harrison glacier retreat – From a Glaciers Perspective

Snowcover extent in Landsat images from August, 1998 and 2018. S=Sperry, H=Harrison, J=Jackson, B=Blackfoot and P=Pumpelly

Five of the eight largest glaciers in Glacier National Park are clustered in a small area: Jackson Glacier, Sperry Glacier, Pumpelly Glacier, Harrison Glacier, and Blackfoot Glacier. The USGS in Glacier National Park has over the last 15 years maintained an extensive glacier monitoring program led by Dan Fagre. This program has led to consistent mass balance observations on Sperry Glacier, and repeat mapping of the 37 named glaciers, 25 of which still qualify as glaciers. The repeat mapping indicates the area lost from 1966 to 2015, (USGS, 2017). There is considerable variation between glaciers , some have lost more than 80% of their area and others having lost less than 20% during this 50 year period. Snowcover extent in late summer is a good indicator of glacier mass balance, which controls changes in glacier volume/glacier area. Glaciers that lack a persistent accumulation zone cannot survive current conditions (Pelto, 2010). Observations of the snowcover extent in years of limited snowpack illustrate which glaciers do have a persistent snowcover and can survive vs those that cannot (Pelto, 2011). Here we examine Landsat imagery from mid to late August in 1998, 2005, 2015 and 2018, all years of extensive mass loss to identify the difference in snowcover extent, which will drive mass balance loss and subsequent retreat. These images are not at the minimum snowcover extent, which usually occurs n September. For a glacier to be in equilibrium it needs more than 50% of its area to be snowcovered at the end of the melt season.

In 1998 glacier mass balance losses were significant in the region, in mid-August the accumulation area (snowcovered area) on Harrison Glacier exceeded 80%, Blackfoot Glacier was ~60 % snowcovered, Jackson Glacier ~50% snowcovered and Sperry Glacier ~30% snowcovered. In 2005 another year of minimum mass balance in the region in late August, Harrison Glacier had ~80% snowcover, Blackfoot Glacier~60% snowcover, Jackson Glacier ~40% snowcover and Sperry Glacier ~20% snowcover. In 2015 glacier volume losses in the region were again large, with Sperry Glacier having a loss of -1.22 m. The retained area of accumulation on Harrison Glacier in mid-August of 2015 was ~60%, on Blackfoot Glacier 50%, on Jackson Glacier 30-40% and on Sperry Glacier less than 20%. In mid-August of 2018 snowcover extent was greater than 75% on Blackfoot, Harrison and Jackson Glacier, while Sperry Glacier had ~40%.

The USGS identified the area of Blackfoot Glacier in 2015 as 1.5 km², a reduction of 18% from 1966-2015 and 8% from 1998-2015 (USGS, 2017). Harrison Glacier had an area of 1.7 km², losing 17% of its area from 1966-2015, and 10% from 1998-2015. Jackson Glacier had an area of 0.8 km², losing 40% of its area from 1966-2015, and 6% from 1998-2015. Sperry Glacier had an area of 0.8 km²in 2015 having lost 40% of its area from 1966-2015, and 16% from 1998-2015. The persistent pattern of limited snowcover extent on Sperry Glacier indicates why the percentage of area loss has been greater than on the other glaciers. The lack of significant retained snowcover indicates Sperry Glacier cannot survive current climate. The retention of significant snowcover on Blackfoot and Harrison Glacier even in low snowpack years indicate these glaciers, unlike most in the National Park, can survive the current climate. The Jackson Glacier is in between these two scenarios.

Jackson is the lowest elevation glacier and Harrison is the highest. Blackfoot, Jackson and Sperry are all north facing. Pumpelly which faces south has lost the least area of the five glaciers from 1998-2015. This underscores the utility of Landsat imagery in assessing glacier mass balance response. Three of the glaciers that retain significant snowcover indicates these glaciers are not as vulnerable to warming and will continue to persist until 2050 at least.

USGS Data on glacier area.

Observation Year	LIA	1966	1998	2005	2015
Jackson	3.1	1.3	0.8	0.8	0.8
Harrison	3.5	2.1	1.8	1.7	1.7
Blackfoot	5.0	1.8	1.6	1.6	1.5
Sperry	3.8	1.3	1.0	0.9	0.8

Snow cover extent in Landsat images from August, 2005 and 2015. S=Sperry, H=Harrison, J=Jackson, B=Blackfoot and P=Pumpelly

**Sholes Glacier August, 24 2016 with the snowcover extent vs the exposed glacier ice.**

USGS Topo Map of the area with the blue line indicating the 8000 foot contour.

There continues to be a persistent misconception that all glaciers in Glacier National Park will be gone by 2030, I get asked that by journalists frequently and when I point out that is not the case they are surprised. An examination of 15 Glacier National Park glaciers using the recently published Alpine Glacier Survival Forecast method, indicates that 10 of the 15 glaciers are experiencing a disequilibrium response and will disappear, the other five have been shrinking little. A simpler and more visual look at the survival issue, illustrates why though they all are diminishing the glaciers will not all be gone by 2030. Blackfoot and Harrison Glacier are the two largest glaciers and show minimal changes in the accumulation zone. Both glaciers continue to retreat with the main termini retreating approximately 100-120 m since 1966. In this post we take a close look at the Harrison Glacier the most vigorous and slowest receding of the few remaining Glacier National Park glaciers. Key and Fagre (2003) utilized a model to construct the future of glaciers in the Blackfoot-Jackson watershed, and determined that all would be gone by 2030 with continued substantial warming, but not with limited additional warming. Based on the slow recession and equilibrium response of Blackfoot and Harrison Glacier to recent climate over the last 40 years these two glaciers are not going to disappear within the next 30 years. Harrison Glacier has according to the ongoing work of Northern Rocky Mountain Science Center (NOROCK) Has lost 9% of its areas between 1966 and 2005, a 40 year period. In the first image below the glacier is outlined in the 1966 map of Harrison glacier overlaid in Google Earth. The orange outline is left on the following three images all from Google Earth’s historic imagery files. The map indicates the area of crevasses above the main terminus. A look at the glacier over the last two decades indicates the glacier remains vigorous in terms of flow, as indicated by the many crevasses. In every image from 1991 second image to 2003 and 2005 last two images, even in these later summer images the glacier retains snowpack in its upper accumulation zone. This suggest a glacier that can survive current climate at a diminished size. The above images indicate the slow recent recession of the Harrison Glacier, which unlike the majority in the park is only slowly receding. This is in contrast to nearby Shepard Glacier and Grinnell Glacier which often are devoid of snow and are losing area at a rate of 10% per decade, four times that of Harrison Glacier. Why the difference? Most of the glaciers lay on the east or northeast slopes-lee side of the mountain ridges and have significant avalanching from the slopes above. Grinnell Glacier has a significant accumulation area at 7000 feet and Harrison Glacier at 9000 feet. Hence, the greater change in area as seen between the 1996 orange margin and 2006 recent margin for Grinnell Glacier