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Firefighters were battling California's largest wildfire of the summer on Monday, a blaze near famed Yosemite National Park that has forced thousands of people to evacuate, officials said.
The Oak Fire in central California broke out on Friday and is raging while parts of the United States remain in the grip of a sweltering heat wave.
The fire in Mariposa County has engulfed 17,241 acres (6,977 hectares) and is 16 percent contained, Cal Fire, the state fire department, said.
"Fire crews are working aggressively using bulldozers, hand crews and aircraft," with only "minimal growth on the fire" seen on Monday, the department said.
It is the most destructive blaze so far this fire season, according to Cal Fire, destroying more than three times the acreage of the nearby Washburn Fire, which has been nearly 90 percent contained.
Greece on Sunday was battling three major wildfires in the north, in the south and in the east, amid high temperatures raising fears for more blazes.
But it pales in comparison to last year's Dixie Fire, which burned nearly one million acres.
"What we're seeing on this (Oak Fire) is very indicative of what we've seen in fires throughout California, in the West over the last two years," Jon Heggie, a Cal Fire battalion chief, told CNN.
"These fires are burning with just such a velocity and intensity, it makes it extremely challenging and extremely dangerous for both the public and the firefighters," Heggie said.
"It's moving so quickly it's not giving people a lot of time, and they sometimes are just going to have to evacuate with just the shirts on their back," he said.
Jonathan Pierce, a spokesman for the fire department, said low humidity and high temperatures were stoking the blaze.
The high number of dead trees and steep slopes in the area was also "leading to extreme fire behaviour," he added.
The Oak Fire has forced the evacuation of some 3,000 people so far, officials said.
More than 2,000 firefighters backed by 17 helicopters have been deployed against the fire near the southwestern edge of Yosemite National Park.
'Direct result' of climate change
California Governor Gavin Newsom declared a state of emergency in Mariposa County on Saturday, citing "conditions of extreme peril to the safety of persons and property."
In recent years, California and other parts of the western United States have been ravaged by huge and fast-moving wildfires, driven by years of drought and a warming climate.
"What I can tell you is this is a direct result of what is climate change," Heggie told CNN.
"You can't have a 10-year drought in California and expect things to be the same," he said. "That drought is what drives what we are calling megafires."
And extreme temperatures could be seen elsewhere in the country, as 60 million Americans were under a heat advisory on Monday.
The National Weather Service said heat advisories were in place in the Southern Plains and Lower Mississippi Valley, while stifling temperatures would ease on Tuesday in the Northeast and mid-Atlantic.
The usually cool Pacific Northwest will see temperatures surpassing 100 or more degrees Fahrenheit (37 degrees Celsius) in the Columbia River Gorge and Columbia River Basin.
The NWS said daily record highs will likely be broken from northern California to the Portland, Oregon and Seattle, Washington areas on Tuesday.
Cities have opened cooling stations and increased outreach to at-risk communities such as the homeless and those without air conditioning.
Various regions of the globe have been hit by extreme heat waves in recent months, such as Western Europe in July and India in March and April -- incidents that scientists say are an unmistakable sign of a warming climate.
The extreme weather prompted former US vice president Al Gore, a tireless climate advocate, to issue a stark warning on Sunday about "inaction" by US lawmakers.
Asked whether he believes US President Joe Biden should declare a climate emergency, which would grant him additional policy powers, Gore was blunt.
"Mother Nature has already declared it a global emergency," Gore told ABC.
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