Hotter climate may be too warm for wine

STANFORD (US) — The prime real estate for growing premium wine grapes in Northern California could be cut in half in the next 30 years if global warming continues as predicted.

The new study, published in the journal Environmental Research Letters, follows an earlier climate study in 2006 that projected that as much as 81 percent of premium wine grape acreage in the country could become unsuitable for some varietals by the end of the century.

“Our new study looks at climate change during the next 30 years—a timeframe over which people are actually considering the costs and benefits of making decisions on the ground,” says Noah Diffenbaugh, assistant professor of environmental earth system science at Stanford University.

Climate change, from global to local

California alone produces on average more than 5 million gallons of wine per year, accounting for about 90 percent of the nation’s total wine production, according to the Wine Institute, a trade organization representing California winemakers. The estimated retail value of the state’s wine industry in 2010 is $18.5 billion.

The new study focused on premium wines—the 25 percent most expensive wines on the market—and how global warming might affect growing conditions in four premium wine-producing counties by 2040: Napa and Santa Barbara counties in California, Yamhill County in Oregon’s Willamette Valley, and Walla Walla County in Washington’s Columbia Valley.

“We focused on these counties because their mild climates have made them major sources of high-quality grapes, and because they represent both cool and warm growing conditions,” Diffenbaugh says. But that could change, and soon.

“There will likely be significant localized temperature changes over the next three decades,” Diffenbaugh says. “One of our motivations for the study was to identify the potential impact of those changes, and also to identify the opportunities for growers to take action and adapt.”

Change for lovers of fine wine

The study was based on the assumption that there will be a 23 percent increase in atmospheric greenhouse gases by 2040, which could raise the average global temperature by about 1.8 degrees Fahrenheit (1 degree Celsius), a conservative scenario.

“World governments have said that to reduce the negative impacts of climate change, global warming should be limited to an increase of 1 degree Celsius,” Diffenbaugh says.

To predict how much land area will be suitable for premium wine grape cultivation in coming decades, researchers used a computer model that incorporated local, regional and global conditions, including factors such as coastal wind speeds and ocean temperatures and then compared the simulations to actual weather data collected between 1960 and 2010 to see if it could accurately replicate past temperatures.

The climate model and the historical weather data predict that by 2040, all four counties are likely to experience higher average temperatures during growing seasons, along with an increase in the number of very hot days when the thermometer reaches 95 F (35 C) or above.

Grape varieties were divided into separate categories based on tolerance to different temperature ranges. For example, Napa Valley—known for its pinot noir and cabernet sauvignon—has historically experienced growing seasons with an average temperature of less than 68 F (20 C) and fewer than 30 very hot days.

The average temperature in Napa Valley during the growing season could increase as much as 2 F (1.1 C), with the number of very hot days increasing by 10. Such a change would make the amount of land with historically hospitable growing conditions shrink by half over the next three decades.

In Santa Barbara County, the amount of suitable grape-growing acreage with similar climate conditions is projected to decline by more than 20 percent as temperatures rise.

“I was surprised that local temperature changes could have such a big impact on an important industry with only 1 degree Celsius of global warming,” Diffenbaugh says.

Higher temperatures are also predicted in Oregon and Washington by 2040, but with potentially different outcomes for winegrowers.

Oregon’s Willamette Valley could see a slight increase in the amount of total suitable acreage and a large increase in area suitable for more valuable varieties, according to the study. But in Washington’s Columbia Valley, varietals that are sensitive to severely hot days could see a 30 percent reduction in suitable land area.

Risky business

Suitable acreage in Napa and Santa Barbara counties could be increased if growers are able to produce quality grapes that can tolerate up to 45 very hot days and average temperatures of 71 F (22 C) in the growing season, the study finds.

But varieties currently grown in those conditions tend to produce considerably lower wine quality and value.

With their knowledge of which cultivation techniques are most appropriate in a given climate, growers could benefit from the study’s forecasts of temperature conditions.

“Climate change over the next few decades is of particular relevance for the wine industry,” Diffenbaugh says. “It’s a big investment to put plants in the ground. They’re slow to mature, and once they mature they’re productive for a long time.”

By moving vineyards to areas where less drastic temperature change is likely, growers may be able to maintain the quality of their grapes by using existing cultivation and winemaking techniques, Diffenbaugh says.

Possible strategies include providing special trellis systems that shade vines, using irrigation to cool plants and adjusting fermentation processes in the winery.

“It’s risky for a grower to make decisions that consider climate change, because those decisions could be expensive and the climate may not change exactly as we expect,” Diffenbaugh says.

“But there’s also risk in decisions that ignore global warming, because we’re finding that there are likely to be significant localized changes in the near term.”

Researchers from Utah State University and Southern Oregon University contributed to the study, that was supported by the National Science Foundation.

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