In the center of Glacier National Park, Mount Gould's rounded ridge cradles Grinnell Glacier. On a September afternoon, Dan Fagre walks over a smooth patch of bedrock toward the ancient slab of ice. Below it, newly splintered icebergs fill an opaque blue lake.
The U.S. Geological Survey research scientist, who studies the retreat of the park's namesakes, stops at a small boulder and taps it with his trekking pole.
"You can tell that this was only recently uncovered by the retreating ice," Fagre says, pointing out the dusty rock flour left behind by the slow grinding of the glacier. He's likely the first person to ever touch the boulder.
Fagre moves forward, climbing up four-foot rock ledges like stairs, making his way to Grinnell Glacier's melting margin. The glacier and its brethren are the park's most iconic features. They're also, Fagre says, "icons of change."
"The fact that they're disappearing suggests that even the nastiest weather pockets in Glacier are becoming more benign, they're warming up," he says.
Fagre and his research team document the glaciers' hastening retreat, a symptom of climate change. When Glacier National Park was established, in 1910, there were about 150 glaciers. Now there are only 25. The scientists analyze historic photographs and conduct margin surveys to monitor the glaciers' melting. They also measure changes in total mass; at Sperry Glacier, for example, the team has noted a 34-percent loss over the past 40 years.
Worldwide, the melting of mountain glaciers, like those in Glacier National Park, accounts for about 40 percent of sea-level rise, according to researchers at the University of British Columbia.
"Normally you think of glaciers in a geologic time scale, and what we've experienced is in a very human time scale," Fagre says.
The park's glaciers would be melting even without the rapidly rising concentration of carbon dioxide and other heat-trapping greenhouse gases in the earth's atmosphere. The glaciers, which formed about 7,000 years ago, have been retreating since the Little Ice Age ended in about 1850. But the rate of change is what alarms scientists. They expect the glaciers to disappear in the next few decades.
"That's anomalous, that's weird," Fagre says. "It's accelerated and we need to have an explanation for that."
Grinnell's retreat has accelerated even in a matter of weeks. Fagre stops as he crests a moraine. Grinnell clings to its headwall to the left, and the smaller Salamander Glacier sits 250 feet above. Before the mid-1900s, the two glaciers were connected by a bulge of ice nearly 1,000 feet thick. Today they're separated by a waterfall filling a glacial lake.
Fagre had hiked to this spot a week before. "All that was open water," he says. But now the lake is full of drifting icebergs. "We had a major event in the last week and now Grinnell is smaller." Fagre shrugs. "Another day in the life of monitoring glaciers."
The icebergs nix the on-water survey Fagre and his fellow researchers, who carry inflatable boats, had planned, but they can still map the glacier's margin on foot. Erich Peitzsch and Adam Clark, also USGS researchers, assemble a six-foot-tall GPS antenna and begin walking up from the glacier's base.
"We're able to map from year to year the location of the glacier terminus with fairly high accuracy, of 20 to 50 centimeters," Peitzsch says. "So we'll basically just follow this entire terminus up the landform."
He means they'll follow it up a steep talus slope broken only by tall ledges of jagged bedrock.
"It takes a position every second, basically drawing a line around the glacier," Clark adds.
The two researchers move into a melted-out crevasse. Water echoes against the rippled ice. They round the corner and keep climbing up, over scree, boulders and ledges, all covered in rock flour. The glacier's watery edge cuts a straight line. The ice itself is mottled with streaks of rock and debris. After a half hour, they finally reach the top.
GPS mapping of Grinnell began only a few years ago. It's the repeat photographs over the last century that best illustrate the glacier's rapid retreat: An image taken in 2006 shows Grinnell roughly a quarter of its size circa 1940.
"Glaciers are not partisan," Fagre says. "They don't have any belief systems. They just sit there and reflect whatever is happening to them."
This reporting was supported by Science Source, a project of the University of Montana School of Journalism.