Hang on to those wines from Napa Valley because in 30 years, they could be a rare commodity.

A new study appearing in the journal Environmental Research Letters indicates that by 2040 the amount of lands suitable for growing wine grapes could shrink by 50 percent. And it's all due to global warming.

"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," said Noah Diffenbaugh, an assistant professor of environmental Earth system science at Stanford, who co-authored the study.

Focusing on premium wines, the 25 percent most expensive wines, Diffenbaugh and his team looked at how a slight shift in global temperatures, 1.8 degrees Fahrenheit, would affect Napa and Santa Barbara counties in California, Yamhill County in Oregon and Walla Walla County in Washington.

"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 said.

With rising temperatures, however, those conditions could change soon.

"There will likely be significant localized temperature changes over the next three decades," Diffenbaugh said.

"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."

To determine how much land would be suitable for grape growing, Diffenbaugh and his team constructed a computer simulation that factored in not only changes in temperature, but coastal wind speeds and ocean temperatures.

Then they divided grape varieties into categories based on their ability to thrive in certain temperatures.

Napa Valley and Santa Barbara both saw a decline in suitable land, according to the simulations, up to 50 percent loss in some cases.

Walla Walla likewise saw diminished acreage, up to 30 percent. Yamhill, however, actually saw an increase in acreage and of varietals capable of growing in the area.

Wine risks

The study did point out that Napa and Santa Barbara could see an increase in acreage if they started growing more heat-tolerant grapes, but such grapes tend to produce lower quality wines.

"Climate change over the next few decades is of particular relevance for the wine industry," Diffenbaugh said.

"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."

Decisions that growers make now could influence their crops for the next 30 years, especially as they consider whether or not to move vineyards to cooler areas, switch varietals or even change growing techniques.

"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 was supported in part by a National Science Foundation CAREER award to Noah Diffenbaugh.