Wildfires in the western United States are getting worse by almost every metric. They get bigger, spread faster, and reach higher mountain heights that were previously too wet and cool to support such violent fires.

They also become more intense, killing a large number of trees and destroying entire areas of forest.

“We didn’t really see fires like this ten years ago,” said Lenya Quinn-Davidson, a fire advisor at the University of California Cooperative Extension, referring to the bootleg fire in 2021, which began on July 6 and eventually consumed more than fifty thousand hectares in a single day.

Here’s what is driving these changes and what can be done about them.

Forest fires require a spark and fuel. In the forests of the western United States, half of all forest fires are caused by lightning. The other half is man-made – often caused by power lines, cigarettes, cars, campfires, or arson.

There has been an abundance of very dry fuel in recent years. Drought and extreme heat can kill trees and dry out dead grass, pine needles, and any other material in the forest floor that acts as a lighter when a fire sweeps through a forest.

Forest fire experts see the signature of climate change in the drought, high heat, and longer fire season that made these fires even more extreme. “Without climate change, we wouldn’t be seeing this huge surge in fire activity anytime soon,” said Park Williams, a climate scientist at UCLA. “There’s just no way.”

These conditions were exacerbated by fire-fighting measures. Prior to the modern settlement of the American West, forested land in the region was naturally burned by lightning or was deliberately burned down by indigenous communities as a form of forest maintenance. But for the past hundred years, most western states have suppressed fires. This has led to ever denser forests and abundant bushes on the forest floors.

“We are prepared for fire,” said Ms. Quinn-Davidson.

Al Lawson, an incident commander for the Oregon Bootleg Fire, described his behavior as “one of the most extreme you can find.” But what does that mean?

Experts think about forest fires based on metrics like intensity, rate of spread and severity.

Fire intensity refers to its power or the energy released from its flame. Satellites measure the energy and temperature of fires, and research has shown that the strength of these flames has increased.

The rate of spread is one of the most important factors as it suggests that a fire is less predictable. Although the size or area of ​​a fire is important, Ms. Quinn-Davidson said it is more important to be aware of how fast it is moving.

Severity relates to the consequences of a fire, such as how many trees are killed. If a fire is big and burns all the way into the treetops and kills them, it might be more difficult to control.

Inhaling forest fire smoke is risky.

Like air pollution, forest fire smoke – and especially the concentration of PM 2.5 or particles smaller than 2.5 microns – can affect the respiratory and cardiovascular system, said Colleen Reid, an environmental epidemiologist and health geographer at the University of Colorado Boulder.

In healthy people, the smoke can cause a sore throat, cough, shortness of breath, or decreased lung function. Those who already have cardiovascular or respiratory diseases are at risk of flare-ups and should take extra precautions even if the air quality is moderate.

Scientists are still studying the chemical makeup of forest fire smoke, which depends on what is burned.

For example, trees and biomass will produce a combination of carbon dioxide, methane and nitrogen oxides, while burning houses or cars can produce a whole range of compounds, including heavy metals.

Dr. Reid said a first step in protecting yourself is to monitor the air quality around you using resources like this Environmental Protection Agency map or the New York Times tracker.

On days with particularly poor air quality, stay indoors, keep windows and doors closed and use HEPA filters if possible. If you don’t have access to a HEPA filter, there is evidence that adding a quality filter to a box fan can be protective. Wear a body-hugging N95 mask outdoors.

Between 60 and 90 percent of houses lost in forest fires are due to embers carried by the wind in front of a fire. If embers land on a house, on mulch under a window, or ventilated into an attic, it can ignite and set the house on fire.

In California, homes built in the past decade at what is known as the wildland-city intersection – the areas that are, for example, between forests and cities – were required to have fireproof features such as non-flammable siding and double-sided hardened windows.

But for any home, experts say that even small steps, like keeping gutters and roofs free of leaves and debris, can be remarkably effective. As a next step, it’s valuable to replace things like vents with a finer mesh screen, or replace siding and roofs with more fire-resistant materials.

“We have good science that shows homes retrofitted or built this way are more likely to survive forest fires,” said Susie Kocher, forest consultant at the University of California Cooperative Extension.

Landscape changes can also make a difference. Fire experts think in terms of the 0-5 zone, which refers to the five feet of circumference around a house. This zone should be kept free of debris, firewood, plants, or mulch. “It looks nice to put a bush under our window, and that’s exactly the wrong thing to do,” says Ms. Kocher.

Experts agree that mandatory burns – deliberately set fires that periodically extinguish undergrowth or other fuel – are key to reducing the severity of forest fires in the future. State and federal authorities have already committed to carrying out more prescribed burns.

However, experts also emphasize that there needs to be more federal and state laws that prioritize this technique. There are currently bills in the US Senate and California Assembly to provide more funding and training for mandatory burns.

Another important step is maintaining the landscape to remove dead trees and other fuels. After a major death in the Sierra Nevada in the 2010s, an estimated 150 million trees fell, but only 1 percent of those trees were removed, creating more fuel for future fires.

But a long-term solution requires big changes, say experts. What is important is that the mindset is shifting from fighting fires to reducing the risk of extreme events that make them worse. “We have treated fire as long as we could fight it. We don’t fight hurricanes, earthquakes or floods, ”said Ms. Quinn-Davidson. “We need some radical changes in the way we do things in order to adapt, but yeah, I think we can.”