The Perito Moreno glacier, which, according to one expert, appears to be "in equilibrium," is more than 200 feet high as it empties into Lake Argentino.

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SANTA CRUZ, ARGENTINA — Dawn at Los Glaciares National Park in Santa Cruz province, at the southwestern end of Argentina.

Massive icy blue shards lunge skyward in the pale pink-orange light of sunrise. The ice creaks and groans as the glacier continues its centuries-old crawl forward, tons of momentum contained within its epic mass. A sharp crack echoes as a 200-foot tower of ice crashes into the lake below, shattering the surrounding calm. And not a moment too soon.

A distant rumble from the land has crescendoed, and suddenly the small parking lot 500 yards from the glacier's leading edge fills with dust as 15 buses disgorge hundreds of tourists bearing cameras. The morning's peace dissolves into multilingual chatter broken by gasps and yells each time another chunk of ice calves off the glacier.

"I had no idea what to expect. It's beautiful, magical," says Gary Numan, who's traveled with his wife from San Francisco. The glacier, called Perito Moreno, attracts a half a million visitors every year —a 10-fold increase since 2000, according to park officials. Its appeal includes easy access via wooden walkways leading to its 2.5-mile-long, 20-story face.

It is also one of the few glaciers in the world that isn't receding. In March, a block of ice almost the size of Tucson broke off Antarctica. As world temperatures rise along with concerns about the Earth's melting ice sheets, Perito Moreno is an apparent exception.

To reach it, tourists travel an hour and a half along a winding road along the shore of Lake Argentino, the largest in the country. The landscape moves from flat, scrubby steppe and vast sheep ranches into deep-green Andean forest. At scenic overlooks of the distant glacier, condors hover overhead. Only upon entering the park does Perito Moreno's immensity become apparent.

Los Glaciares National Park was declared a UNESCO world heritage site in 1981. It lies within the Southern Patagonian Ice Field, which spans the Argentine-Chilean border and contains some 47 glaciers. Perito Moreno is Argentina's most famous. Its name honors Francisco Moreno, the perito — surveyor — who helped resolve international territorial disputes with Chile in 1902 by determining national boundaries, which granted Argentina 26,000 more square miles.

As glaciers worldwide recede alarmingly, there's no clear consensus for Perito Moreno's apparent uniqueness. According to glaciologist Pedro Skvarca of the Argentine Antarctic Institute, Perito Moreno is "in equilibrium," meaning it gains the same amount of ice at its back end as it loses in the front: about six feet every day. The ice at the rear is formed by compacted steady mountain snowfall. Part of the reason that Perito Moreno is less vulnerable to climate change, Skvarca says, is that almost 1,300 feet is submerged in the freezing waters of Lake Argentino.

Perito Moreno is constantly shedding ice, a spectacle that has hundreds of tourists packing a boat that makes six daily trips navigating along the glacier's face as falling chunks rock the vessel.

Besides its tourist appeal, according to Skvarca, Perito Moreno and other glaciers are a major source of fresh water in Argentina's arid Santa Cruz province, where average annual rainfall is around 8 inches, even less than Tucson's 12 inches.

Since 2004, Greenpeace has photographed the recession of the Upsala and Viedma glaciers, two of Argentina's largest, about 60 miles north of Perito Moreno, to promote a national campaign for energy-efficient light bulbs. The group began its efforts, according to Greenpeace-Argentina's Rosario Espina, after realizing that in just six years, Upsala had receded eight miles.

El Calafate, a town 50 miles away, is the base camp for visitors and has developed in parallel to glacier tourism. Surrounded by miles of yellow brush, it is a verdant oasis on the shore of Lake Argentino with its non-native ornamental trees and shrubs that require constant watering. Its log-cabin restaurants, chocolate shops and gift stores all feature calafate berry jam, a local specialty.

Until recently, visitors had to drive 200 miles of unpaved roads to get here. Unlike the rest of the country, however, El Calafate benefited from a 2001 economic crisis when Argentina defaulted on its foreign debt and the value of the peso plummeted.

"Argentina became affordable for foreigners," explains Alexis Simunovic, undersecretary of tourism in Santa Cruz. "And it made Argentines travel domestically because they couldn't afford to go abroad." Today, a new airport has reduced travel time to a three-hour flight from Buenos Aires, 1,200 miles away. The fact that President Cristina Fernández de Kirchner and her husband, past-president Néstor Kirchner make their home here has also enhanced investment in El Calafate. In a recent visit, President Kirchner boasted that in 1991 there were 700 beds for tourists, and now there are more than 7,000.

Over the past eight years, the estimated population of El Calafate has grown from 6,000 to 22,000. While the town has benefited economically, residents have learned that tourism comes at a price.

"El Calafate has no efficient sewage treatment plant," says local biologist Silvina Stuzerbaum. "So all the runoff goes to Laguna Nimez, a bird reserve." Tourism consumes much of the town's water and electricity while driving up prices for locals. Yet there is no indication that Perito Moreno's lure will diminish anytime soon, especially as famous glaciers elsewhere are shrinking.

Yet Lonnie Thompson, a noted paleoclimatologist and glacier expert at Ohio State University, believes that the resilience of Perito Moreno and a handful of glaciers in Alaska that are also not receding might be a temporary trend. Thompson cites glaciers in Sweden and Norway that were growing until 1999 and are now in retreat. "Any one glacier can behave in many different ways. Many factors can affect it," says Thompson. "But looking at glaciers around the world, on a global scale, temperature is the dominant factor causing them to retreat now."

Some Argentine environmentalists believe that Perito Moreno is already showing effects of global warming. "The glacier hasn't receded in length, but it is melting internally," says Greenpeace's Espina.

Glaciologist Skvarca insists that Perito Moreno is still at equilibrium. But he admits that he doesn't know what lies ahead. "I don't have a crystal ball; I can't predict the future."

Meanwhile, tourists flock to the town of El Calafate.

"Let's pray for another piece to fall!" yells a tourist with a video camera, who missed the last calving. Despite the freezing winds, he and dozens of others cling to the railing, scanning the jagged wall of ice. Finally, with a thunderous boom, Perito Moreno concedes another piece to the lake.

Ariana Brocious is an honors student with a double major in Latin American Studies and Spanish literature. A native Tucsonan, she is a senior and plans to attend graduate school.

David Tamayo is from Tijuana, Mexico. He holds a bachelor's in history from the University of California-Berkeley and graduated in May from the UA with a master's in Latin American Studies.