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Climate Change, 'Chill Hours' And California Crops

UC Regents / Courtesy

A walnut orchard near Winters, California. If the current trend of warmer winters continues in Yolo County, a UC Davis study says chill hours may be insufficient for many walnut varieties by the year 2100.

UC Regents / Courtesy

Agricultural economists at UC Davis used Yolo County as a case study to show how warmer winter temperatures may alter the types of crops planted in California.

Dr. Hyunok Lee is Research Economist in the Department of Agricultural and Resource Economics at UC Davis.

Daniel Sumner is Director of the UC Agricultural Issues Center and Frank H. Buck, Jr. Professor in the Department of Agricultural and Resource Economics at UC Davis.

The two economists studied 12 crops in Yolo County, using 105 years of local climate data and 60 years of county planting history.

They found warmer winter temperatures would reduce "chill hours," potentially reducing yields for some crops, while extending the growing season for others.

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Insufficient chill hours can cause a delay in the opening of leaf and flower buds in crops such as walnuts, which may result in reduced fruit yield. UC Regents / Courtesy


Sumner says the study shows walnuts are most vulnerable, while processing tomatoes and alfalfa acreage, may increase due to warmer winters.

He says fewer "chill hours" could also mean farmers plant different varieties of nut crops.

"Growers adapt to the kinds of climate changes as best they can," says Sumner. "Now that means they may adapt the variety of almonds or walnuts they use to minimize potential losses from fewer chill hours. But it could also mean that, if we look out 20 or 30 years, that we may have fewer acres of the crops that are most sensitive to chill hours, and more acres of the trees and vines that are less sensitive to chill hours."


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Winter chill hours required for selected tree and vine crops.  UC Regents / Courtesy


Sumner says if the present trend continues, chill hours would fall from the current trend value of 882 hours to a trend value of 712 hours by the end of this century. He says by 2100 many varieties of walnuts would have insufficient chill hours in years with average weather, and severely insufficient chill hours in some years with usually warm winters, which would mean lower yields.

Sumner says growers have already been adjusting to climate change.

"Those kinds of adaptations are the things that farmers have been doing for a very long time," says Sumner. "And one thing that our study does, by taking a long historical perspective, is to recognize that growers have already been adapting to climate change, climate change is not new news to growers, they've been adapting to it for a long time, and we expect they will continue to do so."

Sumner says in different growing regions in California, warming winter temperatures could see growers moving "away from some crops and toward crops more suitable" for changes in the climate.

He says the study goal was to see how growers have already adapted their crop choices due to climate change and how they might alter those crop choices in the future. 

Sumner says the study did not look at other factors, such as water supply changes based on less snow and more rain, climate variability, extreme weather events, accelerated warming or availability of irrigation water.

The study was published in the peer-reviewed journal California Agriculture

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