By Rachel Ramirez
Following a devastating wildfire season in 2020, new research shows that high-elevation forests in the Rocky Mountains are burning more now than any time in the past 2,000 years amid extreme, climate change-induced drought.
The study, published Monday in the Proceedings of the National Academy of Sciences, concluded that fire activity in subalpine forests of northern Colorado and southern Wyoming is unprecedented in the last several millennia -- a clear signal that the climate crisis is increasing the severity and extent of wildfires in the West.
Rising temperatures and prolonged drought in the West will continue to exacerbate and accelerate wildfire activity for at least several decades, scientists say. Philip Higuera, lead author of the study and fire ecology professor at the University of Montana, told CNN last year's wildfire season was a game changer.
"After 2020, it's clear we're in uncharted territory," Higuera said. "People are being negatively impacted by these wildfires either directly or indirectly. Climate models suggest that this trend is only set to continue."
The current drought sets the stage for another brutal fire season in 2021, particularly in California where rainfall deficits and dead vegetation are already breaking records that scientists didn't expect until August, Higuera said.
The 2020 wildfire season pushed Higuera and his colleagues to analyze historical fire records to understand how 21st century activity differs from the past. In addition to historical records, they also used charcoal found in lake sediments around the subalpine forests -- or high-elevation forests -- to compare how often fires have occurred in the area on average in the last two millennia.
Higuera's team found that last year's wildfires accounted for 72 percent of the total charred area in the subalpine forests since 1984. They also found the current rate of burning is 22 percent higher than the maximum average rate over the past 2,000 years -- a period of time the temperature in the Northern Hemisphere was actually slightly higher than it was during the 20th century.
The study's authors say the increase is a particularly significant climate impact, since subalpine forests typically burn less frequently than lower elevation forests.
Higuera called the results "sobering."
"Understanding how ecosystems have changed in the past is one of our best ways to learn more about how our forests change as climate changes," Higuera said. "Studying the past is so important because it really helps highlight the degree to which we are changing the landscapes that we live in now."
Jennifer Marlon, a climate scientist at the Yale School of the Environment who was not involved with the study, said these results suggest the seasons are shifting.
"When you get extreme heat and drought together, that's a recipe for really severe wildfires," Marlon told CNN. "The fingerprints of global warming are all over this kind of fire behavior."
Concerningly, Higuera said, what has worked to prevent wildfires at low elevations -- controlled burns -- is not an easy solution for subalpine forests.
"In lower elevation forests, it's an easier proposition to say we need to return prescribed fire to these forests to help get them back to conditions similar to how they were before fire suppression," Higuera said. "It's not as feasible in high elevation forests."
"Fire managers are faced with challenging decisions," he added, "whether having to modify the way that fire exists in these systems versus accepting these high severity fires, which is hard when they burn close to human communities."
Forests are vital to addressing the most dire effects of the climate crisis. They not only protect biodiversity, but also absorb and store carbon dioxide emitted from human activities. But as wildfires worsen, the carbon stored in these forests is increasingly released back into the atmosphere, an impact compounded by bad air quality.
Scientists say that forest ecosystems including the subalpine in the Rocky Mountains could soon reach a tipping point unless climate change is addressed. But Higuera warns that the solution isn't to eliminate fire from management systems because it has historically been part of the life cycle of forests.
"One of the challenges of living in the West is that we know that fire is an important component on these landscapes," he said. "If we remove it, that will take away a lot of the things we've come to expect such as species composition. The challenge for us is to be able to learn how to live with fire on the landscapes in ways that do not turn into human disasters."
Climate change leads to unprecedented Rocky Mountain wildfires
IMAGE: THE MULLEN FIRE LOOMS NEAR A PROPERTY IN CENTENNIAL, WYO., LAST FALL. BRYAN SHUMAN, A PROFESSOR IN UW'S DEPARTMENT OF GEOLOGY AND GEOPHYSICS, WAS A MAIN CO-AUTHOR OF A PAPER,... view more
CREDIT: JASON SHOGREN
June 14, 2021 - 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.
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