When Paul Dolan first began purpling his hands with the juice of Mendocino County grapes, he was a young winemaker producing organic wines in a challenging, cool-weathered zone. It was 25 years ago, and he remembers struggling under the mild, late-summer sun of the era to bring his Cabernet Sauvignon, Syrah and other red wines into the 12 and 13 percent alcohol range.

Today, as owner of Paul Dolan Vineyards, an older, more experienced Dolan still struggles—struggles to keep his wines below 15 percent ABV, for the climate, says Dolan, isn't what it used to be. Higher temperatures and more sun produce higher levels of sugar in his ripening grapes, and winemakers around the world have felt similar effects of the same warming trend. Dolan's consulting winemaker, in fact, is a Frenchman, Christian LeSommer, who works at Chateau Latour in Bordeaux. LeSommer once had to encourage and coddle his grapes to produce enough sugar just to hit 10 percent ABV. Now, the very same vineyards are producing fruit bombs as big as some California Zins.

In the Napa Valley, too, numbers are rising. According to Dr. Greg Jones, a geography and environmental studies professor at Southern Oregon University, wines in the Napa Valley increased in average alcohol levels from 12.5 percent by volume to 14.8 percent between 1971 and 2001. Winemaker decisions have surely contributed to these hotter wines, but Jones, whose research focuses on natural ecosystems and agriculture and their relation to climate, attributes at least 50 percent of the change to warmer weather each season. In fact, temperature stations in the Napa Valley have recorded an increase in low nighttime temperatures of 3 degrees Celsius since 1930.

Jones predicts that further change of the same magnitude could dramatically shake up California's wines, and in the Napa Valley, where wine has made many rich, those invested in grapes have plenty to lose. Fine wine, which depends so much on terroir, looks likely to be affected first and most dramatically, and such wines can serve as an extremely sensitive barometer to environmental change. In fact, historians frequently cite the geographical north-south shifts of prime European wine country during the past millennium as evidence of the tremendous changes caused by the Little Ice Age and, before it, the Medieval Warm Period, when even Scots and Scandinavians grew grapes. Today, wine culture may be similarly changing, with a prosperous new region already in development in southern England.

But why not just read the outdoor thermometer to decide if our globe is warming? Because, says Dr. Jones, chronologies of the natural world—including grapevines and grape culture, growth patterns of other plants, and the activity of glaciers—serve as plain, easy-to-read measurements of changes that may be too complex for a thermometer to express.

"Changes in temperature or precipitation tell you one thing, a numerical plus or minus, but knowing how aspects of the natural environment respond can give meaning to the numbers," Jones explains.

Dr. Kim Nicholas Cahill, a researcher at UC Davis, has been studying climate change and its potential effects on winegrowing since 2003. She knows that glaciers and ice caps are melting, and countries far away face desertification and flooding. She acknowledges that wine is a luxury item we can live without. "But it's a touchstone crop, something that we put on the table every day," Cahill says. "People often have a hard time conceptualizing what these little differences in temperature mean, but wine is basically climate change that we can taste."

In September 2009, the international environmental watchdog group Greenpeace reported that the optimal latitude for growing wine grapes could advance northward (and southward down under) by roughly 600 miles in the coming decades. The implication is that zones too cold today to produce quality grapes could become prime wines zones in the near future.

Kent County, England, seems already to be prospering from this expected shift. Here, a small and growing collection of winemakers are now at work, winning awards for various white wines and praising their good fortune. The 2009 vintage is already being recognized as their finest on record. But local winemaking only began in the 1970s. Prior, a balmy era called the Medieval Warm Period that began around the year 800 and lasted roughly five centuries produced temperatures believed to have been comparable to those we're seeing today. (Jones even believes temperatures today have surpassed those of the Medieval Warm Period.) During this fruitful time, winemaking occurred as far north as 60 degrees latitude from the equator. Winemaking occurred in Scandinavia at the time, and some records suggest that grapevines grew in Greenland.

Today, areas currently considered excellent for fine winegrowing could become too hot. The Iberian Peninsula, say experts, seems to be taking a hard hit from global warming, and by some accounts things aren't looking good for legendary Pinot Noir regions in France. Across Europe, records in 11 regions dating back 50 years show an increase in average temperature of about 1.7 degrees Celsius. In California, climatologically a mirror of Portugal, a 3 degree Celsius increase in average temperature could render the entire Central Valley, including prime Zin country near Lodi, relatively useless for producing grapes.

The same prediction, based on a 1997 climate projection model designed in part by Dr. Peter Thornton, would place the Napa Valley on the fringe of becoming too hot for grapes. Another degree or two, one could speculate, might push the valley over the edge. Some scientists have even predicted that temperatures in California could increase by 10 degrees Fahrenheit in the next 100 years.

"That's basically the difference in temperature between Los Angeles and Death Valley," says Dr. Cahill at UC Davis. "Obviously, those are two extremely different climate zones, and that gives you an idea of what these changes might mean."

Cahill studied Pinot Noir grapes in the North Bay in the summers of 2005, 2006 and 2007. Her observations indicate that grape yields are liable to decrease under the stress of increased heat. Content levels of grape skin pigments and tannins could also decline, significantly lessening the durability and complexity of the region's fine wines, Cahill says.

But in the Sierra foothills, John MacCready isn't sure that global warming has dramatically affected his line of work. The winemaker at Sierra Vista Winery, based southeast of Sacramento, believes the high-alcohol-wine syndrome of California and, increasingly, the world is more a symptom of winemakers leaving their grapes to hang longer before harvest than it is a symptom of global warming. MacCready says he has not personally experienced any increased challenges to growing fine wine, and he isn't particularly concerned about the near future.

"People talk about climate change impacting winemaking, but I just don't see it," MacCready says. He suspects that renowned wine critics who favor high alcohol fruit bombs wines have spurred the trend toward the stronger, hotter wines of California, Australia and even France.

Marco Cappelli, winemaker at Miraflores Winery near Placerville, also believes the effects of climate change won't affect business as usual for two decades or more. As a winemaker on the slope of the Sierras, Cappelli sees room to adapt if needed by simply planting grapes at higher elevations. Experimenting with new varieties will also be an option. For the present, says Cappelli, consumer and critic preferences will be among the leading evolutionary forces on winemaking.

Greg Jones says he wouldn't expect all winemakers, especially not those with a firm stake in the state's best winemaking regions, to publicly announce that they may have a problem, but a group of Napa Valley's winemakers has officially done so. In 2006, the Napa Valley Vintners Association founded a climate change task force charged with the mission of determining what may occur, weather-wise, in the long-term future and how the valley's winemakers might respond to the changes. Napa Valley winemaker Chris Howell, a task force member, sees nothing to lose by addressing what could become a problem.

"I see no point in people in the Napa Valley going into denial over climate change," says Howell, who makes several red wines at Cain Vineyard and Winery. Howell hasn't experienced significant winemaking difficulties yet, but he expects to. "It doesn't help to say it isn't happening."

In a valley famous for $100 bottles of wine, says Howell, vintners and growers "have every interest in maintaining the status quo."

"Of course we could adapt," he says. "But do we want to? We'd rather learn to grow the same varieties differently."

David Graves, Saintsbury Vineyard winemaker and a co-member on the task force, says growers have room to adjust to warming.

"We have a lot of tools in the toolbox that we can use before we even have to think about changing varieties," says Graves, who makes several wines from Chardonnay and Pinot Noir grapes. Training the vines to produce fruit farther from the ground, which absorbs the sun's heat and releases it day and night, could counteract warming trends. Boosting irrigation in the fall could inhibit grape shriveling, and encouraging excess foliage could shade the fruit from the sun, but eventually warming effects may be impossible to stave away.

Howell agrees. "Winegrowing techniques can adjust for warming, but beyond a point of technical adaptation, we couldn't make it work anymore."

Up north in Mendocino County, Dolan says in the worst-case scenario he might become a peach grower.

"There's no running away from [climate change], no escaping it," says Dolan. "And at some point in time, it's going to become a game-changer."

In December, the Carnegie Institute of Science, UC Berkeley and the California Academy of Sciences published a report predicting that within the next century animals and plants will have to move as much as six miles per year in order to keep within their temperature comfort zones. Of all organisms, birds may be best suited to maintain such a pace, while wild plants may suffer.

But grapes, at least, are in good hands, tended by men and women with tremendous financial incentive to maintain business as usual. Still, even the powerful wine lobby can't stop climate change if it should steamroll forward on projected paths. For people invested emotionally and financially in the region, paradise could be lost, says Howell.

"When people decide to put down roots in a place, they do it because they like the place and would prefer that it remain that way," Howell says. "We're a society fearful of change."

Dolan too has a lineal reason to fear a warming globe.

"The land we own is really important. We don't plan on running away from the ground we're working. My sons are fifth-generation and they hope to be here for a long time, and we don't want to have to grow peaches."

Howell sees two possible outcomes for Napa Valley. Which, he asks, will it be?

"Will our wines taste like ambrosia, or will it taste like it was grown in Modesto?"

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