Portland (AFP) -
Dense smog from US wildfires that have burnt nearly five million acres and killed 27 people smothered the West Coast Saturday, as presidential challenger Joe Biden warned climate change is becoming an existential issue.
US officials girded for the possibility of further fatalities as wide stretches of land in California, Oregon and Washington remained cut off by flames fueled by tinder-dry conditions of the kind caused by climate change.
Unprecedented infernos have now destroyed an area roughly the size of New Jersey.
'The science is clear, and deadly signs like these are unmistakable -- climate change poses an imminent, existential threat to our way of life,' said Biden, the Democratic nominee who will face President Donald Trump at the polls on November 3.
'President Trump can try to deny that reality, but the facts are undeniable,' he said.
The political row comes as Trump prepares to visit California on Monday for a briefing on the wildfires, which he has previously sought to blame on mismanagement by state officials.
Worsening the sense of environmental catastrophe, all five of the world's most air-polluted cities Saturday were on the West Coast, according to IQAir, with dense smog and ash from the blazes coating the atmosphere from Los Angeles up to Vancouver in Canada.
In Portland, thick, choking smoke blanketed the downtown area Saturday morning.
'It is as if I had smoked 100 cigarettes. I've never seen this but we try to stay positive as conditions are getting better,' said a 37-year-old man who gave his name only as Jessie.
- 'Dropped out of the sky' -
More than 20,000 firefighters are battling the blazes, with officials warning that a respite provided by the arrival of cooler weather could end Monday with the return of warmer, drier weather.
Emergency official Andrew Phelps warned Oregon is 'preparing for a mass fatality incident based on what we know and the number of structures that have been lost.'
In California, Butte County sheriff Kory Honea said additional officials had been brought in to check for human remains, but 'right now, the areas that we need to search are too hot.'
Oregon has suffered another four deaths in the past 24 hours, bringing the West Coast region's death toll this week alone to 19.
Among them was a 13-year-old boy found in a car with his dog in his lap in Oregon. The road was so hot it had melted the tires as he tried to flee.
One 56-year-old woman told AFP she and her daughter don't know if their ash-caked house in Aims, a short drive east of Portland, is still standing.
'To be honest, if we have to rebuild, we have to rebuild... I am just thankful that we're alive,' she said.
'We saw a bird that was flying and then all sudden it just completely dropped out of the sky... if it's killing God's creatures, I don't want to die too. So we left.'
- Anatomical skeleton -
California has already seen more than 3.2 million acres burn this year -- an annual record, approximately the size of Connecticut -- with nearly four months of fire season still to come.
Governor Newsom painted a grim picture of California as the canary in the climate change coal mine.
'This is a climate damn emergency,' he said in televised comments as he toured the damage in Butte County on Friday. 'This is real, and it's happening.
Huge wildfires are becoming more common, with the World Meteorological Organization saying the five years to 2019 were unprecedented for fires, especially in Europe and North America.
Climate change amplifies droughts which dry out regions, creating ideal conditions for wildfires to spread out of control and inflict huge material and environmental damage.
Nine people have been confirmed dead from a blaze in California's northern Butte County, which was driven at unprecedented pace earlier in the week by strong, dry winds and soaring temperatures.
In a rare sliver of good news, a believed tenth victim from the blaze turned out to be a burned anatomical skeleton from a local classroom.
Two more people were killed near the rural community of Happy Camp this week, while eight more perished in California last month.
In Washington state, a one-year-old boy perished while his parents suffered severe burns as they attempted to flee an inferno 130 miles (210 kilometers) east of Seattle.