The current rate of forest fires burning through the Rocky Mountains in the United States is the highest it’s been in the past 2,000 years, a recent study found.
Studying burn scars on tree rings and charcoal in lake sediments, researchers found that the region’s high-altitude forests burned at double the rate in the last two decades that they did in the past two millennia.
The research suggested that warmer, drier weather caused by climate change was contributing to more frequent forest fires in the region.
The Rockies are the largest mountain range in North America and stretch down the western side of the continent. In 2020, fires blazed through the high-elevation forests in the state of Colorado. Across the western United States, fires in the states of California, Washington and Oregon also burned through millions of hectares, cost billions of dollars and displaced many lives.
“The 2020 fire season was record-setting across the West [of the United States],” said Philip Higuera, a fire ecologist at the University of Montana and lead author of the study. “Colorado broke the previous record for largest fires in the state three times last year.”
At the forest’s high elevations, the environment normally keeps the trees cool and wet relative to forests at lower elevations. Naturally occurring forest fires may burn every few hundred years, but even then, fires need unusually warm, dry weather to first dry out the vegetation to serve as fuel.
'Sentinels' of climate change
The areas known as subalpine forests are also less sensitive to fire suppression and less impacted by past land management than lower-elevation forests, Higuera said. “They’re better sentinels of climate change.”
Ecologists wondered if the fires of 2020 were unprecedented, even relative to the forests’ history of fires stretching back over millennia. They studied historical records to understand if the recent fires were an anomaly and published their results June 14 in the Proceedings of the National Academy of Sciences.
Modern human records of forest fires go back only a few decades. The study authors used government satellite records to track fires and how far they spread from 1984 to 2020. For fires before 1984, they relied on records of fires that had been preserved by nature.
Tree rings and their fire scars or charcoal deposited and preserved in lake sediments, for example, are like signatures left by past fires. Each layer of lake sediment, deposited at a certain time in history, provides information about conditions during that time. Similarly, a tree’s rings, which correspond to its age in years, can retain marks that act as a time stamp for past fires.
To see if the pace of forest fires had recently changed, the authors estimated how long it would take to burn all of Colorado’s subalpine forests at the rate they burned in a given era. Accounting for fires over the past two millennia, that number was about 204 years or more. Fires from 2000 to 2020 cut that time almost in half, to 117 years.
“It puts into perspective how far of an anomaly the 2020 fire season was,” said Julie Korb, a forest and fire ecologist at Fort Lewis College in Colorado, who was not involved in the study. “In these high-elevation forests, up until this point, we thought they were kind of protected from the changes that we’ve been seeing in the climate.”
Weather heightened danger
According to the study, warmer, drier weather made the trees more prone to burning.
Although the results showed that climate change has pushed the Rocky Mountain forest systems into uncharted territory with respect to historical wildfires, Higuera said it was important to keep in mind that the specific conditions of these high-elevation forests shouldn’t be generalized across all forests.
“Forest fires are not necessarily all negative,” said Larissa Yocom, a fire ecologist at Utah State University, who also was not involved in the study.
Wildfires, which burn outside human control, are often naturally part of the long-term life cycle of environments such as forests or grasslands.
Studies have shown that many lower-elevation forests on the West Coast of the United States were experiencing a “deficit” in fire activity compared with historical patterns, partly because of the suppression of fires through human intervention. At the same time, those land management practices likely helped accumulate fuel for even more intense wildfires, such as the ones that occurred in 2020.
When fires occur repeatedly or too severely, forests can have trouble recovering to healthy states, Yocom said. The resulting smoke can also impact human communities and cause health issues like lung damage.
“This really emphasizes that we can’t rely on what we’ve seen in the past as a good strategy for what to expect and what to plan for,” said Kyra Wolf, a fire ecologist at the University of Montana and co-author of the study. “We need to rethink or reimagine what it looks like to live in these fire-prone areas in the West.”
Scientists expect more extreme weather and human intervention will continue to change the planet’s environment.
“It’s no longer a projection. It shows that change is already occurring. We’re living in the middle of it,” Yocom said of the study.