WASHINGTON-Snow cover in the Sierra Nevada is expected to shrink overall as the climate warms, but avalanche frequency could remain the same or even increase at high elevations. That's bad news for bighorn sheep that live there, according to new research to be presented today at the American Geophysical Union's 2024 Annual Meeting.
In John Muir's 1894 book "The Mountains of California," he praised the "perfect strength and beauty" of the wild Sierra Nevada bighorn sheep, "leaping unscathed from crag to crag." Those bighorn sheep are now an endangered species. After surmounting its first threats-diseases from and overgrazing by domestic sheep settlers introduced to the Sierra Nevada-the species made a slow comeback, crawling from 100 individuals in 1995 to 600 in 2015. But much of that conservation work is being undone by a new threat: avalanches. Together with mountain lion predation, the Sierra bighorn sheep population has halved since 2023.
Those environmental pressures make it all the more important that researchers like Huilin Huang, a contributor to the new work and Earth scientist at the Department of Energy's Pacific Northwest National Laboratory, work to forecast how a changing climate could influence the endangered sheep.
"I think our intuition might tell us that, as the world warms, less snow will accumulate and avalanche risk could decrease," said Huang. "There may be less snow accumulation at lower elevations due to warmer temperatures. But this is not the exact case at higher elevations. Unfortunately for the sheep, the avalanche risk remains pretty stable."
Along with PNNL scientists Yun Qian and Jianfeng Li, Huang modeled future changes in snow cover under a high-emissions scenario. Leveraging climate model outputs and remote-sensing data, they developed a machine learning model to downscale snow cover from 2 kilometers to 30 meters, providing high-resolution insights of how climate change affects snow cover. This finer-scale data enabled the larger team to better predict snow responses to future wet and dry years. Though much of her work entails modeling a changing climate, Huang was surprised to learn how sensitive bighorn sheep are to snow conditions.
"If there's a particularly wet year," Huang said, "there might be so much snow that the sheep are unable to navigate it and access the food they need to survive. If there's too little snow, that means lower levels of moisture could result in less grass to sustain the sheep. It's a delicate balance."
While bighorn sheep exist in other western mountain ranges, the Sierra Nevada subgroup is genetically distinct-the remaining 350 sheep are all that remain of the species. The few remaining herds of the sheep live at rocky, high elevations, and forage on grasses, herbs and shrubs.
"It's a hard population to sustain," said Ned Bair, a researcher at the University of California, Santa Barbara, who led the work. "I think there are some real challenges with being assured of at least my children being able to see a bighorn sheep in the wild when they're my age."
To learn more about this work, click here.
