Democratic presidential hopeful Joe Biden on Monday slammed rival Donald Trump as a "climate arsonist" whose refusal to take global warming seriously would exacerbate the crisis if the Republican incumbent is re-elected in November.

"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 in western US states.

"We need a president who respects science, who understands that the damage from climate change is already here," Biden added, presenting the opportunity to battle global warming as a necessary choice for Americans.

"We can commit to doing this together, because we know that climate change is the existential challenge that is going to determine our future as a country," Biden said, on the grounds of the Delaware Museum of Natural History.

"Or we can do it Donald Trump's way: ignore the facts, deny reality, which amounts to a full surrender and a failure to lead."

The former vice president delivered his scathing remarks as wildfires ravage parts of California, Oregon and Washington, so far leaving at least 35 people dead and burning nearly five million acres (two million hectares). 

Trump meanwhile arrived in California for a briefing and a short tour of some of the scarred landscape.

"Climate change is real," California Governor Gavin Newsom told Trump.

The US president has made only brief comments about the blazes in recent weeks, and he returned Monday to the theme of improving "forest management" as a way to reduce fallen trees and leaves that can fuel wildfires.

"When trees fall down after a short period of time, about 18 months, they become very dry. They become really like a matchstick," Trump said. 

"There's no more water pouring through, and they become very, very -- they just explode."

Biden, speaking before Trump, knocked the president for the forest management theory that he has espoused for years.

"The west is literally on fire and he blames the people whose homes and communities are burning," Biden said.