Grapes are extremely malleable. Plant the same grape variety in different soils, alter the length of time they’re fermented or vessel they’re aged in, and the taste of the resulting wine will be vastly different.
But the climate grapes grow in, and temperature fluctuation from day to night called diurnal shift, have some of the biggest impacts on a wine’s quality. Just a few degrees’ difference can spell success or doom for sensitive varieties like Pinot Noir, which needs an average growing-season temperature of 57–61°F to thrive.
From frosts and heat domes, to wildfires, floods and droughts, climate change upends the wine industry in countless ways. But the more subtle effects of diurnal shifts are also important, if less frequently discussed.
“A lot of people aren’t putting two and two together when it comes to the diurnal shift,” says Greg Jones, climatologist and CEO of Umpqua Valley’s Abacela Winery. “There are multiple factors at work here. Many warmer regions are now ripening much earlier in the summer, and harvest is happening in August, when it used to happen in September or October. But cool nights are key to correct ripening, especially in September and October, and without that, the sugar, acid, flavor, aroma and phenolic characteristics of the grape will be thrown off.”
Recent climate change research confirms this. According to scientists at the University of Exeter, who looked at temperature patterns between 1983 and 2017, global warming is affecting daytime and nighttime conditions differently, with greater increases in nighttime temperatures being registered.
“Over the past century, nighttime temperatures increased by 1.5°F, while daytime temperatures increased 1.1°F,” says Jack Sillin, climate researcher at Cornell University. “That may not sound like a lot, but that’s a difference of 20%.”
Effect on Grapes
Winemakers depend on the diurnal shift to lock in flavor and brightness, especially in white grape varieties. Diurnal shifts can also impart higher tannins and more complete phenolic maturity, says Julio Sáenz, technical director at Spain’s La Rioja Alta.
“We don’t have exact data, but we have noticed a significant change,” says Sáenz. “Grapes are higher in sugar, lower in acid, there is delayed phenolic maturity and grapes are developing less intense and fresh aromas.”
Franco Bastias, agronomist at Domaine Bousquet in Mendoza, Argentina, notes that changes in diurnal shift are far from uniform.
“In the Uco Valley, the shift is still pretty nice, around 59–68°F, which helps us harvest grapes with remarkable natural acidity and a concentration of fruity flavors,” says Bastias. “But in the center and east of Mendoza, many growers are suffering, and the increase in night temperatures are producing grapes with less acidity and higher pH.”
Resolution in Field
Winegrowers are experimenting with a variety of short- and long-term fixes to contend with these challenges.
At Clos Mogadar, which has 100 acres under vine in Priorat and Montsant, Spain, Winemaker Rene Barbier Meyer says that the changing nighttime temperatures are pushing them to “abandon non-autochthonous varieties, especially the ones that have to be picked early. We’re replanting autochthonous varieties like Grenache and Carignan, and recovering old ones like Picapoll and Xarel-o,” for their lower alcohol levels and higher acidity. New vines are also being planted at higher elevations.
In Paso Robles, Daou Vineyard’s Winemaker Daniel Daou, who has 170 producing acres in Adelaida, says his vineyards are in a better position than most in the region because they’re planted as high as 2,200 feet above sea level and 14 miles from the ocean. Even so, Daou was alarmed enough by changes in the diurnal shift that in 2017 he developed a three-pronged approach to counter the effects.
“We use an organic product called BluVite to activate microorganisms in the soil and strengthen the vine’s ability to withstand thermal stress,” he says. “After a three-year trial, we noticed that it helped lower the temperature of the grapes between 3–5°F during heatwaves.”
Daou also uses shade cloth during the afternoon to shield grapes from sunburn. “It can make between a 5–7°F difference,” he says.
The third element is monitoring moisture in the generally dry-farmed vineyards. “During heatwaves, we water in microbursts, sometimes giving a half-gallon or so during heatwaves in August,” says Daou.
By reducing the impact of the extreme heat spikes on the grapes, Daou says he can maintain some of the freshness lower nighttime temperatures usually lock into place.
Resolution in the Cellar
According to Eric Glomski, founder and winemaker at Page Spring Cellars in Cornville, Arizona, when “the gap between day and night closes” at any of the 30 acres of vineyards he works with, he’s forced to change procedures in the cellar.
“The acid structure gets mucked up, and for our warmer sites, we have to acidulate the wines,” says Glomski. “Picking earlier isn’t always a solution for us, because we want the grapes to be phenologically mature.”
Glomski worries that Syrah, Arizona’s “trademark grape,” will soon no longer be viable because “the acid just can’t hold up to these changes.”
He’s already planting more high-acid grapes like Picpoul Blanc, Gamay, Barbera and Alicante Bouschet, anticipating their increased importance in blending as well as stand-alone bottlings.
In New York’s Finger Lakes region, Red Newt Cellars’ Winemaker Kelby Russell says that, in 2021, he “didn’t have a frost until Thanksgiving. We normally get our first frost in early to mid-October. We depend on those colder temperatures to relieve disease pressure and kill off pests like fruit flies.”
Delayed frost also impedes acid development and aromatic potential of their grapes. Short term, that means increasing spraying regimens in the vineyard, something Russell says he hates doing, and on rare occasions, “adding tartaric acid because the acid is so lacking. We added just enough so our Riesling could be correct for the region.”
In the long term, he says, “low acid can also lead to microbiological issues, which may mean we have to switch up our spontaneous fermentation. We are not looking for malo.”
To contend with the current state of climate change, growers may have to continue to cultivate higher-acid grape varieties, move plantings to higher ground and deploy a range of products in vineyards and cellars to ensure customers get the style of wines they’ve come to expect.
“The changes in the diurnal shift represent a pernicious problem,” says Jones. “There’s no easy solution or shortcut, unfortunately. We must reduce emissions drastically to prevent more warming.”