Rocky Mountain Forests Are Burning More Than Ever

Data reveals that the climate crisis is transforming conditions in the region.

Forest fire creating large amounts of smoke

Don Grall / Getty Images

2020 was an unprecedented year for many people and places, and this was especially the case for the Rocky Mountain forests of northern Colorado and southern Wyoming. 

A study published in the Proceedings of the National Academy of Sciences last month found the extreme fires that raged through the alpine woodlands last year meant the area is now burning at greater rates than at any point in the last 2,000 years. 

“This work is clear evidence that climate change is pushing our forests outside of the range of variability that they’ve experienced for millenia,” study lead author and University of Montana professor Philip Higuera tells Treehugger. 

 The research revealed that 2020 was both a “tipping point” and part of a growing trend, as study co-author and University of Montana Ph.D. candidate Kyra Wolf tells Treehugger in an email.

 “[W]ith the 2020 fire season included, the rate of burning since 2000 was nearly double the average over the past 2,000 years, and even exceeded the maximum,” Wolf says.

Memory Banks

To assess fire conditions in the region over such a long period of time, the researchers turned both to the ground and the sky. 

First, they studied more than 20 sediment records from lakes in the region. During fires, ash falls on the lakes and sinks to the bottom. By searching the sediment for charcoal, scientists can therefore determine when fires occurred over a 2,000 year period. 

“Lakes are amazing memory banks,” study coauthor Bryan Nolan Shuman of the University of Wyoming tells Treehugger. 

For the region’s more recent history, the scientists looked at satellite images of burn extent from 1984 to the present. Taken together, the data revealed that the climate crisis is transforming conditions in the region. 

“We’re these geologists and ecologists who study longterm change and we’re accustomed to looking at the consequences of natural climate change and it’s really striking to see how what’s going on today is beyond our experience, the perspective we can bring from looking over thousands of years,” Shuman says.

In the lab, the sediment cores are split open and examined in detail. The color variation reflects differences in the material the fell into the lake at different times over the centuries.
In the lab, the sediment cores are split open and examined in detail. The color variation reflects differences in the material the fell into the lake at different times over the centuries. University of Montana

Loading the Dice

But how do the researchers know climate change is to blame for the 2020 fires? The sediment record indicates that high-altitude forests tend to ignite in a big blaze once every few centuries. 

 “This is kind of the way that they burn,” Higuera says. 

So what makes 2020 different? The researchers have established a clear link between warmer weather and fire activity in the past, and the current moment is out of range on both counts. Before the current century, the biggest burst of fire activity occurred during the Medieval Climate Anomaly, when temperatures were about 0.5 degrees (0.3 degrees Celsius) higher than the 21st-century average, the University of Montana explained. In 2019 and 2020, temperatures were 2.2 degrees (1.2 degrees Celsius) above the 20th-century average. 

Several other studies have established a link between drier, warmer weather and increased fire risk, which means that 2020 is unlikely to be an anomaly. 

“Human-caused climate change resulting in increasingly warm, dry summers ‘loads the dice’ to make extreme fire seasons more likely in any given year, leading to a general trend of increased frequency of extreme fire seasons like 2020 across the West,” Wolf says.

Flammability Barrier 

The extreme fire season in the Rockies also occurs within the larger geographic context of the U.S. West, which has been increasingly transformed by drought and wildfire. Another study also published in the Proceedings of the National Academy of Sciences last month found that the “flammability barrier” between lowland and highland forests has moved uphill in mountain regions across the West. 

Higher elevation forests were thought to be protected from wildfires because, as study lead author and McGill University Ph.D. student Mohammad Reza Alizadeh tells Treehugger, “the forests were supposed to be too wet to burn.”

However, during the past few decades, the fire line has moved up slope at a rate of 7.6 meters (approximately 25 feet) per year. In addition, dry conditions between 1984 and 2017 exposed an estimated 81,500 square kilometers (approximately 31,467 square miles) of previously protected forests to fires. Further, the higher elevation forests are now burning at a higher rate than the lower elevation woodlands, Alizadeh tells Treehugger. 

Alizadeh and Higuera both note that the two studies are complementary. Alizadeh points out that fires are advancing uphill fastest in the Southern and Middle Rockies, as well as the Sierra Nevadas. Further, Higuera affirms that it is exactly high-elevation forests that were most impacted in 2020. Across all elevations, 44 percent of the area burned since 1984 burned in 2020. For higher elevation forests, however, that percentage skyrocketed to 72 percent. While the dataset used by the broader, regional study cut off before 2020, both Alizadeh and Higuera agree its results would have been even more dramatic if that year had been included. 

Why This Matters

Why does it matter that fires are climbing uphill across the West? 

“These high elevation fires have implications for natural and also human systems,” Alizadeh explains. 

These include:

  1. Drinking Water: Mountains act as a “kind of natural water tower” for downstream communities, but the water these mountains pour into reservoirs could be altered in timing, quality and quantity if fires and hotter weather reduce the snowpack. 
  2. The loss of trees due to fire could also destabilize the snowpack, increasing the chance of avalanches.
  3. Over time, fires could transform the mountain landscape, leading to biodiversity loss. 

Because these changes are already in motion, policymakers, agencies, and communities need to learn to adapt. 

“Given an ongoing trend of warmer, drier summers, we can expect future rates of burning to continue to exceed those experienced in the past; thus, we need to re-think our planning around fire at all levels of decision-making,” Wolf says. 

 This could include measures like using less flammable roofing materials, reducing the amount of potential fuel around homes, improving evacuation plans, and making sure people in vulnerable communities have access to masks and air filters to protect them from smoke. 

However, the fact that burning will continue does not mean it is too late to act on the broader causes of the climate crisis. Shuman notes that the Wyoming Rockies are projected to experience weeks of 90-degree weather even if emissions are reduced. However, if nothing is done to reduce emissions, those same areas could experience two months of 90-degree weather instead, which would likely wipe out the snowpack. This means tackling the climate crisis at its source is essential for protecting alpine woodland ecosystems.

“Any policy that posits to address increasing wildfire activity that does not recognize the role of climate change in driving increasing wildfire activity is going to come up short,” Higuera adds. 

View Article Sources
  1. Higuera, Philip E., et al. "Rocky Mountain Subalpine Forests Now Burning More Than Any Time in Recent Millennia." Proceedings of the National Academy of Sciences, vol. 118, no. 25, 2021, p. e2103135118, doi:10.1073/pnas.2103135118

  2. "UM Research: Rocky Mountain Forests Now Burning More Than Any Point in Past 2,000 Years." University of Montana, 2021.

  3. Alizadeh, Mohammad Reza, et al. "Warming Enabled Upslope Advance in Western US Forest Fires." Proceedings of the National Academy of Sciences, vol. 118, no. 22, 2021, p. e2009717118, doi:10.1073/pnas.2009717118