Grape Expectations

Producers of high-quality wine around the world have been bracing for warmer temperatures. Wine makers in Spain and Australia have been confronted with temperature increases during recent decades, which is raising concerns for the US wine industry. In California, this anticipation has increased with a cascade of studies projecting that California, along with the rest of the world, could warm substantially in the next half-century.  Adding to this, studies that have considered climate stations in Napa Valley report a warming trend over the last several decades. All of this could portend a decline in the ability of Napa Valley to produce grapes for fine wine as climate warming advances through the 21^st century.

But the Napa vintners gained a somewhat different perspective when a recent study led by Scripps Institution of Oceanography, UC San Diego, found that although there has likely been significant warming in the region, issues with the placement and exposure of temperature gauges in and around Napa might have led to an overstatement of temperature increases.

The Napa Valley Vintners, a non-profit trade association, commissioned the detailed study of climate patterns in Napa Valley. The study was led by Scripps climate researcher Dan Cayan and included Kimberly Nicholas, a graduate student at Stanford University when the study began and who is now an assistant professor of sustainability science at Lund University Centre for Sustainability Studies (LUCSUS) in Lund, Sweden, and Scripps researchers Mary Tyree and Michael Dettinger.

 “Taken at face value, temperature records from available long standing weather stations would lead us to think that the Napa region has warmed by more than 3Ëš F (1.6Ëš C) since 1930”, said Cayan.   “However, in comparing these with records collected in and near Napa vineyards, the warming in the Napa region appears to fall in line with broader based trends across the western United States and the eastern Pacific Ocean, between 1 and 2Ëš F (0.5 and 1.1Ëš C) since the 1930s.”

Wine grapes are a unique crop in terms of the extensive attention that is required to understand how environmental conditions affect timing of grape growth stages, from budbreak to final grape composition and flavor.  Temperature increases of more than 3Ëš F may push some important wine grape species into conditions outside of their optima for production, flavor and other characteristics.

“Probably corn, potatoes, a carrot or a strawberry are just as sensitive to the growing conditions, but with wine, we pay really close attention,” said Christopher Howell, a winemaker at Cain Vineyard in Napa Valley. “The main point is that humanity has a long tradition (10,000 years) of harvesting grapes, making wine, storing it in vessels, marking the vintage year and vineyard, and then being able to taste the same wine again and again, sometimes over many years, and being able to compare that wine with other vintages and other vineyards.”

The research team used these detailed records as a part of their examination of Napa Valley climate. Records of the timing of grape growth stages as well as the overall sugar content of the grapes were both analyzed for their potential to estimate past climate.

Working with the Napa Valley Vinters, the team collected an unprecedented set of observations from growers and other community members. These records came from diverse sites within Napa Valley and from wide-ranging sources, including private weather stations and hand-written private records. Data from NOAA’s Cooperative Observer (COOP) stations and State of California CIMAS weather records supplemented the local data.  Previously, the COOP stations, whose records date back to the 1930s or earlier, had been used in studies of Napa Valley climate, but the team found that these key stations were installed in problematic locations including one near an air-conditioning outlet and another on the roof of a building. Comparing local community temperature records with those from the COOP stations, the team found that while there has very likely been warming over the last several decades, the overall warming trend was significantly less than the trend obtained using the COOP stations alone.

Consistent with other observations of warming drawn from stations across western North America, the observations indicate that warming in Napa Valley occurs primarily during the nighttime. The strongest warming has occurred in spring but is also found during winter and summer months.

The reduction in estimated historical warming is good short-term news for Napa Valley. However, Napa Valley is very diverse in both geography and climate, and this study highlights the need for better monitoring of an accurate representative of this diversity.

“The lack of precision that we can obtain from the Napa area historical records underscores the importance of high-quality climate observations and establishing a deliberate mechanism to maintain them over future generations,” said Tyree.

 Grapevines are a multi-decade, even a multi-generational investment. To prepare for the future, vintners will benefit most from understanding the most likely scenarios of change, including which locations in the valley or times of day or seasons may be most prone to warming, the researchers said.  Rex Stults, industry relations director with the Napa Valley Vintners, said that this study will be incorporated into the group’s new three-year strategic plan, requiring collaboration between local government, vintners, and growers.

The reduction in estimated historical warming is good short-term news for Napa Valley. Rex Stults, industry relations director with the Napa Valley Vintners, said that this study will be incorporated into the group’s new three-year strategic plan, requiring collaboration between local government, vintners, and growers.

Ultimately the fate of Napa Valley, like all other agriculture communities, will be affected by larger-scale warming trends. Nicholas noted that California is projected to experience warming in the coming decades and century.  The pathway of future greenhouse gas emissions will likely determine how easy it will be to sustain traditional wine grape growing.

“The more warming we experience, the more difficult and expensive it will be to continue to grow fine wine grapes,” she added, “and for that matter, for other species and ecosystems to continue to function at their current state. It’s not just about wine.”

— Kelley Gallagher is a second-year student with microbial ecologist Paul Jensen.