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124 { THE SOMM JOURNAL } OCTOBER/NOVEMBER 2015 { wine science } Temperature In general, California is expected to see increases in temperatures as a result of climate change. For the Central Coast, models are predicting a 0.6–1.7 o C increase, while the North Coast is expected to see increases of 0.5–1.1 o C. Research funded by the Napa Valley Vintners found that since 1931, temperatures have been increasing in that region by about 0.03 o F every year, though there is significant varia- tion between urban and non-urban areas within the valley. Between 2000 and 2039, some models are predicting increases in growing degree days from 140 to 340 in the Central Coast, and from 60 to 220 on the North Coast. Additionally, the number of growing degree days over 35 o C are expected to increase by up to 17.5 and ten days on the Central and North Coasts, respectively. One major problem with increasing tem- peratures in California is that many regions are already at the upper limit in terms of optimal temperatures for premium winegrape growing. Most wine grapes are grown within a 10 o C range, between 12 o C and 22 o C, while some varieties are more sensitive and can only grow within a much smaller temperature range in order to produce premium quality wines. Using the Winkler Index for vineyard suitability, which evaluates an area based on growing degree days, a 2010 study found that there are several places in California, particularly in the Central Valley, that are already bordering on "too hot" for grapes and that further warming would be detri- mental for grape growing in these regions. For example, 78% of the vineyard sites in the Lodi AVA fall into the Winkler Index Class IV (hot), with the remaining 22% falling into Class V (very hot). The Madera AVA is even hotter, with 100% of the vineyard sites falling into Class V, the hottest class possible before an area is considered unsuitable for winegrape growing. As a comparison, 56% of sites in Napa Valley AVA fall into Class III, right in the middle of "too cold" and "too hot", with just 30% falling into Class IV. Similarly, in Paso Robles AVA, 49% of vineyard sites are considered to be Class III, while 37% are in Class IV. As of 2010, none of the sites in Napa Valley or Paso Robles fell into the "very hot" category. With increasing temperatures and the associated increases in growing degree days, many vineyard sites in Class IV and in particular Class V may find themselves producing lower quality grapes and wine, necessitating immediate mitigation and adaptation. Precipitation and Soil Moisture It is unclear exactly how precipitation will change in California, but the general consen- sus is that some regions may see increased precipitation, while others (e.g. more south- ern regions) might see less. Additionally, changes in ocean dynamics due to climate change may affect fog production, a meteo- rological phenomenon important for some grape growing regions in California, though more research on this is needed. Changes in soil moisture will be inevita- ble simply based upon predicted increasing temperatures. As temperature increases, evaporation rates will increase, leading to drier soils and ultimately more stress on the plants themselves. Climate Change Predictions by 2039 In California Wine Country Since 1931, temperatures have been increasing in Napa Valley by about 0.03°F every year. NORTH COAST • Increases in temperature by 0.5–1.1°C • Increase in growing degree-days from 60 to 220 • Growing degree-days over 35°C are expected to increase by up to 10 days CENTRAL COAST • Increases in temperature by 0.6–1.7°C • Increase in growing degree-days from 140 to 340 • Growing degree-days over 35°C are expected to increase by up to 17.5 days 2,500 degree-days or less 2,501 to 3,000 degree-days 3,001 to 3,500 degree-days 3,501 to 4,000 degree-days Over 4,000 degree-days Climate Regions As measured by degree-days I II III IV V In the Napa Valley AVA, 56% of sites fall into Class III, right in the middle of "too cold" and "too hot." In the Lodi AVA, 78% of the vineyard sites fall into the Winkler Index Class IV (hot). In the Paso Robles AVA, 49% of vineyard sites are considered to be Class III, while 37% are in Class IV. GRAPHIC: TOM DONCHEZ