Greg Jones downloading temperature data from one of 24 sensors located at Abacela. The data help document the vineyard’s spatial variation in temperatures / Andrea Johnson Photography
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.