Abstract
Every decade, a suite of standardized climatological metrics known as climate normals are updated, providing averages of temperature and precipitation data over the previous 30-year period. Although some of these climate normals are directly applicable to agricultural producers, there are additional agroclimate metrics calculated from meteorological data that provide physiologically relevant information for on-farm management decisions. In this study, we identified a suite of energy-based agroclimate metrics and calculated changes over the two most recent normal periods (1981–2010 and 1991–2020), focusing on specialty crop production regions in California. Observed changes in agroclimate metrics were largely consistent with broader global warming trends. While most metrics showed small changes between the two periods, during the 1991–2020 period, the last spring freeze occurred ~5 days earlier as compared to the 1981–2010 period, contributing to a >6 day longer frost-free period in the Sacramento and Salinas Valleys; likewise an additional 6.4 tropical nights (Tn > 20 °C) occurred in the Coachella Valley during the 1991–2020 period. A complementary trend analysis of the agroclimate metrics over the 1981–2020 period showed significant increases in growing degree days across all agricultural regions, while significant increases in heat exposure were found for the Salinas and Imperial Valleys and over the Central Coast region. Moreover, summer reference evapotranspiration increased approximately 40 mm in California’s Central Valley during 1981–2020, with implications for agricultural water resources. Quantifying the shifts in these agroclimate metrics between the two most recent 30-year normal periods and the accompanying 40-year trends provides context for understanding and communicating around changing climatic baselines and underscores the need for adaptation to meet the challenge that climate change poses to agriculture both in the future and in the present.
Highlights
The calculation of standardized climate metrics averaging climatological data over periods of 30 consecutive years was established by the World Meteorological Organization in 1956 and adopted by the U.S Weather Bureau in the early 1960s [1,2]
Our crop mask includes locations with almonds, walnuts, winegrapes, lettuces, and tomatoes, which represent climatic, geographic, and cropping system diversity in high-value specialty crops, allowing our results to illustrate the relationships between our selected agroclimate metrics and crop production; the majority of total U.S production of these crops comes from California, and they are economically important for the state and the regions in which they are grown (Table 2)
The changes in agroclimate metrics between normal periods are shown in Figure 2 and the 40-year trends in metrics are shown in Figures 3 and A1–A7; Table 3 presents metric changes, trends, and significance
Summary
The calculation of standardized climate metrics averaging climatological data over periods of 30 consecutive years was established by the World Meteorological Organization in 1956 and adopted by the U.S Weather Bureau ( the National Weather Service) in the early 1960s [1,2]. A formal assessment of changes in agroclimate metrics can provide an improved understanding of recently observed climate changes in a context relevant for agricultural producers, a means of communicating around climatic baselines against which farm management decisions are made, and aid in guiding forward-looking adaptive management plans and associated investment allocation strategies at multiple scales. Given this relevance, and in light of the absence of such an assessment within the literature, we (1) analyzed changes in agroclimate metrics in California between the two most recent normal periods (1981–2010 and 1991–2020) and (2) assessed trends in the metrics over the 40-year period. We focus on these changes within the context of high-value specialty crop production
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