A climate model
Led by Noah Diffenbaugh, an associate professor of earth sciences at Stanford, the study took the climate panel's "middle of the road" predictions for the concentration of greenhouse gases in the atmosphere and ran a computer climate simulation to see what daily maximum and minimum temperatures would be generated across the world over the next 30 years.
Their climate model, basically a complex mathematical equation, incorporates the effects of variables like coastal winds, atmospheric pressure and topography, and was tested against observed temperature data from a network of weather stations to correct any glitches. "It's the most detailed climate model simulation for the United States that exists," Diffenbaugh said.
That's not detail enough for winemaker Christopher Howell of Cain Vineyard & Winery, west of St. Helena, who described studies showing average temperature trends as "too broad (of a) brush" for an area with weather that can vary within a single vineyard. Though analyses of historical temperature trends can be valuable for farmers who "live season by season," Howell said he has yet to see a study that fully describes how temperature connects to the grapevine.
The connection is important because, while most climate models signal at least a slight warming in Napa, not all of that warmth would matter to the grapes, said James Wolpert, a viticulture extension specialist at UC Davis.
"It could be warmer in January, February, April, and March and I don't care," he said. "Talk to me about August and September and October," the final ripening months in the growing season - the months when too much heat, especially in the daytime, can make or break a grape berry.
In the midst of these alarming predictions, a team of scientists commissioned by the Napa Valley Vintners released a study showing that so far, Napa may not have warmed as much as other studies suggest. Napa Valley's longest-running weather stations - and so the only stations suited to evaluating past climate patterns - may have recorded artificially warmer temperatures because of their locations close to urban heat sources.
"There's a number of climate models that would suggest that temperatures are going to rise by 5 degrees or more. That's quite a lot," said Daniel Cayan, a climate researcher at the Scripps Institution of Oceanography and the study's lead author.
He estimated that Napa has probably seen closer to a 1- to 2-degree increase in the past 60 years instead of the 4 to 5 degrees recorded by the weather stations throughout the valley. That could mean that Napa's temperatures, while still on the rise, will proceed at a slower pace, and may be further off into the future, than scientists originally forecasted.
"In some sense, it's probably a little bit of a relief," he said.
Some studies even predict that the warming of California's interior may draw cooler air from the ocean and actually cause Napa Valley to cool down, which seems to correspond to recent years of chillier weather. But both Diffenbaugh and Cayan, at least, think of those years as variations on the overall theme of a steadily warming Napa.
Even with the moderating effects of the coastal marine layer on Napa's hot seasons, "It's probably overly optimistic to think that Napa is not going to warm," Cayan said. It's just a matter of how soon Napa will warm and by how much - and whether that will matter to the grapes.
The NVV's study, said communications director Terry Hall, showed "that we have a lot more to learn."
"You're healthy now, but in 30 years you may have cancer. You're not going to get chemo today," Hall said. "You can't treat a problem that's not there yet."
"It's like crying wolf, so to speak, right now," Dolan said. "You're not going to stir anybody to change. It will be an evolution of consciousness. It will happen person by person by person."
It is one thing to accept an abstract idea of global climate change. It is another to acknowledge its impacts on what, for California, amounts to a $51.8 billion industry - an industry, Hall notes, that is largely made up of family-owned businesses - and begin to respond.
But Diffenbaugh, who devotes much of his time to studying the effect of climate change upon health and humans living in poverty, is confident that grapegrowers can adapt to climate change, and, indeed, have been adapting for some time.
"We have tremendous ability to not only cope with but optimize a huge variety of climatic conditions," he said. "Independent of any climate change, humans are tremendously ingenious in coping with and taking advantage of a diversity of environments."
And if Napa winemakers can adapt their vines to average growing season temperatures of 71.6 degrees F and weather a month of temperatures 95 and higher, his study showed that Napa will actually gain land for premium grape-growing.
The world reacts
Other parts of the world are already adjusting to the sorts of heat that may be on the way for Napa. Spanish winemaker Miguel Torres has begun to develop vineyards at higher elevations in the Pyrenees Mountains as a climatic "backup plan." His daughter, Mireia Torres, is leading the push to develop wine grape varietals that can tolerate greater heat. Vineyards in South Australia have teamed up with the government in a similar effort. Jones, for his part, is studying regions in India and China that may prove well suited to premium wine grape-growing. Nor is climate the only harbinger of change, Jones said. This generation's changing tastes could have more of an effect on wine flavor than even the warming planet.
Some areas that were once too cold to produce much premium wine are now reaping the benefits of warming. Germany, which lost much of its wine-producing acreage to the cold as part of "the little ice age" that hit Europe in the 16th to 19th centuries, "is loving it," Dolan said. Parts of Oregon and the East Coast may benefit, while the Central Valley "would simply be too hot to produce a usable crop," wrote wine specialist Oz Clarke.
With statements like that, it might be easy to imagine a Dust Bowl-type scenario with growers loading their gear into a truck and driving off in search of cooler climes. Or pulling up their vine rows to make way for more heat-hardy grapes - a costly step to take because it takes three years for vines to bear fruit. Replanting a single acre can cost between $15,000 to $25,000, Sinskey estimated.
For now, growers already have a variety of tools to help their vines cope with the heat. They can change their trellising systems so leaves shade and protect grape clusters from sunburn, or set up shade cloths between the vine rows. They might even set up misters to routinely spritz the vines when it gets too hot.