Grape expectations

Climate change has already shifted harvest dates for wine by up to two weeks. Within the next few decades, many wine growers will need to adapt methods or migrate to higher latitudes. Michael Gross reports

English wine is on the rise, as Masters of Wine Susie Barrie and Peter Richards reported in Waitrose Drinks magazine this summer. In 50 years, production has increased by more than three orders of magnitude, from a negligible 1500 bottles/year to a respectable 5.3m. Since 2000, the land area for viticulture in England has tripled, while in 2017 alone, more than 1m new vines were planted in English soil.

‘It’s hard to ignore English wine today,’ Barrie and Richards conclude. The category has real momentum, fuelled by praise from eminent critics and awards from reputable competitions.’ White wines and champagne-style sparkling wines are among the strongest contenders of this nascent industry. Barrie and Richards have recently teamed up with Hampshire winery Hattingley Valley to make 1,000 bottles of their own sparkling wine.

Meanwhile, on the other side of the English Channel, grapes are harvested around two weeks before the traditional dates. In the Champagne region, harvest kicked off on 26 August 2017, while the average date for previous years was 10 September. In Bourgogne, home of Beaujolais wines, harvest began on 23 August, also two weeks ahead of schedule. Harvest workers in that area are also doing night shifts to reduce heat stress for the sensitive grapes.

Both phenomena, the success of English wine and the earlier harvests in France, are linked to climate change. A fact that Barrie and Richards omitted to mention in the Waitrose article. They are also linked to each other. In a few decades, the favourable wine-growing conditions historically enjoyed by the Champagne region may have migrated to England. As the life cycle of the grapevine, and quality and quantity of the wine obtained, are extremely sensitive to temperature and weather extremes, wine growers have already been noticing the effects of climate change for years. Researchers have detailed how conditions have changed, how they are likely to change further, and what vineyards can do to adapt.

High value product

All agricultural products are likely to be affected by climate change at some point, but wine occupies a special position due to its high value. Therefore, wine growers have always watched the weather and its effects on their vineyards very closely, and recorded their observations.

Climate scientist Benjamin Cook from Columbia University at New York and ecologist Elizabeth Wolkovich from Harvard University, have analysed harvest data spanning more than 400 years, from 1600 to 2007, from seven regions in France and one in Switzerland, together with the weather data.1 While many studies have covered the last few decades, this one reaches back to the time before the Industrial Revolution, thus establishing a baseline measured before the onset of man-made climate change.

The time of the harvest is determined by the ripening of the grapes. The life cycle or phenology of the grape normally lasts around 180 days from the appearance of the first buds to the point at which grapes are ready to be picked. Higher temperatures in spring and summer generally speed the whole process and lead to earlier harvests, like the one in 2017, while cool and rainy summers can delay the phenology and thus the harvest time. Traditionally, the observation was that a warm summer and a period of drought just before grape picking is the best recipe for an early harvest.

The data analysed by Cook and Wolkovich show that the harvest dates follow these rules and oscillate with the weather for most of the period studied. The most prominent spikes in their graphics are the ‘year without a summer’ of 1816, when the eruption of Mount Tambora in Indonesia shrouded the Earth in a veil of ashes and led to a noticeable cooling effect, and the extreme heat wave of 2003, which brought harvests in Western Europe forward by more than a month.

Climate change becomes obvious in the charts post 1980. The curve leaves the range of the previous standard deviation and takes a sharp turn towards earlier harvests. All annual harvests since 1980 have taken place before the historical average date.

The authors explain this turn as a decoupling from the drought requirement. Previously, a late drought was needed to give the grapes the last push towards an early harvest. At least in part, this effect was due to the higher local temperatures above dry ground, in the absence of the cooling effect of evaporation. Since 1980, rising temperatures have generally been sufficient for an early harvest regardless of the rainfall.   

In the past, earlier harvests were generally welcome as they often yielded better quality wines. However, as Wolkovich points out, there is a limit to this beneficial effect. For instance, the extremely early harvest of 2003 is not generally known as an exceptionally good vintage. Thus, if these extremes become the new norm, wine growers are likely to feel negative consequences.  

‘Our research, and other work, has clearly and unequivocally demonstrated that climate change is already affecting viticulture worldwide,’ explains Cook, adding that: ‘There are lots of opportunities for adaptation in various locations, such as planting different varieties, but the most important thing is for people to starting planning for the next several decades, when conditions are likely to get even warmer still.’

Grape composition

Even wine growing areas that have so far felt benefits of the warmer climate have reasons to worry about their future as temperatures continue to rise and the frequency of extreme weather events increases. The regular hurricanes in the Caribbean are becoming deadlier with higher sea surface temperatures, while the South of France has also suffered weather extremes linked to warmer Mediterranean waters.

In a recent review article, Cornelis van Leeuwen from Bordeaux Sciences Agro and Philippe Darriet from Bordeaux University, France, have summarised the known effects of climate change on wine quality and yield and attempted to extrapolate likely developments in the near future.2 Apart from the advancing harvest dates, the temperature increases have also changed grape composition, driving up alcohol levels by up to two percentage points and increasing the pH by 0.2 units.

As with the harvest dates, the first, moderate changes in this direction were often welcome as they improved the quality of the wine. However, higher temperatures are soon projected to drive these parameters away from the optimum, making it more and more difficult to produce good quality wines.  

Even the taste of a specific wine can be altered by temperature changes. Thus, the concentration of various aroma chemicals such as 3-isobutyl-2-methoxypyrazine (IBMP, responsible for a bell-pepper aroma) or rotundone (a sesquiterpene also found in pepper and other spices) are known to decrease with a rise in temperature. On the other hand, 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN, known to develop in aging  Riesling wines and produce petroleum flavours only appreciated by the more adventurous connoisseurs, has long been known to increase with higher temperatures during the ripening phase. Terpene alcohols have been reported to respond in various ways, increasing, decreasing or remaining unaffected.

These changes may make the wine better, worse, or just different, but in a market where consumers are used to associating the name of a village or mountainside with a very specific taste experience, any change to the established link between terroir, grape, and the taste and quality of the wine is certain to upset a well-established business.

It is clear that much of the planetary surface is getting warmer, but changes in precipitation are much harder to predict. As climate change disrupts established hydrological regimes, some places will get drier, others wetter. Both ‘unprecedented’ floods and catastrophic droughts are among the hazards of climate change some parts of the world are already experiencing that won’t go away.

Van Leeuwen and Darriet modelled water balance in the Bordeaux region using a published model. They find that vineyards have become dryer between 1952 and 2012, although this is in part due to rising temperatures and increasing evapotranspiration. Grapevines are fairly drought-resistant, and in many parts of Europe a relatively dry year is generally considered a good year. As with other parameters, this only works to a certain extent, before the plants actually begin to suffer from the drought.

Changes in precipitation are also likely to involve extreme weather events, including torrential rain and hailstorms, both of which can damage crops. The south of France has seen an increasing frequency of such events, which have already caused damage.

A third changing parameter over the past few decades is the exposure of vines to UV-B radiation. In the right amounts, this part of sunlight creates desirable flavour compounds in grapes, but excess UV-B can produce off-flavours in white wines and even ‘sunburn’. The risks of radiation exposure limit the extent to which growers can move to higher altitudes to escape increasing temperatures.

Adapt or move?

Climate change affects wine growers around the world, in Chile and Australia as much as in Europe and California. A recent conference on wine growing under climate change held in Bordeaux3 had contributions from 20 different countries, including studies of the challenges growers face in Portugal4 and China.5

The only reason why France and other traditional European wine growing regions have more to worry about than relative newcomers like Australia lies in the tight connection between the established labels, the terroir they grow on, and the grape varieties used. This combination optimised for the climate of the past four centuries will no longer work in the 21st century. Any changes to the recipe, however, are bound to cause trouble with the hard-fought recognition of labels like ‘appellation controlee,’ and with customers.

So what could be changed? Short of pulling up Pinot Noir vines in Champagne and replanting them in Dorset, van Leeuwen has a few suggestions of adaptive measures that wine growers can make in situ.6

To stop the harvest time from moving further away from the ideal timeframe, which in the northern hemisphere starts in early September, growers could change their viticultural techniques and/or their plant material to delay ripening of the grapes.

For instance, growers could add a few days to the ripening cycle by delaying the spring pruning, or by allowing the vines to grow higher above the ground, where the air is slightly cooler than just above the soil. While these changes are benign, other measures, such as reducing the leaf area, may have complex consequences that could interfere with the quality of the wine.

In selecting the plant material, growers could reverse the trends of the 20th century, when it made sense to select rapidly ripening varieties. Simply by adapting the choice of variety from among the range of varieties already used in a given region to the changing climate, growers can to some extent mitigate the anticipated effects.

Van Leeuwen believes relatively conservative measures compatible with keeping the established regional styles may keep wine growers in business until 2050. However, he writes6: ‘In the second part of the 21st century, more invasive adaptations, like severe reductions in leaf area to fruit weight ratio, or the introduction of non-local, later ripening varieties will, most likely, be unavoidable.’

Alternatively, wines may by then have started to migrate towards the poles. Wines now coming from California may be produced in Washington State, and the premium fizz we now call Champagne may be known as Devon or Kent.

Michael Gross is a science writer based at Oxford, UK.

References:

1. B. I. Cook and E. M. Wolkovich, Nature Climate Change, 2016, 6, 715.
2. C. van Leeuwen and P. Darriet, J. Wine Econom., 2016, 11, 150.
3. N. Ollat et al., OenoOne, 2017, 51, 59.
4. H. Fraga et al., OenoOne, 2017, 51, 61.
5. Y. Li, I. Bardaji, OenoOne, 2017, 51, 71.
6. C. van Leeuwen and A. Destrac-Irvine, OenoOne, 2017, 51, 147.