High above the desert floor, this little alpine town has long served as a natural air-conditioned retreat for people in Tucson, one of the so-called sky islands of southern Arizona. When it is 105 degrees in the city, it is at least 20 degrees cooler up here near the 9,157-foot summit of Mount Lemmon.

But for the past 10 years or so, things have been unraveling. Winter snows melt away earlier, longtime residents say, making for an erratic season at the nearby ski resort, the most southern in the nation.

Legions of predatory insects have taken to the forest that mantles the upper mountain, killing trees weakened by record heat. And in 2003, a fire burned for a month, destroying much of the town and scarring more than 87,000 acres. The next year, another fire swept over 32,000 acres.

“Nature is confused,” said Debbie Fagan, who moved here 25 years ago after crossing the country in pursuit of the perfect place to live. “We used to have four seasons. Now we have two. I love this place dearly, and this is very hard for me to watch.”

The American Southwest has been warming for nearly 30 years, according to records that date to the late 19th century. And the region is in the midst of an eight-year drought. Both developments could be within the range of natural events. But what has convinced many scientists that the current spate of higher temperatures is not just another swing in the weather has been the near collapse of the sky islands and other high, formerly green havens that poke above the desert.

Fire has always been a part of Western ecology, particularly when the land is parched. But since the late 1980s, the size and reach of the fires have far exceeded times of earlier droughts. And the culprit, according to several recent studies, is higher temperatures tearing at a fabric of life that dates to the last ice age.

“A lot of people think climate change and the ecological repercussions are 50 years away,” said Thomas W. Swetnam, director of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson. “But it’s happening now in the West. The data is telling us that we are in the middle of one of the first big indicators of climate change impacts in the continental United States.”

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.

For the squirrel, the frog and other species that have retreated ever higher, there may be no place left to go. “As the climate warms, these species on top of the sky islands are literally getting pushed off into space,” Dr. Overpeck said.

The Coronado National Forest, which includes Mount Lemmon and Mount Graham, lists 28 threatened or endangered species. Heat has greatly diminished the web of life that these creatures depend on, and they “have not evolved to tolerate these new conditions,” Forest Service officials wrote in a report on the declining health of the sky islands.

For people moving to the breezy pines to escape desert heat, the fires that swept through places like Summerhaven can be terrifying. Fire comes much earlier, and much later, in the season.

(By Timothy Egan, NYT, 27/03/2007)