Observed changes in agroclimate metrics relevant for specialty crop production in California

Authors: PARKER, LAUREN - University Of California, Davis; ZHANG, NING - University Of California Agriculture And Natural Resources (UCANR); ABATZOGLOU, JOHN - University Of California; Ostoja, Steven; PATHAK, TAPAN - University Of California Agriculture And Natural Resources (UCANR)

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2021
Publication Date: 1/15/2022
Citation: Parker, L.E., Zhang, N., Abatzoglou, J.T., Ostoja, S.M., Pathak, T.B. 2022. Observed changes in agroclimate metrics relevant for specialty crop production in California. Agronomy. 12(1). Article 205. https://doi.org/10.3390/agronomy12010205.
DOI: https://doi.org/10.3390/agronomy12010205

Interpretive Summary: Climate "normals" are 30-year averages of climate conditions that provide information about what the typical climate is for a location. Climate normals are calculated every 10 years, and the most recent normals were calculated in 2021 and cover the years 1991-2020. For agricultural producers there are additional measures of climate, known as agroclimate metrics, that give a more meaningful picture of climate as it relates to growing crops. When averaged over 30-year periods, we refer to these as agroclimate normals. Understanding how these agroclimate normals change over time can be important for farm management decisions. This research calculated 12 agroclimate metrics for the two most recent normal periods (1981-2010 and 1991-2020) and determined the change between these two periods. The results showed that heat exposure is more frequent and that growing seasons are longer in the new normal period (1991-2020) as compared to the previous normal period (1981-2010). Similarly, the 40-year trends in agroclimate metrics were consistent with what we would expect given global warming. The changes in agroclimate normals and the 40-year trends provide observational evidence of a changing climate. These changes underscore the need for adaptation in order to maintain agricultural production.

Technical 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.