- Forest fires are getting larger and hotter in the western U.S., shrinking the mountain snowpacks vital to communities and ecosystems.
- When a wildfire burns on mountain slopes, snow that falls later in the winter is more exposed to sun and wind, making it melt or evaporate faster and earlier than ever before.
- Burned land is recovering more slowly as the region warms, leaving less water for trees and plants to regrow and increasing the risks of erosion and flooding.
In the western United States, natural periods of fire and snow are cyclical. The summer brings wildfire season, and the winter brings ski season. But as the globe warms, these cycles have become erratic and less reliable, with dramatic impacts on the region’s vital water supply.
Now, researchers have shown that severe wildfires are diminishing many snowpacks on mountain slopes by leaving them exposed to sun and soot, according to a recent study in the Proceedings of the National Academy of Sciences. As those winter blankets shrink, the communities that rely on the mountains as both a source of water and recreation are facing deeper droughts – and higher fire risks.
“We’re seeing that these fires are hotter and larger, and they’re having a bigger impact on our water resources and water availability,” said Steven Fassnacht, a snow hydrologist at Colorado State University and co-author of the study.
Wildfires are particularly increasing in the snowiest parts of the western U.S. In 2020, for example, more than 4 million hectares burned during the summer and fall, casting a smoky haze over the region.
The new study focused on three fires that scarred the southern Rocky Mountains in 2020. The blazes raged in the “late snow zone,” where snow is deepest and lasts the longest on the highest slopes of mountains. So much forest burned that it exceeded the total burned area over the previous 36 years combined.
When a super-hot fire rampages through a forest, trees fall and the underbrush ignites. This frenzy leaves the forest floor bare and exposed to the elements. Later on in the winter, snow that falls on this stark land becomes more exposed to sunlight and wind, leaving less snow on the mountain.
These shifts mean there is less snow to melt in the springtime when mountain communities get most of their water. And as the snowmelt tap runs dry, the next fire season gets a head start as plants and grasses dry out earlier.
This negative feedback loop – more fires causing less snow causing more fires – became apparent to Fassnacht and his colleagues as they analyzed snowpack levels spanning the last 20 years, on both burned and unburned land. Wildfire impacts have significantly increased and spread up mountain slopes into late snow zones, they confirmed.
“This research brings more nuance to understanding how wildfires are impacting our watersheds,” said Mojtaba Sadegh, a climate modeler and water resources expert at Boise State University in Idaho, who was not affiliated with this study. More intense fires now mean that “the entire energy balance of higher elevations is thrown off,” he said.
The study’s authors, led by ecosystems scientist Stephanie Kampf of Colorado State University, are now examining the timing of when snow tends to fall and melt in the western U.S. Noticeable shifts are clear, their work suggests. They also hope to study how well mountain forests can bounce back from wildfires and altered snowpacks.
“We’re working on collecting more data to understand how things recover,” said Fassnacht. “Three years out, five years out, ten years out, how do these forests change? Fortunately, nature is resilient, but how long does it take to recover and how does that affect the ecosystem?”
- Kampf, S. K., McGrath, D., Sears, M. G., Fassnacht, S. R., Kiewiet, L., & Hammond, J. C. (2022). Increasing wildfire impacts on snowpack in the western U.S. Proceedings of the National Academy of Sciences, 119(39). https://doi.org/10.1073/pnas.2200333119
Isabel Swafford (@isabel_swafford) is a graduate student in the Science Communication program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here.