Warmer temperatures may threaten California vineyards, study says

In the next 30 years, high-value vineyards in California could shrink by 50% because of global warming, according to a Stanford University study released last week.

Scientists applied scenarios from the Intergovernmental Panel on Climate Change in a computer model and found that Napa and Santa Barbara counties could experience 10 more very hot days — 95 degrees or higher — during the growing season.

As a result, the amount of grape-growing land is projected to decline over the next three decades, the authors wrote.

“There will likely be significant localized temperature changes over the next three decades,” said Noah Diffenbaugh, coauthor of the study and a Center Fellow at the Woods Institute for the Environment at Stanford. “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.”

Growers of high-value varietals in California may need to account for warmer weather and integrate climate information into their cultivation and practices, Diffenbaugh said. Two areas forecast to have cooler temperatures, Yamhill County in Oregon and Walla Walla County in Washington, can prepare for more optimal growing seasons, he said.

“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 said. “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.”

The study, which has been peer-reviewed but not yet scrutinized by the larger scientific community, is based on the Copenhagen Accord’s target of holding global warming to no more than 3.6 degrees above pre-industrial levels. Under a conservative scenario, that could put the average global temperature 1.8 degrees higher than it is today, Diffenbaugh said.

Researchers compared the computer model’s simulations with weather data collected between 1960 and 2010 to make sure their model could accurately replicate past temperatures. They combined new and historical data and found that the four counties surveyed were likely to experience higher average temperatures during growing seasons.

Certain varietals, such as Pinot Noir and Cabernet Sauvignon in the Napa Valley, grow at average temperatures of 68 degrees, with fewer than 30 very hot days. With more very hot days and an average temperature of 70 degrees, that region would be less hospitable for growing those varietals.

On the other hand, Yamhill and Walla Walla counties will see more land suitable for high-value varietals.

These projections could heavily affect California’s $16.5-billion wine industry, which produces more than 5 million gallons per year on 500,000 acres of vineyards and accounts for nearly 90% of the nation’s total wine production, according to the Wine Institute, a state winemakers trade organization.

Diffenbaugh suggests winemakers adapt to warmer conditions by planting heat-tolerant vines that can survive up to 45 very hot days and average temperatures of 71 degrees. But such varietals can produce lower-quality wine. Growers can also use trellis systems to shade vines, use irrigation to cool plants and adjust fermentation processes.

ashlie.rodriguez@latimes.com