According to simulations run by Noah Diffenbaugh of Stanford University and his colleagues at Utah State University, Southern Oregon University and Oak Ridge National Laboratory, if adaptation strategies are not adopted, the area suitable for premium winegrape production in California could be reduced over the next 30 years, by as much as 50% in some counties. In addition to decreases in suitable area, the researchers project increases in the area that falls in the lower-quality hot temperature classes within some counties of California.

"The temperature regime in the high-value areas of California is near optimal at present, so any substantive change in those areas could be expected to have negative consequences," Diffenbaugh told environmentalresearchweb. "Recent temperature changes have already impacted grapevine phenology and wine quality, and further temperature changes are likely to occur over the next three decades, including intensification of severe hot events."

However, some areas of the western US are currently cold-limited. That suggests that warming could provide some benefit, particularly if the positive effects of decreasing cold limitation outpace the negative effects of increasing hot limitation over the near-term decades. The researchers found the potential benefits of near-term warming to be most prominent in the Willamette Valley of Oregon, which is currently known for its pinot noir styles.

A key motivation for Diffenbaugh and his colleagues was to quantify the potential effectiveness of different adaptation strategies in the face of a changing climate. They found that strategies that enable increased tolerance of severely hot conditions could substantially reduce projected losses of suitable area in California and Washington.

Specific adaption strategies for growers/producers include planting in new locations, planting different varieties or clones, altering vineyard design and/or management and adjusting winery processing. For those that are able to consider new locations, areas that are higher in latitude, higher in elevation, and/or closer to the coast could potentially provide climates that would allow maintenance of style and quality. For those that are not able to consider new locations, the relatively narrow climate suitability of certain varieties could cause even small changes in climate to require shifts to different varieties or newer, more heat-tolerant clones of the same variety.

"There are limits to what can be done to overcome the effects of climate change," said Diffenbaugh. "We know from the present distribution that the highest-value grapes are grown in a narrow climate envelope, and are therefore likely to be sensitive to changes in climate."

The area studied by the researchers was divided into climate classes that are associated with growing conditions for specific winegrape varieties, yielding different wine styles and quality. One metric was the number of growing degree days (GDD), which ranged from Region I climates (1,111–1,390 GDD), which are suitable for early ripening varieties that achieve high quality, through to Region V (2,221–2,499 GDD) climates, which typically make low-quality bulk table wines or fortified wines, or are best for table grape varieties destined for early-season consumption.

The researchers found that although Napa County exhibits very little change in total GDD suitability over near-term decades, substantial losses are projected in the high-quality Region III class. These projected losses are compensated by projected gains in the low-quality warm-to-hot Region V and Va classes, suggesting a decrease in the overall quality and value of the producible area. In contrast, in Oregon's Willamette Valley, losses in the very cool Region Ia class are offset by gains in the higher-quality Region II class, suggesting an increase in the overall quality and value of the producible area.

This recent work is in agreement with previous work by the authors, which projected the effects of late-21st century climate change on wine production in the US. However, the new work suggests a much more immediate emergence of temperature-related changes. "We wanted to look at the changes in the near-term and were surprised by the magnitude of what we found over the next three decades," said Diffenbaugh.

As with all modelling, however, Diffenbaugh points out that there are still improvements that can be made. While the climate model that he and his colleagues used is able to capture the spatial details of temperature in the western US with far more fidelity than the current generation of global climate models, the 25-km horizontal resolution is not sufficient to capture all of the microclimatic features that determine temperature suitability. "An area of particular concern is the dynamics governing changes in the sea breeze and coastal fog, which can ameliorate severely hot temperatures but also lower heat accumulation and increase disease pressure," said Diffenbaugh.

The researchers published their research in Environmental Research Letters (ERL).