Donald Trump dismisses climate concerns as he visits fire-ravaged western US

President Donald Trump suggested global warming will reverse itself and dismissed climate change as a cause of the ferocious fires engulfing the west of the United States.

As the focus of the US election campaign switched to the deadly blazes, Democratic challenger Joe Biden slammed the incumbent as a "climate arsonist" and said America would not be safe from the ravages of a warming planet unless it elects "a president who respects science."

Infernos across California, Oregon, and Washington state have burned more than five million acres (two million hectares) this year, killed dozens of people, and forced hundreds of thousands from their homes.

Whole towns have been ruined by the blazes, which have pumped out tonnes of noxious fumes that have hung in the air for days, blocking out the sun and making breathing difficult.

But Trump, in Sacramento on the third day of a re-election campaign swing, pushed back against state officials arguing that a heating climate underlies the ever-stronger fires.

"It will start getting cooler. You just watch," Trump said, offering no evidence for the theory.

"I wish science agreed with you," responded Wade Crowfoot, the head of the California Natural Resources Agency.

"I don't think science knows, actually," Trump said.

Reputable scientists around the world are almost unanimous in their belief that the world is getting warmer because of human activity.

This man-made climate change amplifies droughts, which dry out regions, creating ideal conditions for wildfires to spread out-of-control and inflict unprecedented damage.

Trump also repeated his argument that wildfires are caused by poor maintenance of forest areas, making them more combustible.

"There has to be strong forest management," he said.

"With regard to the forests, when trees fall down after a short period of time, about 18 months, they become very dry. They become really like a match stick," he added. "They just explode."

Biden, speaking at his own campaign event on the opposite coast, said Trump's re-election would be catastrophic for the environment.

"If you give a climate arsonist four more years in the White House, why would anyone be surprised if we have more of America ablaze?" Biden said, savaging Trump for failing to "take responsibility" for the ongoing wildfire crisis.

"We need a president who respects science, who understands that the damage from climate change is already here."

California Governor Gavin Newsom, who has argued that the fires are driven mostly by global warming, acknowledged as he met with Trump that better forest management was needed, but insisted the overwhelming cause of the problem is far bigger.

"We submit the science is in and observed evidence is self-evident: that climate change is real and that is exacerbating this," he said.

Much of the West Coast has been blanketed in choking smog for days, with Portland on Monday the world's most air-polluted city according to IQAir.

Washington state Governor Jay Inslee said it was crucial voters elect a "climate president" come November 3.

"This moment requires action, not denial," he said in a tweet. "We must come together around science to build a better future for all Americans."

Most of the 35 deaths so far confirmed have occurred in California and Oregon, where the wildfires are the worst in decades and have been fueled partly by record heat.

More than 30,000 firefighters are battling the blazes, with wind gusts and drier weather on Monday threatening more destruction.

Residents of Arcadia, on the outskirts of Los Angeles, were ordered to evacuate Sunday as the nearby Bobcat Fire spread south through forested terrain toward the metropolis.

Two new deaths were confirmed from the North Complex Fire, which swept at unprecedented speeds last week into areas already ravaged less than two years ago by the Camp Fire -- California's deadliest ever blaze.

"There are still active fires, power lines are down, trees are down, there are roads that are impassable," said Butte County Sheriff Kory Honea, warning evacuees it could take "weeks and weeks" to return to their homes.

He said seven people remained missing.

Paul Clement described to AFP how he fled his home in Berry Creek.

"When I came around the bend, everything was on fire -- an entire hillside. So I ended up driving through and you couldn't see 50 feet (15 meters)."

"It was worse than the Camp Fire, which I didn't think was possible."