HISTORICAL AND PROJECTED CHANGES IN CLIMATE METRICS AFFECTING APPLE PRODUCTION ACROSS THE U.S.

Shawn Preston

doi:10.7273/000007080

This thesis investigates changing climate risks to apple production across the United States using observations and climate model simulations. Six metrics are analyzed to capture climate conditions over the complete phenological cycle of apples. These climate metrics include cold degree days, last day of frost in spring, chill accumulation, growing degree days, extreme heat days, and warm nights that affect apple production from dormancy through maturation and harvest. Changes in these climate metrics have the potential to negatively affect apple quality, yield, and production as well as increase or alter growing phases. In the first chapter, I examine the spatial climatology and historical trends over the last four decades using a daily, high spatial (4km) resolution gridded dataset - GridMET. This study includes an analysis of the spatial patterns and trends at the national scale, as well as a more detailed investigation of the top three apple-producing counties in the U.S.: Yakima County, Washington, Kent County, Michigan, and Wayne County, New York. While previous studies have examined trends in specific regions of the U.S., a comprehensive analysis of the three most important apple growing counties in the U.S. has not yet been done. In the second chapter, we expand our investigation to examine climate projections in a future warmer climate utilizing a large ensemble of simulations from the Community Earth System Model-Large Ensemble version 2 (CESM2-LE). Large ensembles simulate internal or natural climate variability along with the response to external climate forcings, thus enabling a more robust and comprehensive understanding of the range of plausible future outcomes. Biases in simulating all six metrics in the CESM2 model are evaluated relative to observations (GridMET). Then, climate projections are analyzed for the mid-21 st century under a medium emissions pathway - Shared Socio-Economic Pathways 3.-7.0 scenario - relative to the present climatology. The mid-21st century time period aligns with the planning horizon of stakeholders, and thereby this research on the changing climate risks to apples and similar tree fruits can inform planning and decision-making in the near-term. Overall, our findings indicate that the western and southern regions of the United States have already experienced compounding climate risks, including enhanced warming during early winter months, increases in chill portions in the Northwest from recent trends, and increases in extreme temperature thresholds vital for healthy apple fruit set and production. Secondly, among the top three apple-producing counties in the U.S., Yakima County, WA has experienced the largest and most significant changes since 1979 for cold degree days, last day of spring frost, growing degree days, extreme heat days, and warm nights. Third, using CESM2-LE, we found that all six metrics analyzed are projected to change substantially with warming, with significant trends observed in most regions across the U.S. The Northwest is projected to experience a significant increase in extreme heat days, warm nights, and growing degree days. Additionally, while maximum trends may not be significant in certain areas, there are significant decreases projected in the last day of spring frost and cold degree days. Simultaneously, the Midwest and Northeast regions, key apple production areas in the U.S., are projected to show significant increases in extreme heat days, warm nights, and growing degree days, along with significant decreases in cold degree days and last day of spring frost. Our approach demonstrates how to leverage large-ensemble climate simulations to make comprehensive projections of climate risks, incorporating irreducible uncertainty from natural climate variability. Findings from our analysis provide a baseline for developing effective adaptation strategies to address these impacts.

HISTORICAL AND PROJECTED CHANGES IN CLIMATE METRICS AFFECTING APPLE PRODUCTION ACROSS THE U.S.

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