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Light snow began to fall around sunrise on New Year’s Eve, but as I began to clean up the debris from the previous day’s severe windstorm at my home in Boulder, Colorado, I was struck by the heaviness of the air. Not the usual moisture-laden crispness one might expect before a snowstorm, but instead there was an overwhelmingly burnt, acrid smell.

The context was immediately sobering: I was smelling the still-smoldering remnants of more than 1,000 homes that had burned in the the previous day. This same fire had raced up the canyon just down the road from our home, in perfect alignment with winds gusting more than 100 miles per hour, to bring about a catastrophic suburban firestorm in the downwind towns of Superior and Louisville.

Drone video taken Sunday morning is providing a closer look at the devastation caused by the Marshall Fire. Denver7 coordinated with the FAA, state and local authorities before and during the flight

— Denver7 News (@DenverChannel)

The fire , home to big-box stores and sprawling parking lots—stores we ourselves had patronized just minutes before the fire ignited. It jumped a six-lane freeway with ease before continuing even deeper into subdivisions out on the plains, destroying entire neighborhoods, whose residents fled by any means possible, many by car, but some on foot—the fire moved too fast for some to pull their vehicles out of the garage. I watched with horror as it roared out of the grassland and into backyards and then leaped from rooftop to rooftop.

https://twitter.com/RebeccainCO/status/1479388699997650947

As a scientist who studies the relationship between wildfires, weather, and climate—and as a lifelong resident of the ever-flammable American West—I have more than a passing familiarity with fire. In recent years, catastrophic wildfires have become an uncomfortably familiar experience. Once seen as a distant threat, wildfire has taken center stage in the public consciousness.

And there’s good reason for this rising discourse: the American West and other areas have experienced a rapidly escalating wildfire crisis over the past decade. In California alone, 15 of the 20 most destructive fires in a century of record-keeping —having together cost more than 150 lives and destroyed more than 40,000 structures. More broadly, across the contiguous U.S. West, the average annual area burned by wildfire in the past 40 years.

views from Avista Hospital in Louisville today.

— BrianOnTheNet (@OriginalStalkie)

But as both and fire ecologists emphasize, the trouble isn’t actually that there’s too much fire in the West—it’s that the fires are burning with rising intensity under ambient conditions that fall increasingly far outside of historical bounds. Combine that with the of towns and urban fringes into heavily vegetated areas (known as the wildland-urban interface, or WUI), and it becomes easier to understand the problem at hand.

We’ve returned to the Louisville neighborhood where we spent the afternoon watching firefighters try to hold the Marshall Fire at McCaslin Blvd. This is looking south over Harper Lake where homes are burning from one end of the neighborhood to the other.

— Kyle Clark (@KyleClark)

I often describe the wildfire problem as a stemming from the simultaneous escalation in risk brought about by three key factors. The first of these is the unfortunate legacy of forest and fire management policies from the 20th century; natural fires, which would have thinned forest understory and reduced vegetation density, known as “fuel loading,” were continuously suppressed, across most of the West’s forests. The second factor is the rapid expansion of populated areas into wildfire risk zones, whether in the form of scattered homes deep in the woods or sprawling subdivisions on the margins of rural vegetated tracts of land, which puts far more people and structures potentially in harm’s way when a fire comes along. The third factor is climate change, which has length and caused substantial drying of vegetation, that fires can attain.

The first two factors are somewhat conditional: historical forest fire suppression and increased fuel loading are mostly relevant in forests, after all, and many Western fires (like the Marshall Fire) burn in vegetation types other than forest; the WUI has not expanded everywhere, and scientists are still observing well away from population centers. Climate change, on the other hand, is decidedly more pervasive: it’s happening everywhere, and no part of the West remains untouched. Thus, the relative importance of each of these three factors varies from fire to fire, depending on the local geographical, ecological, and climate context.

In the case of the Marshall Fire, it’s pretty clear that historical forest management was not to blame, as this was by no means a forest fire. It ignited in a brushy landscape scattered through the vegetation. In its first 30 minutes, this vegetation fire consumed mainly brush and grass, along with a handful of structures. But soon thereafter, pushed by violent wind gusts of 90 to sometimes more than 100 miles per hour, the fire emerged into a truly suburban landscape. It became, effectively, a self-sustaining urban conflagration—burning from structure to structure and fueled more by the vegetation in backyards, road medians, and city parks than by anything that could be reasonably be considered wildland. Most of the structures that burned had been built in the past 30 years; previously, much of this area had been part of the Denver metro area’s extensive Great Plains grassland that has largely disappeared due rapid development. This expanded suburban footprint is clearly a major part of the story.

One of the more sobering videos taken by BPD Patrol off S. Marshall Rd. in Superior on Thursday afternoon during the height of the fire. First responders from all over the Denver Metro area rushed to help with evacs and traffic control. ⁦⁩

— Broomfield Police (@BroomfieldPD)

So, what about climate change? Well, the data show that the six months leading up to the fire were singularly the along the Colorado Front Range and among the driest. I saw this firsthand: autumn 2021 was eerily balmy in Boulder, with temperatures often into the 60s and overnight lows well above freezing most of the time—conditions more befitting of coastal California than above 5,000 feet elevation in Colorado. The cumulative vapor pressure deficit—a measure of atmospheric “thirstiness” that is strongly related to vegetation dryness and wildfire behavior— prior to the fire’s ignition. That unusual heat and dryness followed what was actually a remarkably wet spring, a sequence of events that allowed for prodigious growth of brush and grass earlier in the year, then subsequently for months-long dehydration of all that new growth that provided fuel for the fire. , as well as and increasing in some parts of the West, have been repeatedly linked to climate change. In other words: long-term climate trends in summer and autumn helped establish the preconditions necessary for a fast-moving winter fire.

https://twitter.com/GRDenver/status/1476757009122873351

Then there is the question of the winds themselves. Strong downslope gusts are actually rather common along the Front Range in winter, though this event was particularly violent. (One nearby gust was clocked at 115 miles per hour.) There isn’t any reason to believe the winds themselves have been accelerated by climate change, although there is little research on the topic. But climate change has dramatically into erstwhile “shoulder seasons” in the spring and autumn—and, increasingly, into winter as well. As I watched the Marshall Fire in late December 2021, I couldn’t help but reflect on a similar experience I had just over one year earlier, in mid-October 2020, when I witnessed the fast-moving roar out of the foothills just north of Boulder and wipe out dozens of homes. Furious downslope winds were also the proximal culprit in that disaster, but the unusually dry vegetation conditions facilitated the fire’s rapid spread in the first place. At the time, my colleagues and I thought it quite late in the calendar year to see a fire of that magnitude in this part of the world. And now, in 2021, we’re having the same conversation yet again—except this time it’s in the heart of winter.

When it comes to meeting the challenge of escalating fire catastrophes amid overstocked forests, an ever-expanding urban interface, and worsening climate change, there are no easy answers. Because the underlying causes are complex and multifactorial, so must be the solutions: there simply is no silver bullet. Many folks would rather there be a singular villain—but the reality is that all of these factors are critically important to varying degrees. Making toward mitigating this crisis means addressing each component head on: using to reduce and improve ecosystem resilience; reimagining how we design neighborhoods and to make them more fire resistant; and, of course, as quickly as possible to eventually halt climate change. None of this will be easy, but given that the alternative is an ever-increasing risk of catastrophic fires, we simply can’t afford not to act.

Daniel Swain is a climate scientist with joint positions in , the , and the . He can be found on Twitter at .��