California’s Wildfires: Why Have They Been So Destructive?

The Coffey Park neighborhood in Santa Rosa, Calif., was destroyed by an intense wildfire.CreditJim Wilson/The New York Times

Intense, fast-moving fires have been raging across much of California since Sunday night. The blazes have barreled through communities like freight trains, turning homes to dust in a blink and leaving at least 23 people dead. The largest of the fires are in the state’s wine country north of San Francisco.

“We’ve never seen anything like this,” said Trina Grant, 40, who grew up in California but was not prepared for the ferocity of this year’s fire season. Her parents, Arthur and Suiko Grant, died on Monday when flames consumed their home.

Why have these fires been so destructive?

Wildfires often break out in California in October after the state’s dry, sunny summers. The fires are worse this year because of record heat over the summer and high winds now, which can swiftly turn the smallest fires into fast-moving infernos.

Weather experts note that this year’s outbreak was a long time in the making.

Drought parched California for years, leaving it littered with fuel in the form of dry vegetation. Then the winter of 2016 and the spring of 2017 brought record amounts of rainfall, which spurred new plant growth. That was followed by months of extreme heat that withered the new growth and turned it into more tinder.

Finally, the autumn winds from the northeast, known as diablo winds, began blowing through the region over the weekend at speeds of 70 miles an hour or more.

All of this is happening in a region where more people are building homes tucked into forests.

What lit these fires in the first place?

Officials are not sure yet.

There were 22 major fires burning in the state on Wednesday, and the specific cause of each fire will be investigated, according to Thom Porter, Southern California region chief at the state’s fire agency.

In general, the vast majority of wildfires are caused by people, Mr. Porter said, and the past few months have been so dry that even seemingly innocuous human activities — a boat-trailer chain dragging on a road, engine heat coming off a car parked in a grassy area — have lit fires. It is possible, he said, that similar things may have started some of the current fires.

Daniel Berlant, assistant deputy director of Cal Fire, the state firefighting agency, said there were no thunderstorms in the region when the fires started, so lightning has been all but ruled out as a cause.

Could it have been downed power lines?

State officials are looking into that possibility. There were some reports of sparking power lines and other electrical equipment problems over the weekend. And poor power-line maintenance by the main utility in Northern California, the Pacific Gas & Electric Company, has been blamed for some past wildfires.

“Power lines are definitely one of the causes we’re looking into,” Mr. Berlant said, adding, “One thing we’ll be looking at is whether the fires started before the lines went down, or after.”

Fuel, oxygen and heat combine to start wildfires, which destroy millions of acres each year in the United States.Published OnCreditImage by Mike Blake/Reuters

Ari Vanrenen, a spokeswoman for the utility, acknowledged that electric lines were down in the region, but said it was too early to say whether they could have sparked the fire.

In 2015, a pine tree in the Sierra Nevada mountains came into contact with a 12-kilovolt conductor and burst into flames, leading to a fire that killed two people, destroyed 550 homes and scorched 70,000 acres, the state Public Utilities Commission concluded. It fined the utility company $8.3 million in April for failing to properly clear vegetation around the power line.

What’s the role of climate change here?

Fire seasons in general have grown longer and more destructive in recent decades, something scientists attribute in part to increased dryness caused by warming temperatures. It is less clear, however, whether the season for this kind of wind-driven fire in California has lengthened.

Researchers from the University of Idaho and Columbia University published a study last year saying that climate change had caused more than half of the dryness of Western forests since 1979.

Parched landscapes can increase fire size and duration, said Scott L. Stephens, a professor of fire science at the University of California, Berkeley.

But it is important to note, he added, that climate change is not necessarily causing specific fires to occur. Wildfires are a natural part of a forest’s life cycle and have been part of the state’s history since long before anyone called it California.

Who is fighting the fires?

About 8,000 firefighters from local, state and federal agencies are on the job, according to Cal Fire.

How do these fires compare with past California wildfires?

Taken together, the current wildfire outbreak is now the second deadliest of the last century, according to state figures. The Oakland hills fire in October 1991 was the deadliest, killing 25 people.

The Bay Area is blanketed in smoke from the fires. Is it bad to breathe?

The short answer is yes.

Residents can check on air quality in their area on the website. Make sure to look both at current conditions and the forecast.

When wood, grass and other materials burn, the flames produce gases and throw fine solid particles into the air. Those particles can burrow deep into lungs, which can be particularly dangerous for people with heart or lung diseases, older adults, people with diabetes, pregnant women, and children whose lungs are still developing.

Over the last few days, parts of the Bay Area have experienced extremely high levels of air pollution from the fires. Tom Flannigan, a spokesman for the Bay Area Air Quality Management District, called the situation “similar to what you see in Beijing, China.”

The agency advises Bay Area residents to limit outdoor activities, keep their windows and doors closed, and set air-conditioners and automobile ventilation systems to recirculate interior air rather than draw in outside air. If residents have masks available, they should use them.

Hiroko Tabuchi contributed reporting.

A version of this article appears in print on , on Page A12 of the New York edition with the headline: On Scorched Hillsides, ‘We’ve Never Seen Anything Like This’. Order Reprints | Today’s Paper | Subscribe