Last fall, the Mullen fire west of Laramie raged for the better part of two months, burning more than 176,000 acres and 70 structures in Wyoming’s Carbon and Albany counties, and in Jackson County, Colo.
Unfortunately, this scenario was typical during the intense 2020 fire season in the Rocky Mountain region, an area of Colorado and southern Wyoming where high-elevation forests are burning more than at any point in the past 2,000 years, according to a study in which a University of Wyoming faculty member was instrumental.
“Global warming is causing larger fires in Rocky Mountain forests than have burned for thousands of years,” says Bryan Shuman, a professor in the UW Department of Geology and Geophysics. “The last time anything similar may have occurred was during a warm portion of the medieval era.”
Shuman was the main co-author of a paper, titled “Rocky Mountain Subalpine Forests Now Burning More Than Any Time in Recent Millennia,” that was published today (June 14) in the Proceedings of the National Academy of Sciences (PNAS). The journal is one of the world’s most prestigious multidisciplinary scientific serials, with coverage spanning the biological, physical and social sciences.
Philip Higuera, a professor of fire ecology in the W.A. Franke College of Forestry and Conservation at the University of Montana, was the paper’s lead author. Kyra Wolf, a Ph.D. candidate in paleoecology and forest ecology at the University of Montana, also contributed to the paper.
Higuera and Shuman conceived and designed the study, while Higuera and Wolf analyzed the data, a unique network of fire-history records, to understand how current fire activity compared to wildfires of the past. The 2020 fire season marks the emergence of 21st century fire regimes with distinctly higher rates of burning, not only from the late 20th century but relative to the past two millennia.
By November 2020, wildfires in southern Wyoming and northern Colorado were responsible for 72 percent of the total area burned in high-elevation, subalpine forests since 1984. During 2020, Colorado had experienced three of its largest fires on record.
“As the 2020 fire season unfolded, we realized we already had a well-defined understanding of the fire history of many of the places burning, based on over 20 lake sediment records our teams had collected over the past 15 years,” Higuera says. “When the smoke settled, we thought ‘Wow, we may have witnessed something truly unprecedented here.’ So, we combined the existing records for the first time and compared them to recent fire activity. To our surprise, 2020 indeed pushed fire activity outside the range of variability these forests have experienced over at least the past two millennia.”
Researchers used charcoal found in lake sediment records to assemble the fire history across the Rocky Mountains. They discovered that, since 2000, wildfires are burning nearly twice as much area, on average, compared to the last 2,000 years.
Over that 2,000-year period, fires in high-elevation, subalpine forest historically burned, on average, once every 230 years. In the 21st century, those fires now occur, on average, every 117 years. This is 22 percent higher than the maximum rate — which took place during the Medieval Climate Anomaly (770-870) — reconstructed over the past two millennia. During the Medieval Climate Anomaly, Northern Hemisphere temperatures were 0.3 degrees Celsius above the average in the 20th century.
“The results indicate that, if fires continue to burn as often as they do now, every forest in the region could be burned by the beginning of the next century,” Shuman explains. “In the past, it would have taken 200 to 300 years, if not longer, for fires to affect that much area.”
In the Rocky Mountains of northern Colorado and southern Wyoming, 840,000 acres burned between 1984 and 2019, Shuman says. Another 660,000 acres burned in 2020 alone. Approximately 1.1 million acres burned in the past decade in the Colorado-Wyoming study area, even though only 400,000 acres — less than half as much — burned in the previous 25 years, Shuman says.
Subalpine forests are becoming less resilient and more susceptible to fires because the climate is warming. Because humidity was extremely low, temperatures were high, and storm events produced high winds, forest management had little impact on the 2020 fires. They burned designated wilderness and national parks with limited fuel management; heavily managed areas with substantial timber removal; and intact forest and areas with extensive beetle kill. The extreme climate completely overrode all types of forest management, according to Shuman.
“Snowfall in our high-elevation forests is lower now than in past decades, and summers are hotter. The changes convert trees into dry fuel, primed and ready to burn,” Shuman says. “With less snow now, the fire season lasts longer than before. When areas burn, the fires are bigger. They can burn longer.
“Then, after the fires, big areas with few live trees mean few seeds to help forests regrow and, even when seeds are plentiful, seedlings can often die from drought and heat,” he continues. “Some forests may never grow back.”
“It isn’t unexpected to have more fires as temperatures rise. Our records show that fire tracked past variations in climate just as it does today,” Wolf adds. “What’s striking is that temperatures and, correspondingly, fire are now exceeding the range that these forests have coped with for thousands of years — largely as a result of human-caused climate change.”
Continual warming will reinforce newly emerging fire activity in these high-elevation forests, with significant implications for ecosystems and society, according to the paper.
“It may sound dire, but it’s critical to remember that we have ample opportunities to limit or reverse climate warming, while still working to adapt to the increasing fire activity expected in upcoming decades,” Higuera says.
Shuman helped plan the study, which came about because of more than $600,000 in grants he was able to obtain from the National Science Foundation to support undergraduate and graduate student research at UW.
“We were able to examine the 2020 fire season because of a decade of student projects at UW that revealed how often our forests have burned in the past few thousand years,” Shuman says.