CLIMATE CHANGE IN THE WESTERN UNITED STATES GRAPE GROWING REGIONS

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Climate is a pervasive factor in the viability of all forms of agriculture and is never more important than with the cultivation of grapes for the production of wine. In the Western United States grapes are grown over an extensive north-south gradient of climate types. In addition, grapevines are typically grown in regions and under conditions that are considered narrow for a specific cultivar’s optimum quality, ultimately putting it at a greater potential risk from climatic variations and change. Therefore, to examine the structure, variability, and trends in climate in the Western United States, this research analyzes growing season average temperatures and degree-days, annual and seasonal frost frequencies, the dates of last spring and first fall frost occurrence, and the length of the frost-free period for the principal grape growing regions in California, Oregon, and Washington from 1948-2002. Results reveal that, on average, most regions have experienced warmer growing seasons, driven mostly by changes in minimum temperatures, with greater heat accumulation, a decline in frost frequency that is most significant in the spring, earlier last spring frosts, later first fall frosts, and longer frost-free periods. While many of these trends may have been beneficial to grape growing and wine production in these regions, an examination of possible future climate change in these same regions indicates an average growing season warming of 1.7°C in the next 50 years. Depending on the seasonal structure and magnitude of climate change in the future, important issues for the wine industry include potential shifts in regional varietal viability and achieving optimum varietal ripeness and wine balance in a warmer environment.