Climate change is altering the chemistry of wine

Following the devastating Glass Fire sparked in California’s Napa Valley in September 2020, Anita Oberholster, a wine chemist,’s inbox was filled with emails from frantic winemakers. They wanted to know whether they could ripen their grapes without fear of harmful effects on their wine: the unpleasant smoke taint.

Oberholster from The University of California, Davis would only say, “Maybe.”

Industries laboratories were swamped with samples of grapes for testing and waited for as long as six weeks. The growers were still determining if it was worthwhile to harvest their harvests. Around eight percent of California wine grapes harvested in 2020 rotted away.

Winemakers aren’t immune to the unpredictable effects of changes in the climate. Warmer temperatures are a blessing for those celebrating the more ripe fruit in cooler regions. However, they are devastating for other areas. Hot, scorching summers, wildfires, and other climate-driven disasters have destroyed harvests across Europe, North America, Australia, and worldwide.

As 2020 demonstrated, climate change can potentially affect the grapes but not directly destroy their nutritional value. Fires and warm temperatures can change the flavor of the wine. Its taste and character depend upon the subtle chemistry that makes the grapes and the climate they’re grown under. Numerous winemakers and growers are becoming more worried that climate change could rid wine of its distinctive flavor and even eradicate vintages.

“That’s the big worry,” says Karen MacNeil, a wine expert who lives in Napa Valley and author of The Wine Bible. “That’s the heartbeat of wine — it’s connected to its place.”

MacNeil says the most significant challenge changes in the weather creates for winemaking is uncertainty. Winemakers used to know what varieties to cultivate, how to grow them, when to harvest the berries, and how to process them into the exact quality wine. Today every step is in the air. The increasing recognition of this is causing scientists and vintners to seek out ways to protect the most beloved grape varieties and their distinctive characteristics in the changing and unpredictable conditions of our changing climate.

To find out about the dangers to our beloved drink, we talked to wine experts from two famous wine regions — Bordeaux and Napa Valley in France and California to learn the ways climate change is destroying the traditional vineyards and wines, and we traveled into California’s University of California, Davis as well as the close by Napa Valley in late 2021 to talk with growers, scientists, and winemakers.

We got an insider’s view of the various stages of winemaking that changes to preserve the desired flavors and aromas. Oh, did we get to sample many wines that ranged from the most prestigious Cabernet Sauvignon to samples spoiled due to heat and smoke?

From trying out new grape varieties to changing fermentation methods, winemakers and growers are looking for ways to reduce the impact of climate change.

The flavor of climate change

Extreme weather can kill the most rigid vines. However, the majority of the threat from climate is invisible chemical alteration within the fruit.

This is because the quality of wine, when it is at its most fine level, is about achieving harmony between three main elements of the berries: sugar, acid, and secondary compounds. Sugar accumulates in the berries when vines photosynthesize, and acid degrades as the grapes mature. Secondary compounds — essentially substances that aren’t essential to the plant’s metabolism — build up throughout the season. Anthocyanins are the ones that provide red grapes with their color and help protect plants from UV ultraviolet rays. The tannins, in turn, give wine bitterness and astringent, dry mouthfeel. The grapes safeguard against predators like grazing animals as well as other insects.

Various environmental factors, including the soil type, the level of rainfall, and fog, influence the three constituents that makeup wine’s taste. All of these are incorporated into the French term “terroir.” Climate -which is the pattern of precipitation and temperature is the most significant component of terroir, Oberholster says.

If the climate of a particular region changes, it could disrupt the balance of acid, sugar, and secondary compounds by altering the speed of their development throughout the growing season, states Megan Bartlett, a plant biologist who studies viticulture at UC Davis. Grapes, as with all fruit, can break down acids and store sugar when they begin to ripen. When temperatures are warmer, the process of ripening gets accelerated, which results in the sweet, raisin-like taste of grapes.

The yeasts consume these sugars during fermentation and excrete alcohol. Therefore, fermenting sweeter berries results in greater levels of alcohol in wine wine from warmer regions like south France becoming more boozy. This is a bad thing for consumers in the area, particularly since it’s being coupled with a decline in acidity, claims Cecile Ha, spokeswoman for the Bordeaux Wine Council. Edge provides freshness and ensures that wines last for many years when stored in cellars.

Temperatures rise and alter the wine grapes’ delicate chemistry.

In some wines, more alcohol levels can cause a burnt taste that masks subtle aromas, according to Carolyn Ross, a food scientist at Washington State University who cataloged wine aroma compounds in the Annual Review of Food Science and Technology. Wines with higher alcohol levels are also more likely to be spicier. So, as temperatures rise, “you’re getting pushed more and more towards that Zinfandel style,” Bartlett states. “Which is fantastic when Zinfandel is something you’re looking for. However, If there’s been a planting of Pinot or Cab it’s not actually expressing the top version of the variety.”

If the tale was merely about acid and sugar, the answer would be straightforward: Get the grapes harvested earlier, prior to them turning too sweet, and when they still have their tartness. But winemakers also require a stew of second compounds to develop as they make the complex aromas essential to high-quality wines. Winemakers may be forced to decide between harvesting earlier with no fully developed tannins and anthocyanins or picking later when the berries are full of these compounds, but they are also overly sweet.

Everything being equal, The changes in the grapes caused by warmer temperatures can produce more mature or “cooked” flavors in wine. MacNeil describes the process in this manner: “an unripe cherry, to an almost-ripe cherry, to a ripe cherry, to cherry juice, to cherries that have been cooked down on the top of a stove like if you’re going to make a pie, to dried cherries that are almost like raisins.” For wines that hail from warmer areas, the effects of climate change are worrying as they’re at risk of losing their identity as increasing numbers of wines turn very raisiny. (“All raisins taste alike,” says MacNeil.)

A blurring between wine is already affecting the wine industry caused by rising temperatures, more significant uncertainty in weather, and increased exchanges of advancing techniques. This has made the certification to master Sommelier a tough test that requires guessing a wine’s type or year of production and its region, making it even more difficult.

“There are a lot of people who are older masters of wine and master sommeliers who will tell you that if they had to take that exam now, especially the tasting exam, they would never pass it,” MacNeil declares.

The taste of fire

The changes in the wine’s taste are not as noticeable compared to other, much-feared impacts of climate change, as smoke impairs the taste of wine. A little smoke from, for example, barrel-aging could improve the flavor of wine; it is a “very characteristic ashtray character at the back of your throat,” as Oberholster describes it, using some notes like “Band-Aid” and “medicinal.”

Volatile phenols, which are compounds created when wood is roasted in the process, are absorbed into grapes and build up within the skins. The phenols are bonded with sugars to form glycosides, which are odorless -up to the point of fermentation, at which point certain compounds break up, giving the distinct, overwhelming taste. (The breakdown continues in the bottle or barrels and mouth.) The most intense flavor comes in the case of berries bathed with fresh smoke, not older smoke.

The sensation can be described as “retro-nasal,” meaning the scent is released into your sinuses when the wine hits your tongue. It’s believed that between 20 and 25% of people don’t smell it, possibly because their saliva isn’t containing enzymes that break down bonds to let the smoky notes out. It’s most likely dangerous to red wines because they are made by fermenting grape skins.

The recent flurry of massive wildfires, exacerbated by the changing climate, has made Napa growers nervous each year as the autumn harvest is nearing. Since 2017, a thick layer of smoke has hung over Napa vineyards for the majority of the time. Grape growers who are worried have contacted Oberholster for advice, and the chemist has prepared a variety of test batches and subjected them to different amounts of fume.

On the day that we meet with her, Oberholster takes us to a 24,000-bottle-capacity library of wine at the UC Davis Robert Mondavi Institute. She sifts two reds from the piles and then hands us some of the vintages for 2020. One is a mildly affected wine exposed for a week to fumes caused by The Glass Fire; the other is a very tainted wine due to grapes exposed to the smoke of a huge collection of lightning-triggered fires that extended right to the vineyard that year.


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