This story was updated on Sept. 14 with firefighting costs setting a record.

Wildfires burned across hundreds of thousands of acres in the American and Canadian West in early September, fueled by scorching temperatures that broke heat and fire records across the region. The fires helped make 2017 the U.S. government's most expensive year for wildland firefighting yet.

In California, at least 15 cities saw record-breaking heat as the state experienced its hottest summer on record. San Francisco hit 106 degrees on Sept. 1, breaking its previous high by 3 degrees. Stoked by unusually high temperatures, fires burned on thousands of acres just outside Los Angeles that week, while firefighters in Washington, Oregon and Montana battled dozens of blazes across those states.

On Sept. 5, at least 81 large fires were blazing across 1.5 million acres of the U.S. West, from Colorado to California and north to Washington. Over the Canadian border, British Columbia has already had a record-breaking fire season—and it's not over yet. Cities including Seattle were shrouded in a smoky fog. In satellite pictures, the smoke could be seen traveling the jet stream and reaching the East Coast.

As firefighters battled the blazes, climate researchers pointed to studies finding that a warmed global atmosphere, with increasingly clear human fingerprints, will continue driving a potent mix of heat and dryness that's projected to escalate in the West.

"These unprecedented extreme events, on the daily to the seasonal scale, are exactly the types of events that are more likely due to the global warming that's already occurred," said Daniel Swain, a climate scientist at UCLA. "That's not so much a future projection, but an observational reality, and that's something we expect to increase in the future. When we get these extremes, there's a human fingerprint."

A forest fire spread along the Columbia River Gorge on Sept. 5, 2017. Credit: James C. Kling/CC-BY-2.0

Swain co-authored a study led by Stanford researcher Noah Diffenbaugh published earlier this year that found human-caused greenhouse gas emissions have increased the chances of extreme heat across more than 80 percent of the globe's surface area.

"The increased occurrence of severe heat, and the role of global warming on the occurrence of severe heat—that's already happening," Diffenbaugh said. "It wouldn't be scientifically credible to make attribution statements without analyzing the event. That being said, we can see the odds of setting new records based on the global warming that's already happening."

While drought and high heat aren't the only factors making wildfires more intense and frequent—researchers also blame encroaching development into wild areas and certain wildfire management practices—they are key drivers.

Nine of the 10 worst fire seasons in the past 50 years have all happened since 2000, and 2015 was the worst fire season in U.S. history, surpassing 10 million acres for the first time on record. As of Sept. 5, wildfires in the U.S. had burned 7.8 million acres, and the fire season was far from over. (In 2015, 8.4 million acres had burned by early September.) The average fire season is 78 days longer than it was in the 1970s—now nearly seven months—beginning and extending beyond the typical heat of summer. By April of this year, wildfires had scorched more than 2 million acres in the U.S.—nearly the average consumed in entire fire seasons during the 1980s.

Last fall, researchers published the results of a study that found human-induced climate change accounted for about half the observed increase in fuel aridity, or forest dryness, in the western U.S. since 1979 and had nearly doubled the area of the U.S. West affected by forest fires since 1984.

During that same time period, temperatures across the West have risen. Temperatures are projected to rise further—and along with them, the tinderbox conditions that fuel wildfires.

"We know that global warming has already increased the probability of unprecedented high temperatures in the western U.S., including in California," Diffenbaugh said. "And we know, with high confidence, that continued global warming will continue to intensify those increases."

The wildfire risk is showing up in firefighting costs, as well. The U.S. Department of Agriculture confirmed on Sept. 14 that the Forest Service's firefighting costs this season had already exceeded $2 billion, more than Congress had set aside for the year, and the Interior Department said it had spent at least an additional $391 million. The previous record was a combined $2.1 billion in 2015.

The Agriculture Department acknowledged in its press release that "fire seasons are longer and conditions are worse." At the peak of the Western fire season this year, it said, there were three times more uncontained large fires burning than the five-year average.