The wildfires that exploded over the past few days in California and Colorado show clear influences of global warming, climate scientists say, and evidence of how a warming and drying climate is increasing the size and severity of fires from the California coast to the high Rocky Mountains. 

They may also be the latest examples of climate-driven wildfires around the world burning not only much bigger, hotter and faster, but exploding into landscapes and seasons in which they were previously rare.

For tens of thousands of Californians enduring evacuations, and millions more suffering through smoke that has brought some parts of the state the worst air quality in the world, the recent fire weather has seemed almost biblical. 

The entire state and much of the rest of the West has been, for the last week, in the grip of a “heat dome” that has brought temperatures of 129.9 degrees Fahrenheit to Death Valley, perhaps the hottest temperatures ever recorded on the planet. On Saturday, Aug. 15, the National Weather Service issued its first ever warning for a tornado born of a wildfire, when radar detected at least five spinning vortices in a pyrocumulonimbus cloud rising from the Loyalton fire near the Nevada state line. Witnesses saw a “firenado” dropping from the smoky storm cloud to the ground.

That weekend, nearly 11,000 lightning strikes peppered Northern California and the Bay Area, where thunderstorms are rare, sparking more than 370 new wildfires. By Saturday morning, wildfires had burned more than a million acres in California—an area larger than the state of Rhode Island—and nearly 120,000 residents were evacuated.

Colorado evacuated a tiny fraction of that number of people, but the state had nearly 200,000 acres burning in four large wildfires. The Grizzly Creek fire near Glenwood Springs closed I-70, the most critical artery crossing the state, for more than a week and was the highest priority wildfire in the nation until California exploded. 

The largest of the Colorado blazes, the Pine Gulch fire, is burning north of Grand Junction. On the night of Aug. 18 it grew by nearly 40,000 acres—42 percent—and created its own lightning storm during hours in which lower temperatures and increasing humidity used to calm wildfires in the state. By the morning of Aug. 19 it had become the state’s second largest fire on record.

Scientists Aren’t Surprised by Fires Across the West

Despite what may look like a new world of wildfire, Daniel Swain, a climate scientist with the National Center for Atmospheric Research (NCAR) and the University of California, Los Angeles, said few climatologists and fire scientists have been surprised by the severity of the fires across the West this year.

Federal wildfire forecasters predicted that it’s “going to get bad really quickly, starting with Arizona, Utah and Colorado,” Swain said. “And then the forecast was that Northern California would start getting bad in August and peak in the fall. Well, up through August, so far, that’s looking like an awfully good prediction.”

What’s more, Swain said, the recent fires in the West fit into a trend of groundbreaking yet predictable wildfires around the world, as warming temperatures and diminishing moisture tip over fire regimes, the term fire scientists use to describe the typical timing, frequency, intensity and duration of wildfires on a given landscape. 

“If we go back, this feels like years ago, earlier in 2020, to Australia, all the experts there before that got bad said this looks like it could get bad,” he said. 

During 2019, the hottest and driest year on record in Australia, the usual early season rains didn’t arrive, but extraordinary heat did. “And so places there burned intensely that wouldn’t normally ever burn intensely,” Swain said.

He also pointed to more recent conflagrations in the Arctic, “a wildly different fire regime from either Australia, California or Colorado. That, too, is a region that has had extremely severe fire.”

Siberia experienced unprecedented, 100-degree Fahrenheit temperatures and record-low soil moisture that drove epic fires, from the boreal forests to peatlands, swamps, permafrost and tundra In June. Some of what burned, Swain noted, is usually frozen through most of the year.

“There’s all sorts of examples of weird fire this year,” Swain said.

Climate Change Has Doubled Risk of Extreme Fire in California

Swain was a co-author of a paper published Thursday that showed a steep increase in extreme fire conditions in California during the fall, the state’s most deadly and destructive wildfire season.

“Our paper says that climate change has already doubled the risk of extreme fire weather conditions in California,” he said, “and it will double them again over the next couple of decades…perhaps more than double. That’s a big change.”

Other research, led by Park Williams at the Lamont Doherty Earth Observatory at Columbia University and published last year, identified an even steeper increase in summertime wildfires in California, marked by an eight-fold increase in the amount of land burned annually since the 1970s, caused largely, the authors reported, by human-driven climate change.

Swain said he also sees surprising changes in where and what the current fires are burning. “What’s so interesting about the fires in Northern California right now is they’re burning an incredibly diverse range of ecosystems,” he said, pointing out that in addition to chaparral, brush and grasslands that routinely burn in August, the fires have overtaken areas that rarely catch fire.

“It’s burning in the coast redwoods which have almost no modern fire history,” he said of the forests of huge, moist trees. “That’s kind of incredible that that’s happening at all, but especially in August, in the complete absence of any offshore winds.”

Swain pointed to the CZU (Santa Cruz Mountains) Lightning Complex fire that tore through California’s oldest state park—Big Basin Redwoods—which was founded more than a century ago to begin preserving the state’s iconic trees from logging. 

“That’s a park that has almost continuous old growth redwoods,” he said. Historically, fires in such forests stayed on the ground, rarely burning up the trees. But, Swain said, he was in touch with a firefighting strike team that had to retreat from its attempt to save the park’s buildings when crown fires exploded through the tops of the redwoods. “That is really astonishing to me,” he said.

In Sonoma County, shorthanded fire crews were forced to leave the Armstrong Redwoods State Natural Reserve to their fate in order to defend the town of Guerneville.

The fog that rolls off the Pacific to chill the Bay Area in the summer also helps keep redwood forests moist and cool. But research published in 2010 by James Johnstone and Todd Dawson at the University of California, Berkeley, suggested that the fog, known as the “marine layer,” has declined by a third, possibly increasing the drought stress and flammability of the redwoods and other coastal ecosystems. 

Crystal Kolden, a fire scientist at the University of California, Merced’s Sierra Nevada Institute, noted that the current heat dome has kept that fog away. 

“This heatwave is drying out coastal fuels,” she said. “That marine layer, which normally cools off the Bay Area and the coast during the summer, is actually being repelled off the coast.”

So on top of the almost unprecedented heatwave and “lightning siege,” Kolden said, California’s coasts were unusually dry and ready to burn. Research shows that up to 4 percent of lightning strikes in the region start fires during dry conditions, so the electrical storms alone could have ignited as many as 500 fires in an area that rarely sees such blazes.

“We just don’t see very much fire in coastal California that is lightning ignited,” she said.

And that hot, dry vegetation wasn’t just more prone to ignite, but to burn hot and fast.

“To see this kind of fire growth in the summer is very concerning to me,” Kolden said. “We don’t have large explosive fires in the coast range in the summer. That says to me that we are in more trouble with climate change than we thought.”

Kolden also noted that such unusual fire events are occurring across the country and around the world. 

“The thing that I continue to point to when we see these sorts of extreme events, is the frequency of these extremes and that they’re happening all over the place in the West simultaneously,” she said. “And it’s not just the West, this is globally.”

‘Dry Droughts vs. ‘Hot Droughts’ in Colorado

Last spring, Colorado residents looking at the mountains might not have expected a fiery summer, despite forecasters’ predictions to the contrary. Most of the state’s mountains had around 100 percent of their normal snowpack, which in the past often kept the mountains moist and less prone to burn in the summer. 

But by July, the National Weather Service’s Colorado Basin River Forecast Center was predicting just 55 percent of the normal runoff.

Brad Udall, a climate scientist with the Colorado Water Institute at Colorado State University, explained that as the difference between the “dry droughts” of the 20th century, which were generally caused by reduced precipitation, and the “hot droughts” of today.

“This 21st century drought you can’t entirely explain by precipitation decline,” he said. “You’ve got to pull in some other factor to explain why these stream flows are so low and that other factor is, of course, temperature, because it increases the evaporative load of basically everything. This year was like a supercharged version of that.”

A 2018 study in which Udall collaborated with researchers from the University of California, Los Angeles, showed that half of the reduced stream flows in the Colorado River Basin today compared to a century ago were due to warming, rather than to reduced precipitation. 

Such heat-driven aridity is one of the factors that has expanded the time between the first and last large western wildfires by 78 days since the 1970s, and created a year-round wildfire season in some regions.

And after last winter’s snows, Colorado did see a steep decline in precipitation, along with a sudden rise in springtime temperatures. Then, for the third time in three years, the monsoon that usually drenches and cools the Southwest failed to materialize, leading the state to become even drier and hotter.

“Here we are again with a really, in most places, non-existent monsoon,” said Russ Shumacher, Colorado’s state climatologist and a professor at Colorado State University. He pointed to the failed monsoon as one reason big fires are burning in August, a month in which such fires were once rare. 

The rain that comes with the monsoon can help relieve the drought and moisten vegetation that could fuel fires, making it less prone to burn, he said. But just as important are the clouds that cool the temperature and lower the fire danger. “Even if it doesn’t rain, it’s cloudy, and it doesn’t get up to 100 degrees.”

Instead of the monsoon, Colorado found itself under the same heat dome as California. Schumacher sees more such summers coming to Colorado.

“What we used to consider a hot summer is now going to be an average summer,” he said. “I think we’re kind of starting to get a bit of a glimpse of what the future of hot summers are going to be.”

John Abatzoglou, who studies fire and climate at the University of California, Merced, noted that most bad fire seasons require a variety of drivers that climate change is increasingly lining up.

“The earlier disappearance of snowpack gets compounded by a warm, dry summer and the failure of the monsoon to materialize,” he said. “Those switches are tending to turn on more often than not.”

Jennifer Balch, Director of Earth Lab at the University of Colorado Boulder, said that some big switches have been turned on in Colorado and the West, some of which will be difficult to turn off again.

“In Colorado, half the state is in severe or extreme drought conditions right now,” Balch said. “And the western part of the state is a climate change hotspot that has seen a 2-degree Celsius increase since 1895, twice the global average increase.”

It’s no surprise, Balch noted, that the largest of Colorado’s fires is burning where the state has shown the greatest average temperature increase. Or that Colorado isn’t the only state where hotter temperatures have increased the amount of land burning.

“Across the West, human-caused warming has effectively made fuels much drier and doubled the amount of western forests that have burned since 1984,” she said.

And, Udall added, the warming and drying of the West and the impacts on wildfire, are far from over.

“I have, as most scientists do, real worries about what yet additional warming will do to what is already a bad situation,” he said.