region as well.

It was mentioned that another conference on climate change and wine growing will be held in February 2008 in Barcelona. Al Gore will be a keynote speaker at that conference.

Notes About the Conference Venue

Jose Maria Faci González delivers a paper at the Congress on Climate and Viticulture.

The meeting was held in Zaragoza, which is a city often overlooked by tourists even though it lies roughly in between Barcelona and Madrid. While not yet a major tourist destination, there are numerous high-quality hotels, countless restaurants and many beautiful buildings, including a tremendous 17th century basilica adjacent to the river Ebro. Zaragoza is the site of the ancient Roman city of Caesaraugusta (from which it derives its name), and there are Roman ruins here and there, the highlight of which is the theater, which has been excavated and has catwalks installed for close observation and an excellent museum for historical perspective. The city of over 650,000 people will be the site of a summer-long expo next year called "International Expo Zaragoza 2008," where the theme is to be "Water and Sustainable Development."

Zaragoza is the capital city of the province of Aragón, which lies in the eastern portion of Spain. There are four appellations (denominación de origen) in the province, all of which lie about an hour's drive from the city: Calatayud, Cariñena, Campo de Borja and Somontano. Somontano lies to the northeast of Zaragoza, at the foot of the Pyrenees, while the other three lie to the west of the city. The list of varieties grown in the four DOCs is lengthy and beyond the scope of this article. In short, many Bordeaux and Rhône varieties (plus Chardonnay) along with some other characteristically Spanish varieties are grown. Many vineyards, especially in the western DOCs, are of the old "bush style" vines, with vines that were maintained very short, severely pruned to produce low yields, dry-farmed and with completely clean-tilled soils (not a weed in sight!). There are, however, some vineyards, especially prevalent in Somontano, that are trellised and many that are drip irrigated. It is not difficult to see that there is a trend towards irrigation, and the old bush style vines may eventually be replaced with vines on wires.

The hosts of the congress took great strides to make the vineyard and winery tours very enjoyable, comfortable, informative and quite delicious! And while there was a significant language barrier for some of us in the international group, there were outstanding translators provided during both the working sessions and the tours to make the information transfer very effective. wbm

Disclaimer: The facts presented in this article are accurate to the best of my knowledge. However, it is possible that errors were made during language translation and/or hand-transcription. Before citing any of the data mentioned in this article, I strongly
recommend consulting the authors of the papers.