LOS ANGELES — There’s a dangerous but basic equation behind the killer Yarnell Hill wildfire and other blazes raging across the West this summer: More heat, more drought, more fuel and more people in the way are adding up to increasingly ferocious fires.
Scientists say a hotter planet will only increase the risk.
More than two dozen wildland fires are burning from Alaska to New Mexico, fueled by triple-digit temperatures and arid conditions. In the Arizona mountain town of Yarnell, a blaze apparently sparked by lightning killed 19 members of specialized firefighting squad who had deployed their emergency shelters Sunday when erratic monsoon winds sent flames racing in their direction.
While no single wildfire can be pinned solely on climate change, researchers say there are signs that fires are becoming bigger and more common in an increasingly hot and bone-dry West.
‘‘Twenty years ago, I would have said this was a highly unusual, fast-moving, dangerous fire,’’ said fire history expert Don Falk at the University of Arizona at Tucson, referring to the Yarnell Hill fire. ‘‘Now unfortunately, it’s not unusual at all.’’
Wildfires are chewing through twice as many acres per year on average in the United States compared with 40 years ago, US Forest Service Chief Tom Tidwell told a Senate hearing last month. Since Jan. 1, 2000, about 145,000 square miles have burned, roughly the size of New York, New England, New Jersey, Delaware, and Maryland combined, according to federal records.
A draft federal report released earlier this year said climate change is stressing Western forests, making them more vulnerable to fires.
What’s happening now ‘‘is not new to us,’’ said climate scientist Don Wuebbles of the University of Illinois, one of the main authors of the federal report. ‘‘We’ve been saying this for some time.’’
Communities nestled next to wilderness are used to girding for fire season, which typically occurs in the summer. Compared with decades past, however, the traditional fire season now lasts two months longer and first responders sometimes find themselves beating back flames in the winter.
Rising temperatures all over the West, for one, have created dangerous, dry conditions.