When he saw smoke in the air around Boulder, Colorado on Dec. 30, Tom Veblen walked up a trail near his home to check it out. Veblen, a professor emeritus of geography at the University of Colorado Boulder who has been studying forest ecology, wildfires and climate change since the mid-1970s, said he could see that the Marshall Fire, on the southern edge of the city, was already jumping over distances of several hundred yards.

The winds were so strong that he said he struggled to open his car door, and to stay on his feet in the powerful gusts. Wooden fences separating yards in the suburbs sprawling in the distance looked like burning fuses, as winds gusting faster than 100 mph pushed the flames along them to ignite decks, roofs and residential landscaping. The firestorm would eventually engulf shopping malls and a hotel.

As a resident of a neighborhood he had previously believed to be a safe distance from the fire-prone forests, Veblen felt a sudden and unfamiliar sense of vulnerability.

“Sure, I knew that Chinook winds could drive winter grassland fires to spread very rapidly, but in the past we just did not have all the driving factors align so perfectly—wet spring producing abundant grass fuels, one of the warmest and driest June-Decembers on record and then an ignition at the base of the mountains.” Local topography also contributed to the intensity, with a canyon opposite the fire acting like a nozzle, blasting  winds from the peaks onto the flames and pushing the fire east into suburban neighborhoods.

The Marshall Fire ultimately burned some 6,200 acres, destroying at least 1,084 homes and seven commercial structures, before it was largely smothered by a New Year’s Eve snowfall. On Wednesday, investigators reported they had found partial human remains assumed to be those of one of the two people still missing after the fire. Insured losses are estimated at about $1 billion, making it Colorado’s most destructive fire on record in terms of property loss. 

In the days since the fire, Veblen said he’s had many conversations with neighbors and friends, some feeling a combination of survivor’s guilt and post traumatic stress disorder, and all wondering how worried they should be about wildfires burning into suburbia in the future. 

“I told them that, this winter, we’re probably going to be OK,” he said. But with the warming and drying climate shortening the snow season and desiccating grasses and brush more each year, chances are growing that similar drought, heat and wind will align more frequently to drive wildfires into the cold seasons and developed landscapes where they were once rare. 

In the meantime, few residents of rapidly expanding suburbs in which most of the vegetation has been planted by homeowners and developers realize that they are living in an expanding “Wildland Urban Interface,” or WUI, in which wildfires can threaten their homes and lives. In some areas with little natural vegetation, wooden fences and decks, wood-framed houses, flammable roofs and landscaping are the biggest source of fuel, which can burn down into glowing chunks that are lofted by high winds to help a fire hopscotch through neighborhoods.

“We could have another fire starting in Sunshine Canyon in some of those grassy areas and burn right down into Boulder,” he said. “We could call it a freak event, but we know that it’s not. It’s just a matter of those conditions setting up again.” 

A visit Friday to the towns devastated by the Marshall Fire by President Biden, Colorado Gov. Jared Polis and Rep. Joe Neguse (D-Boulder), could help jump-start the conversations needed to address the threat, he said.

“The important message our society needs to hear from them is that this is an example of a climate-enabled event, and the probability of similar events will continue to increase as we have continued warming,” he said. “Unless we keep fossil fuels in the ground, these events are going to get more frequent and worse.” 

New Climate, New Fuels and New Fires

“It’s clear the climate change is increasing the likelihood of these types of events,” said University of Montana fire ecologist Phil Higuera, who is currently a visiting fellow at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder, studying the relative influence of climate, vegetation and human activity on wildfire trends. 

“What I don’t want to see is a reaction of, ‘Oh, this is such an extreme event that we can’t do anything about it,’” he said. “Yes, this fire was very bad luck, but we shouldn’t be rolling the dice with fire in December.” 

Yet research, including a landmark 2019 study of fire weather indices, shows that global warming is loading those dice for more winter fires. Warmer temperatures and decreasing precipitation increasingly leave fuels like trees and brush tinder dry late in autumn and early winter, and increase the probability that snow-free Decembers will leave grasses, decks and roofs uncovered and vulnerable to wind-driven sparks and embers that could ignite them. 

What used to be the start of the season that brought snow to the West and cool, rainy conditions to many other parts of the country is now sometimes more like late summer. Even if global warming didn’t ignite the Marshall Fire, “there really is a seasonality change that is the main climate factor,” said UCLA climate researcher Daniel Swain, who studies extremes like fires and floods. “Usually by this time of year, there is just more moisture on the ground.”

For more than 20 years the region has endured alarmingly rapid aridification that has shrunk snowpacks, dried up river flows and lowered groundwater levels. Denver, just south of the Marshall Fire, experienced one of its longest snowless stretches on record just prior to the blaze, while much of the West blistered through an extreme autumn heat wave.

Winter fires are not unheard of in Colorado, or in grassland like where the Marshall Fire was first sighted, Swain noted. 

“That is not quite as surprising as what happened next,” he said. “It started there, burned a few hundred acres within 10-15 minutes, then it came across shopping malls … a significant extent of tract homes, a fair bit of vegetation in people’s yards and city parks. This is not a wild place, not a remote place.”

“That’s why we get these eerie images,” he said, alluding to social media posts of people fleeing from shopping mall pizza parlors and medical workers watching the fire from a hospital window as near-hurricane force wind gusts pushed fire and smoke plumes east into the towns of Superior, with a population of  13,077, and Louisville, with 20,860 residents. 

The images of fires around shopping malls are jarring, Swain said, “And yet as bewildering as it is, we’ve seen it in any number of large, wind-driven fires in recent years.”

Swain said several recent California fires were similar to the Marshall Fire, including the 2017 Tubbs Fire that burned more than 5,000 structures in Santa Rosa, the 2018 Camp Fire that killed more than 80 people when it destroyed the town of Paradise, and the Carr Fire, also in 2018, which jumped the Sacramento River in Northern California to spread into Redding, a city of 90,000 people.

Swain said the temperatures on the day of the Marshall Fire ignited were unremarkable for December in Boulder, with highs in the 40s. But that contrasted sharply with a “multi-month period of almost continuously balmy and record-warm temperatures leading up to this event, with many days making into the 60s and 70s during October and November and overnight lows rarely getting below freezing,” he said. “It was those antecedent record warm and dry conditions that were key in setting the stage.”

And the winds that drove the fire were like nothing he had ever encountered before. “The strongest I have ever experienced anywhere in the world while outdoors,” said Swain, who had to wear protective glasses to protect his eyes from airborne pebbles and roof shingles, with gusts “rushing downward over the Front Range foothills, creating very erratic windflow and occasional tornado-strength vortices. At one point, I witnessed one of these clear-air vortices cross the road and uproot a tree.”

With the increasing confluence of extreme fire weather conditions like high winds and extended droughts and heat waves, “there are a lot of places that are at similar risk, including many of the suburban areas around the Front Range,” Swain said. “But it’s really hard to prepare. There aren’t any simple interventions.”

Preparing for Wildfires in Suburbia

One part of the solution clearly lies in revising building codes to ensure that most construction and landscaping materials are non-flammable, even in areas that appear to be far from wildfire threats, said Veblen, an expert in the geography of fire. Such measures are becoming more common in areas where fire hazards are widely recognized, and the destruction of the Marshall Fire could inform how the boundaries of those zones are drawn in the future.

Since fires cross between jurisdictions, Veblen said that state rules would be most useful, but are unlikely to happen in Colorado, a home rule state where most land use decisions are made by local governments. So that leaves it up to county commissioners, “who need to feel they have the political support of the people so they can resist the influence of the building and real estate interests, which nearly always oppose any mandatory measures that make building more costly,” he said.

A meaningful change to building practices could also be spurred by the insurance industry, which could make sure that people who, for example, build with flame-resistant brick, pay less for fire insurance policies than those who build with flammable materials.

Apart from the built environment, he said the Marshall Fire will also trigger some “serious rethinking” of wildfire mitigation and the management of open space and parklands, which are among the key amenities that make the nearby homes desirable in the first place. 

“We know that up until 1950 it was mostly ranchland,” he said, with grazing cattle keeping grasses short and less prone to fire. Residential development started after World War II and  accelerated in the 1970s. 

“The most important thing we’ve done is change the fuels by putting structures all over the foothill ecotone,” Veblen said. Some early reports on the Marshall Fire suggest the fire may have slowed down when it reached one of the few small areas where cattle still munch the grass, so it could be that managed grazing could be a fire mitigation strategy, he added. Restoring wetlands and stream corridors to the point that they sustain live vegetation could also help by adding moist fire buffers to the landscape. 

The Marshall Fire and similar blazes burning in unusual landscapes and seasons could also challenge assumptions about how to reduce the wildfire hazard in areas far from the towns that burned—the fire-prone zone where forests spill off the lower slopes of the Rockies onto the plains. There, the long-standing thinking has been to thin woody fuels.

“But if you thin out ponderosa pine, it increases resources for grass to grow,” Veblen said. “So we said, ‘Sure, let’s have some grass fires, that will be beneficial.’ But no one was thinking about this. Wow, this fire event is changing my perspective on where it is or is not safe from fire.”