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, Diffenbaugh said.

"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," he added.

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

Using the climate model and the historical weather data, the researchers predicted 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.

In the experiment, the scientists divided premium grape varieties into separate categories based on their tolerance to different temperature ranges. For example, Napa Valley – widely known for its pinot noir, cabernet sauvignon and other premium wines – has historically experienced growing seasons with an average temperature of less than 68 F (20 C) and fewer than 30 very hot days. Grapes that thrive in that climate have done well there.

According to the study, 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. As a result, the amount of land with historically hospitable growing conditions could shrink by half over the next three decades, the study found. 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 said.

The study also predicted higher temperatures 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, the results showed.