Few things affect where fruit and nut trees can thrive more than temperature. Nuts and many fruit trees need enough cold hours to produce quality yields, whereas too much cold, especially at the wrong time, can prove disastrous.
Ill-timed frosts have resulted in nearly $400 million in insurance payments to California farmers for damage to perennial orchard crops in the last two decades. But the true cost of frost is much larger, because not all farmers carry insurance and yield losses don’t capture all the money spent on frost prevention.
Now, in a rare bit of good climate news for California farmers, a recent study suggests that orchards may see fewer crop-destroying frosts by midcentury.
Frost damage has always been a risk for perennial tree crops, which undergo a plant version of hibernation that arrests growth to survive freezing temperatures. Farmers can lose an entire crop if a cold snap hits trees in early bloom, after an unseasonably warm winter tricks them into breaking their slumber.
It’s been unclear, however, whether such fateful encounters between frosts and early bud breaks will become an unsustainable cost of business under climate change.
To find out, scientists from two University of California campuses and the U.S. Department of Agriculture used climate models to understand how changes in temperature during the coldest winters would affect frost exposure—and mitigation expenses—for the state’s $7 billion a year almond, avocado and navel orange orchards.
The scientists compared historical frost exposure with projected frost events, based on the most likely scenario for future emissions through midcentury (using a scenario called RCP 8.5). According to the model, the coldest winter temperatures in California will warm by more than 4.5 degrees Fahrenheit (2.5 Celsius) throughout much of the state by midcentury.
According to their estimates, published in the March issue of Science of the Total Environment and online in December, the majority of almond and navel orange acreage will see a 50 percent to 70 percent decline in frost exposure by midcentury, while avocado orchards will experience at least 75 percent fewer frost hours.
Fewer frosts across the growing region could save farmers billions of gallons of water and millions of dollars in energy costs that would have gone toward preventing frost damage, the study found.
There will always be winners and losers with climate change, but the study points to a potential benefit, said Lauren Parker, a coauthor and postdoctoral fellow at the University of Davis, California, and the USDA’s California Climate Hub.
Parker, who specializes in agricultural climatology, is quick to point out that warming winters are likely to cause more harm than good for perennial crops, which may see a reduction in their required chill hours and more pests. “But it’s kind of nice to be able to say, even with all the bad news, there’s at least a little bit of a silver lining with these warming temperatures,” she said.
Almonds and navel oranges are especially vulnerable to frost. A bitter cold spell in 2018 battered almond trees that had blossomed earlier than normal following warm winter temperatures. Growers in the hardest hit regions received nearly $45 million in federal insurance payouts for frost damage. Four years earlier, a freeze cost navel orange growers $25 million in losses.
Most frosts in California happen on clear nights with little wind, when heat rapidly radiates away from an orchard as the sun goes down. Cool, dense air sinks to the ground as the lighter warm air rises above the tree canopy.
Growers spend considerable effort and resources to reduce frost risk, using heaters, wind machines and sprinklers to mix air layers and raise the temperature around the trees a few degrees. Some growers even hire helicopter pilots to buzz an orchard and blend the cold air that hangs in the trees with warmer air floating above the canopy, Parker said.
After the 2018 freeze, Parker spoke to a Sacramento Valley almond farmer who spent many late nights driving around his orchards, sometimes waking up in the middle of the night, to check thermometers when the forecast predicted frost. “He joked about all the coffee he drinks during the bloom season,” she said. “I sent him coffee for Christmas for the upcoming season.”
Kripa Jagannathan, a climate adaptation postdoctoral research fellow at Lawrence Berkeley Lab who was not involved in the research, said the study offers valuable information for growers by showing that a frost-free season is both critical for trees and is extending.
Five years ago, Jagannathan asked Central Valley almond growers, as part of her doctoral research, what kind of climate change information would be most useful for them. Impacts on frosts during blooms were a chief concern, they said, because they could steal a big chunk of their yearly income.
Since a climate projection can’t tell growers something like how many times they’d need to use their sprinkler for frost mitigation in 2050, they might not see its utility, Jagannathan said. “But it helps them understand their regional climate and how it’s been changing.”
The projected reductions in water and energy use from having fewer frosts to manage are clearly a bonus, said Parker. “But it’s not all sunshine and roses.”
Water and energy used for frost mitigation is just a fraction of what’s needed during the growing season, she said. And those demands are likely to increase as farmers face more frequent, severe and prolonged heat waves. Plus, the same warming phenomena that will reduce frost risk will also reduce the chill hours required for proper fruit development, boost insect pests’ reproductive capacity and decrease the Sierra Nevada snowpack that supplies surface water irrigation.
Predicting how climate change might affect crops is complicated, and results vary with crop, location, emissions model and other factors. The study didn’t determine whether plants are likely to bloom earlier, but earlier work by Parker and a colleague also found that climate change would pose minimal frost risk for California almonds, even though new growth was expected to occur an average of six days earlier by midcentury.
Even so, farmers will continue to see earlier blooms run up against frosts, Parker said. “The bulk of California’s orchards are still going to have frosts, just perhaps fewer of them.”
Parker paused to consider what the results might mean for individual growers. “I often think about Will,” she said, referring to her almond grower friend in the Sacramento Valley. “He just might not need as much coffee.”
