2017 has been a bad year for peaches in the Peach State. Georgia’s disruptively warm winter caused the loss of an estimated 85 percent of the peach crop. “We had fruit here in Georgia from the middle of May to about probably the first week of July, and after that we didn’t have anything else,” said Dario Chavez, an assistant professor in peach research and extension at the University of Georgia.

As temperatures rise globally because of climate change, Georgia is not the only part of the country where warm winters are causing trouble for farmers. California’s cherry crop took a hit in 2014 because of a warm, dry winter. And in 2012, after a warm February and March brought early blooms, Michigan’s apple crop was decimated by an April frost. Farmers have always been at the mercy of the environment, but now agricultural catastrophes brought on by warm winters seem likely to occur with greater frequency.

For trees that fruit each year (such as peaches, cherries, blueberries, almonds and other fruits and nuts), cool weather is as important as warm. Cold air and less sunlight trigger the release of chemicals that halt trees’ growth, prepare them to withstand freezing temperatures and enable them to resume growing the following spring. When a tree enters this dormant state, it sets a kind of internal seasonal alarm clock that goes off once the tree has spent enough time in chilly temperatures. This countdown is measured in so-called chill hours — the amount of time the temperature is between 32 and 45 degrees Fahrenheit. When crops don’t get the chill hours they expect, they can’t properly reset. Buds are delayed, and instead of ripening into juicy, delicious fruit, they remain small and underdeveloped.

This last winter, middle Georgia got about 400 chill hours during what Chavez described as the usual dormancy period for peaches (roughly Oct. 1 to Feb. 10). The winter before, while still on the low side, had closer to 600 chill hours. But that 200-hour difference meant several peach varieties that had produced fruit in 2016 never bloomed this year. There are products and techniques that can help stimulate delayed crops, but this year the deficit in chill hours was too large to overcome, Chavez said.

A chill-hour deficit hits places with milder climates, such as the southeastern U.S. and California, especially hard because they get fewer chill hours to begin with.

But Georgia was not the only place with a chill-hour deficit in the last year. According to an analysis by the Midwestern Regional Climate Center’s Vegetation Impact Program, most of the U.S. got fewer chill hours than the average from 1998 to 2013.

Climate change, and the loss in winter chill that can come with it, poses a particular threat to fruit and nut trees and the farmers who depend on them, said Eike Luedeling, a senior scientist at the University of Bonn’s Center for Development Research. Farmers who grow annual crops, such as corn and wheat, replant every year and might be able to adapt more nimbly to a sudden change in the environment, by changing their planting schedule or switching crops (though doing so may be costly). But fruit and nut farmers rely on plants that take much longer than a single growing season to be productive. “You really have to plan for several decades ahead when you plant a tree,” said Luedeling, who has modelled what winter chill hours may look like in the future. “It’s a huge investment.”

Looking ahead, the experience of Georgia peach farmers this year might become more common. Luedeling predicts only about a quarter to a half of California’s Central Valley, which produces much of America’s fruit and nut crops, still will have enough chill hours by the middle of the 21st century to grow walnuts, apricots, plums and most varieties of peaches and nectarines. A separate, global projection from Luedeling shows that while colder areas may not change much over the next century (or may even gain winter chill hours, thanks to more days above freezing), warm areas are likely to see dramatic reductions in chill hours.

Though concerning, these projections are far from certain, Luedeling said. There is still a lot we don’t know about winter chill. Anything above 45 degrees does not count toward the chill hour total in most models, but that threshold is almost certainly not as firm for plants themselves. As Luedeling put it, the cutoff doesn’t “have much biology in it,” but he hopes to build a better model soon that will help fruit producers plan for the future.

Meanwhile, farmers must make decisions now about their plans for the next few years. Peach growers and researchers, for their part, are focused on moving toward varieties that need fewer chill hours to thrive. Chavez, who works closely with growers, some of whose families have been growing peaches in Georgia for three to four generations, said that the time to make changes is now. “The weather is something we cannot control,” he said, but peach farming “is part of the region…. We have to address [it] sooner or later.”