Joe Biden branded US President Donald Trump a "climate arsonist" on Monday for refusing to acknowledge global warming's role in deadly bushfires sweeping the western United States.
Mr Trump blamed lax forestry for the fires and declared: "I don't think science knows."
Dozens of conflagrations have raged across nearly 1.8 million hectares in Oregon, California and Washington states since August, laying waste to several small towns, destroying thousands of homes and killing at least 36 people.
The fires also have filled the region's air with harmful levels of smoke and soot, colouring skies in eerie orange and sepia tones while adding to the public health crisis posed by the coronavirus pandemic.
Ten deaths were confirmed during the past week in Oregon, the latest flashpoint in a larger summer outbreak of fires accompanied by lightning storms, record-breaking heatwaves and bouts of extreme winds.
Those incendiary conditions gave way over the weekend to cooler, more moist weather and calmer winds, enabling weary firefighters to gain ground in efforts to outflank blazes that had burned largely unchecked last week.
Fire managers said that the battle was hardly over. Thunderstorms forecast for later in the week could bring much-needed rain but also lightning. Officials also braced for a rise in the death toll.
As disaster teams scoured the ruins of dwellings engulfed by flames amid evacuations last week, Oregon's emergency management authorities said they had yet to account for 22 people reported missing in the fires.
At least 25 people have perished in California bushfires since mid-August, and one fatality was confirmed in Washington state. More than 6,200 homes and other structures have been lost, according to figures from all three states.
Fires focus climate change campaign
Mr Biden, the Democratic presidential nominee attacked by Republicans for not visiting disaster-hit areas, spoke from his home state of Delaware on the threat of increasingly frequent weather extremes that scientists have pointed to as evidence that climate change is supercharging the fires.
Mr Trump, who trails Mr Biden in national polls for the November 3 election, met firefighters and officials in California after Democrats blasted the Republican president for remaining mostly silent on the bushfires.
"I think this is more of a management situation," Mr Trump said, when asked by a reporter if climate change was a factor behind the fires. Without mentioning large fires that have raged elsewhere around the world in recent years – from southern Europe to Australia and Siberia – Mr Trump asserted that other countries "don't have this problem".
"They have more explosive trees, meaning they catch fire much easier," he said. "But they don't have problems like this."
The president and his administration have long sought to pin the blame for large bushfires on state officials, saying fuel-choked forests and scrub need to be thinned, more firebreaks should be cut and flammable debris cleared from forest floors.
Mr Trump said improved forest management was something that could be tackled quickly, whereas climate change would take more time and require international co-operation that he said was lacking.
"When you get into climate change, well is India going to change its ways? And is China going to change its ways? And Russia? Is Russia going to change its ways?" he said after landing in McClellan Park, California.
'I don't think science knows'
Mr Trump has referred to climate change as a "hoax", and in 2017 pulled the United States out of the Paris accords laying out an international approach to global warming. Mr Biden, the former vice president, has included climate change on his list of major crises facing the US.
Calling Mr Trump a "climate arsonist," Mr Biden said: "If we have four more years of Trump's climate denial, how many suburbs will be burned by wildfires? How many suburban neighbourhoods will have been flooded out?"
California Governor Gavin Newsom said more needed to be done to better manage forests to reduce fire risks. More than a century of aggressive fire suppression, he said, had allowed fuels to build up.
But he countered that global warming was nevertheless a driving factor in newly extreme bushfire behaviour, and he reminded Mr Trump that 57 per cent of forest land in California was under federal ownership.
“We come from a perspective, humbly, where we submit the science is in and observed evidence is self-evident: that climate change is real, and that is exacerbating this,” the Democratic governor said during a meeting with the president.
Mr Trump, who has authorised federal disaster aid for both California and Oregon, questioned that science.
“It’ll start getting cooler, you just watch,” he said. “I don’t think science knows.”
Tens of thousands of displaced residents across the Pacific Northwest continued to adjust to life as evacuees. Around the devastated southwestern Oregon towns of Phoenix and Talent, some people set up food stations in car parks. Others defied evacuation orders to guard their homes from looters.
Reinforcing local law enforcement resources strained by the disaster, Oregon is sending as many as 1,000 National Guard troops to fire-stricken communities.
Two men were arrested in Oregon, one on Friday and another on Monday, on arson charges stemming from a handful of small fires.
But police have cautioned against fake social media reports blaming bushfires on left-wing anti-fascists or right-wing Proud Boy activists.