SCOTT SIMON, host:

All year, NPR is exploring how people are changing the Earth's climate and how the climate is changing us. It's called Climate Connections with National Geographic.

Today, we'll visit some geographic jewels - the Sky Islands of southern Arizona and northern Mexico. As temperatures rise, these mountaintop refuges are starting to look more like the desert below.

NPR's Ted Robbins explains.

TED ROBBINS: Look at a topographic map of North America. You'll see two huge spines in the West - the Rocky Mountain stretching from Canada to the desert southwest, and south of there, in Mexico, the Sierra Madre.

In between are relatively small, isolated mountain ranges rising from the desert, much as islands rise from the ocean. Which is why the ranges - with names like the Huachucas, the Chiricahuas, the Catalinas - are called Sky Island. And they are home to unparalleled diversity.

Mr. MATT SKROCH (Sky Island Alliance): Believe it or not, there's more mammals that occur here in southeastern Arizona than anywhere else in the United States.

ROBBINS: Matt Skroch is head of the Sky Island Alliance, an environmental group trying to protect this intersection between tropical species on the northern end of their range...

Mr. SKROCH: Such as jaguars, ocelots, greyhawks, Mexican possums.

ROBBINS: And temperate species on the southern end of their range like black bear and northern goshawk - not to mention 20 species of hummingbird. But much the way oceans are rising and claiming some seaside habitat, hotter, drier conditions are moving up the slopes of the Sky Islands.

Matt Skroch and I drive from the heat of Tucson to the top of the Santa Catalina Mountains. In less than an hour, we can climb 7,000 feet, the ecological equivalent of going from the desert southwest to Canada. We passed giant saguaro cactus, then oak woodlands, then about halfway up the mountain, we see the first sign of a cooler climate - a ponderosa pine in a small canyon.

(Soundbite of car door closing)

ROBBINS: It's the perfect place to see change.

Mr. SKROCH: Because we're at the fringe of where the pine occurs within the Sky Islands, you also notice, looking across this canyon, that about 80 percent of the pines are dead. And that's because it's getting hotter.

ROBBINS: This is gradual change, creeping up the mountainside like a rising tide. Farther up the mountain, we see the equivalent of a tsunami.

Mr. SKROCH: Now, down there, you don't see any tree that's alive.

ROBBINS: This hillside burned in two swift hot fires in the last five years -the Bullock fire and the Aspen fire. More than 100,000 acres burned in the Catalina range. Very few pine, spruce, or fir are growing back here. Instead, warmer-adapted species like oaks and grasses are starting to come in.

Carol Mack owns the Mt. Lemmon General Store in Summer Haven, a village near the top of the Catalinas. She says she's starting to see animals she's never seen here.

Ms. CAROL MACK (Owner, Mt. Lemmon General Store): We've seen a road runner up here. Now, how far out of their element could a road runner be?

(Soundbite of "Road Runner")

ROBBINS: Hey, even if you've only seen them in Warner Brothers cartoons, you know the road runner belongs in the desert. But they appear to be climbing the mountain to some rates. Carol Mack's first Mt. Lemmon General store burned in the Aspen fire along with nearly 350 other buildings.

Now, above the new store in town, there's little forest covered.

Ms. MACK: I hear from so many people who have rebuilt that the winds are just so bad now. And the sun is just so much more intense. It just feels hotter to us.

ROBBINS: It feels hotter, and it is drier. The southwest is in the middle of a six-year drought. But drought is a natural occurrence.

(Soundbite of chainsaw)

Back in Tucson, scientists at the University of Arizona Tree-Ring labs slice and polish cross-sections of ancient trees to study climate change.

Lab director Tom Swetnam shows me a sample from the Chiricahua Mountains. It has a band of thinner rings, indicating a drought that lasted not six years, but six decades.

Dr. THOMAS SWETNAM (Director, Laboratory of Tree-Ring Research, University of Arizona): And so a sixty-year long drought in the Colorado River basin would exceed anything that we've seen in the modern period, so far, so far.

ROBBINS: Swetnam says that because scientists are beginning to think that the combination of normal drought with human-caused higher temperatures might tip the region into a mega-drought.

Dr. SWETNAM: The effects of global warming might be a trigger to those sorts of extended drought periods.

ROBBINS: As the Sky Island shrink, people are working on ways to protect what's left. More prescribed burns and tree thinning to prevent cataclysmic fires. More than protection for animals that migrate between the Sky Islands, and ultimately, ways to save species at the very top stranded with no place to go.

Ted Robbins, NPR News, Tucson.

SIMON: You can see the latest episode of our animated series, "It's All About Carbon," from NPR's Robert Krulwich and public television's "Wild Chronicles" at our Web site, npr.org.

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