Glacier National Park is losing its glaciers.
The flowing sheets of ice scattered throughout the Montana park shrank by more than a third between 1966 and 2015, according to new data from the United States Geological Survey and Portland State University.
Using aerial and satellite imagery, researchers traced the footprints of 39 named glaciers in the park and surrounding national forest. They found that 10 had lost more than half their area over 50 years.
Ice extent for every glacier in Glacier National Park in 1966 and 2015
Lost 50 percent or more coverage area
Lost 213 acres (54%)
Lost 133 acres (40%)
Lost 129 acres (41%)
Lost 113 acres (45%)
Lost 107 acres (33%)
Lost 98 acres (19%)
Lost 93 acres (26%)
Lost 87 acres (82%)
Lost 83 acres (18%)
Lost 70 acres (56%)
Lost 65 acres (49%)
Lost 57 acres (41%)
Lost 48 acres (85%)
Lost 44 acres (72%)
Lost 41 acres (57%)
Lost 34 acres (57%)
Lost 34 acres (81%)
Lost 33 acres (41%)
Lost 32 acres (37%)
Lost 28 acres (77%)
Lost 27 acres (27%)
Lost 26 acres (10%)
Lost 25 acres (25%)
Lost 25 acres (33%)
Lost 25 acres (49%)
Lost 20 acres (19%)
Lost 19 acres (13%)
Lost 18 acres (53%)
Lost 17 acres (20%)
Lost 14 acres (10%)
Lost 13 acres (42%)
Lost 13 acres (23%)
Lost 13 acres (23%)
Lost 10 acres (36%)
Lost 9 acres (42%)
Lost 9 acres (13%)
Lost 8 acres (26%)
Lost 7 acres (21%)
Lost 2 acres (24%)
“One of the reasons we study glaciers is because they have a simple, visual and easily understood response to climate,” said Daniel Fagre, a U.S.G.S. research ecologist who led the study. “If it gets warmer or if they get less snow, they shrink.”
Glacier National Park’s eponymous ice sheets have been around for more than 7,000 years, and have survived warmer and cooler periods. But they have been shrinking rapidly since the late 1800s, when North America emerged from the “Little Ice Age,” a period of regionally colder, snowier weather that lasted for roughly 400 years. (At its founding in 1910, the park had at least 150 glaciers, most of which are now gone.)
After the end of the Little Ice Age, glaciers across the western United States, Canada and Europe lost ice as temperatures rebounded. But scientists have attributed more recent melting to human-caused global warming.
“With each decade that we go, more of what we see can be attributed to humans, and less to natural variation,” Dr. Fagre said.
Dr. Fagre noted that even under natural conditions, these small, vulnerable mountain glaciers would have lost ground over the past 50 years — but they would have eventually stabilized at a reduced size. Instead, the park is on track to lose its ice sheets within a generation.
Larger, thinner glaciers have lost the most ground
Agassiz glacier, pictured above, has lost more ground than any other glacier in the park: over 200 acres.
The relatively large glacier — which covered nearly 400 acres in 1966 — is also relatively thin, making it more vulnerable to rising temperatures.
“The analogy here is, think of the shoreline where the water is shallow and the slope of the beach is flat, and we have a small drop in sea level, which immediately reveals a whole lot of beach. Take that same amount of sea level drop with a steep slope and deep water, and you don't expose much more beach,” said Joel T. Harper, a glaciologist at the University of Montana. “The same is true of these glaciers.”
But smaller, thicker glaciers have lost mass, too
Unlike Agassiz, the smallest glacier in the park — appropriately named Gem — has not ceded much ground over the past 50 years. But that doesn’t mean it hasn’t lost ice.
Gem is positioned at the top of a cliff, in a small area on a ledge, which means it never had much room to spread out. The glacier formed by accumulating ice upward, becoming thicker. But Gem has visibly thinned in recent years.
“Repeat photographs show it losing volume over time,” Dr. Fagre said. He added that, today, many of the park’s glaciers were “noticeably thinner than they were in the past.”
The recently published U.S.G.S. data measured only coverage area, but a coming study by Dr. Fagre’s team will measure the glaciers’ volume.
“Both processes are going on: thinning and contracting,” he said.
The park’s most visited glacier lost nearly half its footprint in 50 years
The park’s most visited glacier, Grinnell, lost 45 percent of its footprint — more than 100 acres — from 1966 to 2015.
“I’ve been going there since 1991 and remember having to choose carefully how to climb up onto the glacier. It was 20 to 30 feet high at the edge,” Dr. Fagre said. “Now it comes only up to your shins.”