Accumulated winter chill is decreasing in the fruit growing regions of California

We examined trends in accumulated winter chill across the fruit growing region of central California and its internal coastal valleys. We tested the hypothesis that global warming is in motion in California and is causing accumulated winter chill to decrease across the fruit and nut growing regions of California. The detection of potential trends in accumulated winter chill (between 0 and 7.2 degrees C) was determined using two complementary climate datasets. The California Irrigation Management Information System (CIMIS) contains hourly climate data and is suitable for computing accumulated chill hours and chill degree-hours. But, its longest data records extend back only to the 1980s. The National Weather Service Coop climate record is longer, extending beyond the 1950s at many sites. But its datasets only contain information on daily maximum and minimum temperatures. To assess long term trends in winter chill accumulation, we developed an algorithm that converted information from daily maximum and minimum temperature into accumulated hours of winter chill and summations of chill-degree hours. These inferred calculations of chill hour accumulation were tested with and validated by direct measurements from hourly-based data from the CIMIS network. With the combined climate datasets, we found that the annual accumulation of winter chill hours and chill degree hours is diminishing across the fruit and nut growing regions of California. Observed trends in winter chill range between -50 and -260 chill hours per decade. We also applied out analytical algorithm to project changes in winter chill using regional climate projections of temperature for three regions in the Central Valley. Predicted rates of reduced winter chill, for the period between 1950 and 2100, are on the order of -40 h per decade. By the end of the 21st century, orchards in California are expected to experience less than 500 chill hours per winter. This chronic and steady reduction in winter chill is expected to have deleterious economic and culinary impact on fruit and nut production in California by the end of the 21st Century.