Over the past two decades, fire seasons in Colorado have consistently grown larger and more destructive. The three largest wildfires in tracked history ignited within 10 weeks of one another this year, putting the year’s total wildfire-burned acreage above the past six years combined.
It’s a trend caused by several factors, experts and researchers say, and it’s likely to continue.
Colorado has been lucky not to have seen the unusually large fires that characterized this year, said Camille Stevens-Rumann, a Colorado State University professor whose research focuses on fire ecology.
“If you think about other areas like California, or even other Rocky Mountain states, like Montana or Idaho,” she said, “they’ve had huge fires. We’ve not seen those. We had Hayman in 2002, then bad years in 2010 and 2011, but we haven’t had to face this reality until this year.”
Hotter, drier seasons, along with some misguided forest management practices, are to blame, she and other experts agree.
Fires in Colorado are a natural event, they stressed. The lodgepole pine is cited as an example of how the ecology has evolved to coexist with regular fires. The tree’s pine cone opens and releases the seeds when it raises to a certain temperature. And the natural cycle is for the adult lodgepole pines to be burned, which clears the area for the seedlings dropped after fires.
“The natural fire regime, for that system, is a stand replacement fire that kills off all the adults, about every 100-150 years or so,” said Matthew Hurteau, a University of New Mexico professor who specializes in the effect of climate on forests.
What sets this year apart, he said, is the fires’ fast spread. Only a handful of fires have grown larger than 100,000 acres in the past 20 years. The East Troublesome fire grew by 100,000 acres in a single day.
“That’s unusual,” Hurteau said, but the reason is because of higher temperatures and less precipitation.
“We had a decent snowpack this year, but it melted really quickly. The way the snowmelt is happening, and the failure for monsoons or convective storms to materialize in the summer, affects the condition for the way fires burn.”
Higher temperatures — average global temperatures have risen about 1 degree Celsius since the middle of the 20th century — mean the mountain snowpack doesn’t last as long, Hurteau said. The same higher temperatures that shorten the winter then, in the summer, sap moisture from the ecosystem, priming Colorado’s forest vegetation for a fire.
Two key metrics quantify for researchers how much the higher temperatures cause the more rapid drying of the environment.
“Vapor pressure deficit and climatic water deficit: How much moisture does the atmosphere want to pull out of the soil, versus how much there is,” Stevens-Rumann said. “As temperatures increase, there’s more demand in those two metrics.”
The natural process plays out every summer, but with snowpack disappearing earlier in the year and not arriving until later, it happens more intensely and for a longer duration.
“It's been exceptionally warm and dry,” Hurteau said. “That conditions the fuel, particularly the dead fuel, to ignite and to support that kind of rate of spread.”
Bark beetle is another factor that stokes Colorado’s wildfires. If a patch of trees becomes infested, after time, the trees die, leaving dead, drying timber that’s primed to ignite because of the drying pattern, the experts said. And while pine needles, twigs, loose foliage or leaves on the forest floor can burn quickly without burning larger trees, standing dead trees burn hotter for longer, further contributing to more intense fires.
“Satellite data showing temperature signatures and how long areas burned show that they stayed hot, longer than is typical,” Hurteau said, “Which would suggest that more of that standing dead fuel is burning.”
Bark beetle is part of the natural ecosystem, but their spread to more trees has been made possible by the same problematic higher temperatures.
“It's basically when trees get water stressed, they can't kill the beetles when they attack,” Hurteau said. “Then the beetle populations grow and you get an outbreak.”
Forest management practices have contributed to the problem as well. The doctrine of extinguishing forest fires as quickly as possible, without regard to the natural cycle of burning and regeneration for forests, has led to more fire-prone wildland.
“A century of fire suppression, for example in the lower edge of the Front Range, like those areas right around Colorado Springs, has led to much higher forest density and continuous fuels,” Stevens-Rumann of CSU said.
Peter Brown, the director of Rocky Mountain Tree-Ring Research, a nonprofit group focused on forest history, ecology restoration and management, said he’s studied large fires dating back hundreds of years, some that burned significant portions of the western range.
“These are not unprecedented. Fire is inevitable.” he said. “Except in our recent decades they’re happening more. The question is what kind of fires are going to happen given the circumstances?”
He said there are now efforts to bring a better approach to forest management, which lets some of the fuel burn, to better match the natural cycle.
“People like to put just one explanation on this, like saying it’s fire management, or saying it’s climate change. Obviously, it’s not one or the other. It’s both.”
The overall prognosis, the experts agree, is more intense fire seasons to come.
“Will we have worse fire seasons? Absolutely,” Hurteau said. “I don't know anybody who studies this who disagrees.”