A scorching future

Diffenbaugh anticipates an increase in the frequency of heat spikes. Extremely hot days (95 degrees Fahrenheit or higher) in California's existing wine regions could occur between 30 to 50 times more often each year by the end of this century. Research into increased heat spikes in Europe's wine regions, he says, indicates a similar increase in the number of extremely hot days.

Not everyone is buying the doom and gloom. Some of Graves' Napa neighbors believe that scientists are exaggerating the risks. Napa's geography, they say, protects it from climate change. And Sonoma's proximity to the coast will keep it cool. That's assuming, of course, the oceans are immune from warming.

"I don't think anyone is trying to deny global warming," says Jim Verhey, a director and manager of Silverado WineGrowers, which owns 200,000 acres in Napa Valley, Sonoma, Monterey and Lodi. "But we believe that the marine influence from the San Francisco Bay mediates the effects on Napa Valley. We believe we haven't seen changes in Napa's temperatures."

It is tempting to speculate that the increasingly higher alcohol levels of California wines are the result of a warming climate. But, according to Jones and other scientists who have studied the relationship of global warming and wine, that's difficult to prove.

California vintners have altered their viticulture practices during the last two decades, increasing the time that grapes hang on the vine. This increased exposure of the fruit to the sun has raised sugar levels, which raises alcohol levels. How much of the higher alcohol levels are due to longer hang time, and how much to rising temperatures? It is impossible to know, they say.

"Our parameters for ripeness have changed," says Paul Hobbs, a winemaker well-known for his ripe, powerful wines. He no longer even tests for sugar levels, he says. By concentrating instead on the phenolic development, referring to the other grape flavors, he finds himself among the last to harvest in Napa. "I farm completely differently than I did 17 years ago," he says. "I can't see any effects from global warming on my grapes."

While Hobbs does believe the global climate is changing, he doesn't believe that, on average, Napa is warmer. Still, "I've seen more heat spikes, absolutely. If that is an indication of global warming, then yes, even in cool vintages we have them," he says.