Glacier National Park had about 80 glaciers in 1850 and just 32 by 2005, USGS says

Around 1850, at the peak of the Little Ice Age, Glacier National Park in Montana held about 80 glaciers. By 2005 the count stood at 32, a decrease of nearly 60 percent in roughly 150 years. The figures come from the U.S. Geological Survey’s Northern Rocky Mountain Science Center, which has tracked the park’s ice for decades.

Both numbers carry a definition that matters. The USGS counts only ice bodies larger than 0.1 square kilometer, about 25 acres, so the tally measures the survival of reasonably sized glaciers rather than the survival of every patch of ice.

The 0.1 square kilometer threshold behind the count of 32

On its status-of-glaciers page, the USGS defines a glacier as a body of snow and ice that moves under its own weight, with crevasses as a possible sign of movement. For the park, the working cutoff is 0.1 km2, equal to 100,000 square meters. Smaller bodies of ice do not enter the 80-to-32 comparison, although the agency also keeps a broader inventory that uses a 0.01 km2 threshold, following the Randolph Glacier Inventory, to capture very small glaciers.

Most of the park’s glaciers are cirque glaciers, small bodies sitting in alpine basins along the Continental Divide. Small size is the reason the park has little buffer: a glacier with modest volume and a sheltered basin can lose a large share of its area while still being counted, then drop below the cutoff and leave the tally.

Moraines, satellite imagery and the 1850 and 2005 snapshots

The two endpoints were built differently. According to the USGS, the roughly 80 glaciers of 1850 come from a moraine inventory made with satellite imagery, because the rock ridges that glaciers pushed up at their maximum extent remain visible after the ice has gone. The 32 glaciers of 2005 come from an analysis of modern glacier extent. The agency dates the Little Ice Age to about 1400 AD, peaking around 1850 AD.

A separate USGS dataset, released with Portland State University in 2017, follows 37 named glaciers in the park, plus two on adjacent Flathead National Forest land, across the margin years 1966, 1998, 2005 and 2015. The time-series page says all of the measured glaciers lost area between 1966 and 2015. The average reduction was 39 percent. Boulder Glacier shrank the most, by 85 percent, and Pumpelly Glacier the least, by 10 percent.

By 2015, 26 of the named glaciers remained large enough to be considered glaciers, according to the same page. Satellite imagery revised an earlier count of 25. The USGS attributes the differences in retreat rates from glacier to glacier to local conditions, among them geometry, ice thickness, elevation, shading, avalanche input and wind-deposited snow.

Repeat photography at Grinnell, Boulder and Jackson

Numbers are one record of the loss; photographs are another. The USGS Repeat Photography Project pairs historic images, taken between 1887 and 1943, with modern ones shot from the same spot, beginning in 1997 and continuing through 2019. Lisa McKeon, a former USGS physical scientist, took many of the modern frames, and Daniel Fagre is credited on others. The project describes the method as providing “objective visual evidence of landscape change.”

Grinnell Glacier has the most pairs, 23. The pictures document recession, but also fragmentation: Blackfoot Glacier has separated into Blackfoot and Jackson, and Logan and Red Eagle were once a single continuous glacier. At Grinnell, a lake called Upper Grinnell Lake formed in the space the ice vacated.

The National Park Service puts the pattern in a single sentence on its glaciers overview: all of the glaciers in the park have receded. The Kootenai, the same page notes, traditionally called the area the place where there is a lot of ice.

Another USGS count for the same era is higher, and the two figures use different inventories. A USGS fact sheet written by Caitlyn Florentine, published in December 2019 as Fact Sheet 2019-3068, says the park had 146 small mountain glaciers at the Little Ice Age maximum near the mid-19th century and that the glaciers have decreased in number, area and volume since the park’s establishment in 1910. The 80-to-32 pair is the one that holds a single size cutoff constant at both ends.

Projections for the remaining ice

The USGS status page describes two modeling efforts. A previous USGS geospatial model forecast early loss but did not account for existing ice volume or other physical controls. A more recent physical model, led by the World Heritage Programme and published by Bosson and colleagues in 2019, predicts near-total loss of the park’s glaciers by 2100, using the mean of 14 global climate models. Larger glaciers elsewhere may persist beyond 2100 under mitigated emissions, the page says, while the park’s small glaciers have little buffer.

The USGS adds that local factors such as ice thickness, shading and wind may shift the timing of disappearance for individual glaciers. The agency’s published endpoint for the park as a whole, a drop from about 80 glaciers to 32 by 2005 and to 26 named glaciers by 2015, is the baseline against which any later count will be measured.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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