What made this year a record wildfire season?

A hunter goes shooting in the wilderness and a bullet ricochets off a rock. A man hammers a metal stake into the ground. A car gets a flat tire on a freeway, sending sparks into dry grassland. A smouldering campfire is left unattended, a cigarette is tossed out a window, a child plays with matches.

There are innumerable ways humans ignite wildfires, and in a normal year nearly 95 per cent of wildfires in California are started by people, the vast majority of them accidentally.

But this year, as fast-moving wildfires have ravaged communities up and down the West Coast, has been anything but normal. A freakish siege of thousands of dry lightning strikes in Northern California — a weather event on a scale not seen in decades — moved over lands parched in an era of climate change and sparked four of the five largest wildfires in modern state history.

Nearly 14,000 lightning strikes over a 72-hour period ignited more than 900 wildfires in August, state officials said, with relatively little rain to dampen the potential for wildfire — in some cases because the heat over a hot, dry summer caused it to evaporate before it hit the ground. Some of those fires are still burning.

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Lightning was also a factor in the Pacific Northwest, where Oregon officials refuted conspiracy theories that wildfires had been set by left-wing groups. They blamed thunderstorm activity for the blazes that smouldered for weeks before winds stoked them into devastating fires.

“The story this year is the lightning,” said Lynne Tolmachoff, the chief of public education at the California Department of Forestry and Fire Protection, or Cal Fire. “That’s not a normal weather pattern for us.”

This week, two enormous new fires in Northern California have killed three people and destroyed close to 250 structures in Napa, Sonoma and Shasta counties. Though the blazes are still under investigation, authorities believe they were likely caused by human activity.

Most wildfires can be blamed on people. The 22,700-acre El Dorado Fire in Southern California was ignited by a pyrotechnic device at a gender reveal party. A fire that wiped out the city of Malden, Washington, appears to have started when a power line was struck by a tree.

But in some ways, scientists say, the hunt for culprits misses the more important point: the overriding role of climate change in this year’s troubling fire season.

“If the lightning caused the home run, global warming put runners on base,” said Noah Diffenbaugh, a climate scientist at Stanford University.

Across the region, intense heat waves with little rain over the summer left forests brittle and dry, creating the conditions for wildfires to grow out of control and burn with more intensity.

“It’s true that climate change didn’t cause the fire,” said Philip Duffy, president and executive director of the Woodwell Climate Research Center in Falmouth, Massachusetts. “It causes them to get big. It drives the intensity of the fires. That’s really the story, fire size.”

Duffy said climate change had not been conclusively linked to the lightning sieges in the West, a fact seized on by some on the political right, including Fox News’ Tucker Carlson, as evidence that global warming is not to blame for the historic wildfire season.

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But Daniel Swain, a climate scientist at UCLA, whose comments about the lack of a definitive connection between lightning and climate change were misquoted and taken out of context on Carlson’s show, said in an interview later that the continual focus on ignitions “deeply undermines the fundamental point.”

“With climate change, what it is doing is changing the character of wildfires irrespective of ignition sources,” he said. “There are so many ways to start a fire.”

Tim Brown, director of the Western Regional Climate Center at the Desert Research Institute in Reno, Nevada, said he had not seen much consensus in the scientific community about whether climate change was causing more lightning that could ignite fires.

“This particular event would be difficult to link directly back to climate change,” Brown said.

Lightning has been a source of major wildfires in the West many times in the past, though infrequently on the scale seen this year. What happened was an unusual confluence of factors, weather scientists say. Before the lightning hit, Northern California was already in the grip of a heatwave, which had left vast swaths of vegetation parched and combustible. Then, moisture from the remnants of a tropical storm off the Pacific Coast of Mexico destabilised the atmosphere in the San Francisco Bay Area, unleashing a wave of thunderstorm activity.

The result was a siege of dry lightning strikes that Swain described as “just phenomenally exceptional.”

In Oregon, for two weeks after a lightning storm crossed the Cascades in mid-August, fire smouldered deep on the forest floor of the Opal Creek Wilderness — no open flames visible, just smoke wafting through a 10-acre patch of rugged terrain.

Also read: Hundreds of Oregon homes destroyed by US wildfires

Then, as warmer air drifted in from the east, the footprint of the fire grew to 50 acres, then 150, then 200, then 425. As the dry winds began escalating, they sent an inferno racing down the western slopes of the mountains. Ahead of its path, the wind tore down power lines and ignited 13 more blazes, the flames merging with one another to create one of Oregon’s most destructive wildfires.

While the four largest fires of the year in California were caused by lightning, and officials have said criminal charges could be filed in the case of the fire started by the gender-reveal party, the cause of a vast majority of the state’s fires remains under investigation.

When the investigations are complete, there is likely to be a long list of human causes, just as in Oregon and Washington. The largest recorded fire in California history before this year, the Mendocino Complex Fire in 2018, was ignited when a man hammered a metal stake into the ground. The Carr Fire, near Redding, California, in 2018, which set off a fire tornado, was started with a flat tire on a trailer, which set off sparks on the asphalt. And famously, singer Johnny Cash, in 1965, set off a wildfire with his overheated truck, a blaze that threatened endangered condors.

Officials in Oregon and Washington state have provided more detailed data than California on the causes of wildfires this year. In Oregon, there have been 614 human-caused fires and 600 that were caused by nature. Washington has reported 765 human-caused fires and 69 caused by nature.

Arson is rarely the cause of a large wildfire, officials say. Often, arsonists are quickly caught and their fires extinguished before growing out of control. In Oregon this year, authorities reported several fires being deliberately set, many of them quickly put out.

Police said a man in Portland used a Molotov cocktail to start a small brush fire alongside the 205 Freeway; firefighters extinguished the blaze and found the man in a nearby tent. After the man was released from custody, authorities said he started six more small fires, was arrested again and transported to a hospital for a mental health evaluation.

The fire that devastated the cities of Talent and Phoenix in southern Oregon remains under investigation. One man has been arrested on suspicion of lighting a fire that day, but authorities are still looking for information about a nearby fire in Ashland, where human remains were found.

Mark Wallace, a former state fire marshal in Oregon, said arson wildfires, when stacked up against other causes — lightning, unattended campfires, fireworks and other things — tend to make up only a sliver of cases in most years.

“In the whole scope of things,” he said, “that’s pretty rare.”