California's heatwave set records across the state this week, like 115 Farenheit (46.1 Celsius) in the Russian River Valley, which is normally famous for cool-loving Pinot Noir. In Mendocino County the temperature hit 117 on Tuesday. 117! That's Death Valley weather.

Despite that, grapes are being harvested – mostly at night – as grapegrowers adapt to the latest curveball thrown by climate change. The heat will probably lead to a smaller crop, and it's going to force some difficult choices for producers of single-vineyard wines.

"This is going to change the vintage for California," said Patrick Hamilton, viticulturist at Notre Vue Estate Winery in Sonoma County. "For all the big companies – this was not foreseen, but fruit that's sitting in the sun for multiple days at 110 degrees, it's going to be raisins. It's going to fall on the ground."

Here is the good news. Kaan Kurtural, viticulture specialist at UC Davis, said that even though many individual grapes went through different stages of cell death on Tuesday (yikes!), wine lovers need not worry about the overall quality of California wines from this vintage.

"Growers are prepared to deal with these things," Kurtural told Wine-Searcher. "There are many things they do, such as using shade cloth. They will irrigate prior to these events to put some moisture into the vines."

However, it's possible that you will see fewer single-vineyard wines from California this year. Wineries that are not committed to single-vineyard wines might want to blend early-picked grapes, that have higher acid and lower potential alcohol, with later-picked grapes that have denser flavors.

Martha Barra, owner of Barra of Mendocino, sent a snapshot of a thermometer reading 117 degrees at her winery – the hottest it has ever been.

"I do not remember it ever going over 110," Barra said. "The grapes shut down when it gets this hot. We did Chardonnay sugar tests this morning. The pH looks OK but the acid is still way up there. My vineyard foreman said they don't taste like they're ready."

Barra said she expects to get some high-alcohol Pinot Noir from her estate vineyards, but will blend it with some earlier-picked Pinot Noir. She has never picked grapes at night before, but this year the winery made a large light bar that goes over three rows of vines to make it possible.

Barra, who has farmed organically for 33 years, is troubled lately by leafhoppers, a pest that can transmit vine diseases. This is another impact of climate change.

"My vineyard foreman says it's because we don't have the cold winters," Barra said. "Since it's not so cold at night anymore, the little guys want to over-winter with us. That's probably our biggest problem with global warming."

Sonoma County vintner Kathleen Inman normally picks early; her wines like Endless Crush rosé are popular because they're restrained. But she had a crew pick grapes on Tuesday. She hadn't planned to, but she ran into another contemporary California challenge: a labor shortage.

"I brought most of my stuff in last week. I have some new projects with inexperienced growers who have not had all their harvesters lined up," said Inman, while covered in sticky grape skins in the heat. "Nine days ago I told one of the growers I wanted to harvest on Wednesday. At the weekend he said everybody had canceled on him and he had nobody to pick the fruit. You have to make judgment calls about what things can wait by watering them. You have to take a calculated risk based on experience on what has to come in and what doesn't."

Inman chose to harvest some Mount Eden clone Pinot Noir, while leaving other clones in the same vineyard on the vine.

"That clone shrivels if you don't harvest it," Inman said. "It tends to have lots of little berries and those shrivel very easily. The sugar is much higher and when they soak in the rest of the juice, they bring the brix way up."

Kurtural said that the surface temperature for grapes in the sun on Tuesday reached as high as 60 degrees Celsius (140 Farenheit.)

"All the desirable compounds for a wine grape are in the skin of a berry," Kurtural said. "It goes through different stages of cell death. The plant requires a lot more water to keep it hydrated and transpire to cool it down. We started the year in water deficit and we have not been able to catch up."

Shade cloths are a temporary measure, but California growers will have to adapt their planting strategies for the future; many already have.

Kurtural said that if you want to see how growers are adapting to climate change, pay attention the next time you drive up State Route 29, the main drag in Napa Valley. Historically, vines ran parallel to the highway to better capture more sunlight.

"Now they're running perpendicular to the highway to avoid damage from the sun," Kurtural said.

Hamilton said that at Notre Vue, the most profitable grape is Pinot Noir, but he has told the owners not to plant more of it because of the warming climate.

"What's scaring me a little is the night temperatures are higher by an average of 8 degrees a night," Hamilton said. "That's going to change the acidity in the grapes. We're going to have to change the varieties we grow. I've told them we shouldn't plant any more Burgundian varieties. What we have, OK. But plant Rhône varieties. And 20 years from now, plant Spanish and southern Greek varieties. Plant Grenache Blanc."

For now, though, wineries whose fortunes are tied to cool-climate grapes like Pinot Noir and Chardonnay can only bunker down and wait for the heatwave to subside.

"What we're seeing is just a quick accumulation of sugar in just three days," said Scott MacFiggan, owner of Sosie Wines in Sonoma. "It's a decision point. You either want to get your grapes off before the heat spike or you white-knuckle it and see what comes out the other side. It came early enough in the season that the grapes weren't ready to pick. With our Pinot, we're a little worried about whether it might go too far."

But MacFiggan does not plan to blend grapes from different vineyards together.

"We are site and vintage-expressive," he said. "So we'll take it as it comes."

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