Historical and projected changes in chill accumulation and spring freeze risk in the Midwest United States

Authors: FORD, TRENT - Illinois State Water Survey; CHEN, LLANG - University Of Nebraska; FERNANDEZ, EDUARDO - University Of Valparaíso; WAHLE, ELIZABETH - University Of Illinois; LUEDELING, EIKE - University Of Bonn; Todey, Dennis; Nowatzke, Laurie

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2025
Publication Date: 4/5/2025
Citation: Ford, T.W., Chen, L., Fernandez, E., Wahle, E., Luedeling, E., Todey, D.P., Nowatzke, L.W. 2025. Historical and projected changes in chill accumulation and spring freeze risk in the Midwest United States. Agricultural and Forest Meteorology. https://doi.org/10.1016/j.agrformet.2025.110532.
DOI: https://doi.org/10.1016/j.agrformet.2025.110532

Interpretive Summary: Many types of tree fruits in the Midwest require a certain amount of cold each winter at a certain temperature to be able to create blooms the following year. The accumulation of time at these certain temperatures are referred to as chill hours. Changing winter temperatures due to climate change are leading to changes in the accumulation of chill hours that is beginning to impact fruit production because fruits are reaching their chill accumulation before the last freeze event in the spring more frequently. This study reviewed historical changes in chill accumulation and looked at projected changes in chill in the future based on a commonly recognized set of models for climate change projections and used an apple development ("phenology") model to estimate potential future risks to production. Historical changes in chill accumulation are showing quicker accumulation leading to earlier potential bud break in the spring. Future projections indicate continued earlier dormancy break is more likely into the mid-21st century, but phenology model did not indicated an increased risk to fruit production because last spring freezes are expected to also trend earlier in the spring. This study is only a first step in trying to determine potential climate change impacts on perennial crops in the Midwest. Midwest specialty crops and climate change issues have been noted as needing much more research in combination with producers to better understand current and future issue for production. This research will be useful to establishing initial estimates of climate change impacts to tree fruits in the Midwest for other research to compare and for producers to begin testing new management practices.

Technical Abstract: In the Midwest region of the United States, the dormant or cold season has experienced significant changes over the past several decades due to human-caused global warming, and changes are projected to continue or intensify through the end of the century. Changes in chill accumulation and spring frost injury risk are particularly concerning for specialty crop growers in the Midwest. Despite their importance for the industry, relatively little work has been done to assess these changes and help guide crop management strategies accordingly. We use a combination of historical observations and CMIP6 multi-model ensemble projections to assess recent and potential future changes in chill accumulation and phenology for apple in the region. Observations show increased chill accumulation in much of the Midwest over the past 70 years, and CMIP6 projections indicate continued increases through the next 70+ years. The southern Midwest is expected to lose chill, but not at a rate that would require a substantial shift to fruit cultivars with very low chill requirements. Additionally, apple full bloom estimates using the PhenoFlex model combined with CMIP6 model projections show shifts earlier in the spring for both apple phenology and spring freeze dates. We do not find any appreciable change in spring freeze injury risk by mid-century under any scenario. This study provides an important assessment of climate change impacts on specialty crops in an understudied region of the United States for non-commodity agriculture. More collaborative work is needed between scientists, practitioners, and growers to (1) assess the current and future risks to specialty crop agriculture in the Midwest that result from climate change and (2) explore viable solutions to ensure a resilient specialty crop industry in the face of changing climatic, economic, and social systems.