And it comes at a time when millions of Americans are moving to these places. Since 1990, more than eight million homes have been built in Western areas that foresters call “the urban-wild land” interface, also the focus of recent federal firefighting efforts.

The fear is that what happened to Summerhaven is a taste of things to come. As heat-stressed ecosystems provide fuel at the edges of new homes, catastrophic fires could become the new normal. Dr. Swetnam compares it to new developments in hurricane-prone areas in the Southeast.

Others say the projections are overly alarmist, and note that fuel buildup is a legacy of fire repression, not necessarily higher temperatures. They also say the higher reaches of the West may simply be evolving into less alpine settings, and could resemble life that exists at lower elevations.

Still, there is a broad consensus that much of the West is warmer than it has been since record keeping began, and that changes are happening quickly, particularly in places like the sky islands.

“The West has warmed more than any other place in the United States outside Alaska,” said Jonathan T. Overpeck, a University of Arizona scientist and co-author of the recent draft by the Intergovernmental Panel on Climate Change, released last month in Paris.

A trip up to any one of the 27 sky islands shows the ravages of heat on the land. The forests are splotched with a rusty tinge, as trees die from beetle infestation. Frogs with a 10,000-year-old pedigree have all but disappeared. One of the sky islands is the world’s only habitat for the Mount Graham red squirrel, an endangered species down to its last 100 or so animals.