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What’s causing California’s unprecedented wildfires

Extreme heat, dryness, wind, and ample fuel are driving enormous wildfires into new areas.

A chairlift at Sierra-at-Tahoe ski resort sits idle as the Caldor Fire moves through the area on August 30, 2021, in Twin Bridges, California.
The Caldor Fire burning near Lake Tahoe in California is one of two fires in history to cross the Sierra Nevada.
Justin Sullivan/Getty Images

Another explosive wildfire season is underway in California, with more than a million acres already burned in 2021. While still short of the unprecedented 2020 fire season, the blazes this year are well above average and are still gaining ground.

The Caldor Fire burning near Lake Tahoe has forced thousands of people to evacuate as it has spread to more than 207,000 acres, an area larger than New York City. The blaze ignited August 14 and was 23 percent contained as of Thursday.

The Dixie Fire near Chico, California, that ignited on July 14 has scorched more than 847,000 acres. It was 52 percent contained as of Thursday.

Spread by winds reaching 40 mph and fueled by abundant dry vegetation, wildfires across the Golden State have whipped up enormous clouds of smoke and ash. Some have even spawned fire tornadoes.

Across the United States as a whole, more than 2.7 million acres have been charred in wildfires this year, according to the National Interagency Fire Center.

Researchers said that the current fires align with what they forecast earlier this year, noting that the region was parched by a massive drought, was facing severe heat, and had plenty of trees, brush, and grass ready to burn.

Even so, the blazes have proved surprising in other ways. The Caldor and Dixie fires are the first wildfires on record to cross the Sierra Nevada mountain range, posing new challenges for firefighters working to contain them. “These two big fires started in very steep canyons [that are] difficult to access, with very dry, overloaded forests that are burning intensely and so it’s very hard to get a handle on this,” said Craig Clements, director of the Wildfire Interdisciplinary Research Center at San Jose State University.

And as humans continue to drive up wildfire risks — from building in fire-prone regions to suppressing natural fires to changing the climate — scientists are having to rethink what’s possible.

Even by California standards, the current wildfires are surprising

There are several key ingredients needed for wildfires. They need favorable weather, namely dry and windy conditions. They need fuel. And they need an ignition source.

The California Department of Forestry and Fire Protection said that they are still investigating the origins of most of the blazes underway. But other factors this year stacked the deck in favor of massive conflagrations.

California and much of the western US are in the midst of a years-long drought. With limited moisture, plants dry out and turn into kindling. Ordinarily, vegetation at higher altitudes would still hang onto some moisture and act as a barrier to wildfires in places like the Sierra Nevada. However, the severity of the drought has caused even this greenery to turn yellow and gray.

Traffic backs up on Hwy 50 as people evacuate ahead of the Caldor Fire on August 30, 2021 in South Lake Tahoe, California.
The Caldor Fire forced thousands of people to evacuate and burned more than 207,000 acres.
Justin Sullivan/Getty Images

“That’s the unique thing, that these fires have burned over the Sierra Nevada crest,” said Clements. “The fuel moistures are still at record lows across the state of California. That’s allowing these fires to burn at higher altitudes.”

Moisture in the soil and in vegetation can also act as a cooling mechanism as it evaporates. With the drought, this effect is diminished, allowing even more heat to accumulate and pushing temperatures up to the new record highs achieved this summer. The hot weather in turn drove even more drying across the landscape, reaching aridity levels not typically seen until October.

It’s likely that even more fires are in store for the rest of the year across the West. As the autumn Santa Ana and Diablo winds pick up, the risks remain high.

People make wildfires worse, but can take steps to mitigate them

Wildfires are a natural part of ecosystems across much of the western United States. They serve vital functions like clearing decaying vegetation, regulating forest density, restoring nutrients to soils, and helping plants germinate.

But human activity has made wildfires worse at every step. Climate change caused by burning fossil fuels is increasing the aridity of western forests and increasing the frequency and severity of extreme heat events.

People are also building closer to wildland areas. That means that when fires do occur, they cause more damage to homes and businesses. That proximity also means that humans are more likely to spark new infernos. The vast majority of wildfires are ignited by people, up to 84 percent, whether through errant sparks, downed power lines, or arson.

And for hundreds of years, people have suppressed naturally occurring fires. European settlers also halted cultural burning practices from the Indigenous people of the region. Stopping these smaller fires has allowed forests, grasslands, and chaparral to grow much denser than they would otherwise. Paradoxically, that means more fuel is available to burn when fires do occur, causing blazes to spread farther and faster.

The combination of these factors leads wildfires to keep breaking records, forcing scientists to reevaluate what kinds of fires are possible. Decades ago, a 20,000-acre blaze would have been considered massive. Now, wildfires can gain that much ground overnight. “What we’re finding is that even forecasting the fires is so difficult for us because the domain size is huge,” Clements said. “These fires are massive and we keep having to expand the domain of our weather models.”

There are well established ways to reduce the risks of destructive wildfires. Directly attacking a fire once it’s ignited can only go so far, so much of the focus has to be on prevention. One key way is to reduce fuel load. That can take the form of forest thinning and prescribed burns. Towns and cities in fire-prone regions can build defensible perimeters, cutting fire breaks and clearings to reduce the chances of a fire encroaching.

Building codes that demand fire-resistant materials and avoiding construction in the highest-risk areas can reduce the impact of fires as well. Ignitions can be reduced by hardening or burying power lines.

Restoring Indigenous burning practices can also help mitigate wildfire risk — but that will require governments to address historical wrongs, restoring tribes’ access to and sovereignty over ancestral lands.

Over the long term, slowing climate change by drastically reducing greenhouse gas emissions can also stop fires from becoming even more destructive.

The factors that laid the foundation for massive wildfires took centuries to build and won’t be reversed overnight. But the process of reducing these risks can begin now.