Multiple fast-moving and destructive wildfires are burning Los Angeles County in California. These fires have been fueled by unusually dry conditions and hurricane-force seasonal winds. Gusts reached over 145 kilometers (90 miles) per hour. They quickly drove the blazes into urban areas. This has forced more than 100,000 people to evacuate from their homes and killed at least five people as of January 9.   

The largest of the blazes is known as the Palisades Fire. It erupted the morning of January 7 on the west side of Los Angeles. It has since burned around 7,000 hectares (17,000 acres) of land. And it has destroyed around 1,000 structures. The second largest is called the Eaton Fire. It ignited near Pasadena that night and had burned more than 4,290 hectares by the next morning. Three more smaller fires — the Sunset, Hurst and Lidia fires — are also burning in other locations around the area.

“This has the potential to be, at least collectively, the costliest wildfire disaster in American history,” Daniel Swain said during a January 8 livestream. Swain is a climate scientist at the University of California, Los Angeles. “Even just the Palisades fire on its own may become so.”

Here is a quick look at what is driving these extreme blazes.

Why are these wildfires happening in the winter?

Fire season in Southern California is generally thought to stretch from May to October. But recent research has shown that rising temperatures and decreased precipitation trends are expanding the fire season’s duration. That widening of the season is projected to continue in the future. In some parts of the state, fire season is already considered to be an all-year affair.

What is driving these fires to such extreme intensities?

One of the big reasons the Los Angeles area fires are growing so out of control is the Santa Ana winds. These winds typically occur in the fall and winter. Dry wind blows from inland, high desert areas toward the California coast. Along the way, the wind goes over mountain ranges. As the wind travels down the mountains, it becomes compressed. This is due to increased atmospheric pressure. The air also warms. That in turn lowers the relative humidity of the already dry desert air. And it makes that air better at drying out vegetation that can fuel fires.

What’s more, the Santa Ana winds can reach up to 160 kilometers per hour (100 miles per hour). That makes them very good at fanning flames and spreading embers. In the days preceding the fires, the National Weather Service in Los Angeles warned that a “life-threatening, destructive windstorm” would drive extreme fire risk.

The other big driver of the extreme fires is the abundance of dried-out grass and vegetation. The area’s vegetation is so dry because of recent weather patterns. “In a very dry year, there is a lot less grass that grows in Southern California versus a wet one,” Swain said. “Last year was very wet in Southern California. [That led] to what we call excessive fuel loading, a lot of extra fuel for potential fires,” he noted. “Now, since September, Southern California has seen the driest start to the winter on record, period, as well as among the hottest starts to winter on record.”

This combination of the Santa Ana winds and high fuel loads set the stage for the devasting burning occurring now. 

How much is human-caused climate change to blame here?

It will take a little bit more time for researchers to determine exactly how much the burning of fossil fuels has influenced these specific wildfires. However, it’s undeniable that human-caused climate change has worsened the weather conditions favorable for more intense wildfires in California.

“It’s not just that drier conditions are perpetually more likely in the warming climate,” Swain said. Wetter years will probably continue to get even wetter. “At the same time, we’re also getting much hotter summers and drier falls,” he said. There’s also “an increasing likelihood of dry winters.”

Individual wildfires are difficult to tie to climate change, due to the large number of factors that affect their spread. However, it is clear that human-caused climate change has worsened the weather conditions favorable to intense wildfires in California.

In California, this worsening weather whiplash makes conditions for wildfires more favorable. Wetter years grow more vegetation. And the drier years that follow then prime those fuels for burning.

Much of the area now burning in Southern California is dominated by grass and brush. These landscapes are especially sensitive to this weather whiplash. That’s because vast tracts of fire-fueling grasses can grow or disappear within a single year depending on the weather.

With wildfire smoke blanketing the region, what are the health concerns?

Smoke from the Los Angeles wildfires could impact the health of millions of people in the region. Fine particulates within the wildfire smoke can impact the health of even those who live far from fires. These aerosols can damage lungs, increase the risk of heart attacks and even lower life expectancy. Smoke can pose a particular risk to kids and teens. A recent study estimated that California wildfire smoke from 2008 to 2018 caused up to 55,700 premature deaths.

Smoke from the Palisades fire “has caused unhealthy air quality in Los Angeles County.” That’s according to the L.A. Department of Public Health. The worst immediate impacts are expected to affect the northwest coastal parts of the county.

But it is difficult to predict where smoke will travel and how winds will impact air quality. “So it’s important for everyone to stay aware of the air quality in your area, make plans and take action to protect your health and your family’s health,” Muntu Davis, a health officer for Los Angeles County said in its health advisory. “Smoke and ash can harm everyone, even those who are healthy. However, people at higher risk include children, older adults, pregnant individuals and those with heart or lung conditions or weakened immune systems.”