Topography, snowfall affect retreat of cirque glaciers

University of Montana doctoral candidate Caitlyn Florentine is taking a new look at Sperry Glacier’s retreat, and this particular glacier’s near future isn’t quite as bad as previously thought.

Florentine’s data suggest that cirque glaciers, the smallest ice bodies on landscapes, are heavily influenced by topographic processes like avalanches and snow drift.

A cirque glacier is confined in a throne-shaped valley at high elevation. Although larger valley glaciers, ice caps, and ice sheets are found elsewhere in the world, all the glaciers in Glacier National Park are cirque glaciers. In 1910, when Glacier was established, there were 150 glaciers. Now there are 26.

All glaciers in the Park are receding, but some have demonstrated greater loss than others since 1966. Florentine’s interest was piqued when she observed that, in response to a warming climate, large glaciers historically lose relatively more area than smaller glaciers.

Piegan Glacier, for example, is a small glacier that’s already tucked up and contained close to a steep headwall. It’s been this way since 1938, old photographs show. But Piegan only lost 13 percent of its area between 1966 and 2015.

Canadian scientists have similar data indicating that some larger glaciers have lost more area while some small glaciers, especially those that are north-facing, could be more resilient.

“We know that climate is still a part of the story,” Florentine confirmed, noting that in warmer, lower-latitude places like Utah, the cirque topographic features have no glaciers whatsoever.

To test her hypothesis about the impact of topographic processes, Florentine used mass balance data from Sperry Glacier, snowfall and summer temperature data, and historical records of glacial extent.

In general, all models of glacial extent from the early 1900s to today have indicated loss with some variability. With more warm days in a summer, glaciers melt more.

With more winter snowfall, the glacier is better able to accumulate mass. The difference between accumulation and ablation, or melting, is the year’s mass balance.

Since the mid-20th century, Glacier has seen an overall decrease in snow and an increase in temperature. How have cirque glaciers responded?

No matter what the annual weather patterns, in every year since 1950, mass balance is greater near the headwall and lower near the glacier’s edges. And since Sperry Glacier has lost its extended “toe” with gradual warming, it’s shown less ablation at its lower boundary. So the relative mass balance loss is less for Sperry than it used to be, according to Florentine’s research.

“Modern, cirque-contained, avalanche-fed Sperry Glacier is accumulating relatively more mass than it did without the toe,” Florentine summarized. “It really seems like it’s a story about topography.”

She was quick to reiterate that warming is a key factor too, though, and topographic processes could be buffering climate change response.

“Cirque glaciers never completely decouple from climate. Sperry has a positive bias, but it’s a modulation of melt,” she said.

Next, Florentine hopes to conduct a vulnerability assessment of all the glaciers in the Park to evaluate which ones could retain their mass more easily due to topographic processes, and which are vulnerable wholly to the “impending doom” of climate change.