"We know we have to adapt," says Avery Heelan, a winemaker at Larkmead Vineyards in Calistoga. "We can't just pretend that it's going to go away, because all we see is each year it's getting more and more extreme."

Still, blending with other grapes comes with risks. For a U.S. wine to be labeled cabernet, a bottle must contain 75% cabernet grapes or more. Any less, and it's considered a red blend. Blends typically don't command the same prices on store shelves as cabernet, especially since consumers are accustomed to picking U.S. wines by the name of the grape. Moving away from cabernet would be a major financial gamble for Napa's multibillion-dollar wine industry.

"It is a big shift," says Elisabeth Forrestel, an assistant professor of viticulture and enology at the University of California, Davis. "Without the market changing or demands changing, you can't convince someone to grow something that doesn't sell or doesn't garner the same price."

Some grapes growing at Larkmead Vineyards aren't ones that many American wine drinkers would recognize. Long rows of vines are labeled: touriga nacional, aglianico, charbono and tempranillo.

"There's not a huge market for a lot of these varieties," says Heelan, walking among the vines on a hot summer afternoon. "We're really choosing them not from popularity, but for their qualities."

Established more than a century ago, the winery is known for its bottles of cabernet sauvignon. These lesser-known grapes were planted only a few years ago, part of a research vineyard that took the place of cabernet vines.

"Which most people would probably think is a little crazy, considering it's 3 acres of perfect cabernet land," Heelan says. "But certainly with the climate and how dramatically it's changed over even the last 10 years, we really have to start adjusting."

The vineyard is already at the hotter northern end of Napa Valley, but the extreme heat in recent years has been a wake-up call. A late-summer heat wave in 2022 hit temperatures just under 120 degrees at the vineyard, she says.

"When it gets that hot, the vines, they're done," she says. "They're going to go dormant, and when that happens, they're not ripening anymore."