October 4, 2021
Jackson and Blackfoot Glacier in late August and early September Sentinel 2 false color images. Point A indicates exposed ice showing annual layers. Point B indicates exposed firn that had been retained through previous summers. The gray color of the firn indicates how dirty it is and that its albedo would enhance melting. The yellow arrow indicates the one patch of retained snow in 2021 on Blackfoot Glacier with no retained snow on Jackson Glacier.
The exceptional heat of the summer of 2021 across glaciated mountain ranges of the Pacific Northwest, reduced snowcover extent from Mount Shasta, CA north to Mount Baker, WA and west to Kokanee Glacier, BC and Bonnet Glacier, Alberta. In Glacier National Park, Montana retained snowcover by the end of summer in 2021 is the lowest observed on Blackfoot and Jackson Glacier. Snowcover extent in late summer is a good indicator of glacier mass balance, which controls changes in glacier volume/glacier area (Pelto, 2019). Here we examine the percentage of Blackfoot and Jackson Glacier that have a persistent accumulation zone from 1998-2021.
The USGS in Glacier National Park has over the last 15 years maintained an extensive glacier monitoring program including 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 have lost less than 20% during this 50 year period. Blackfoot Glacier is the second largest glacier in Glacier National Park in Montana. Jackson Glacier the seventh largest. The USGS identified the area of Blackfoot Glacier in 2015 as 1.5 km2, a reduction of 18% from 1966-2015 (USGS, 2017). Jackson Glacier had an area of 0.8 km2, losing 40% of its area from 1966-2015. Extrapolating area loss to determine when a glacier will disappear, is typically not a useful approach. Glaciers that lack a persistent accumulation zone cannot survive current conditions (Pelto, 2010). Other glaciers may have a persistent accumulation even if small that allows them to persist.
In 2005 a year of mass balance loss in the region the retained area of accumulation in late August on Blackfoot Glacier was ~60% and Jackson Glacier ~40%. The retained area of accumulation in mid-August of 2015 was ~60%, on Blackfoot Glacier and Jackson Glacier 30-40%. In September 2019 at the end of the melt season an accumulation area covered 10-15% of Jackson and 20% of Blackfoot, the lowest observed since at least 1998. In early September 2020 as summer ended both glaciers had an accumulation area ratio of ~40%. The higher typical end of. summer snowcover extent on Blackfoot Glacier explains why area loss has been less in recent decades..
The June-August 2021 period was the warmest of the 127 period of record for the Western Montana climate division (NOAA, 2021). The result was that by the end of August 2021 0n Aug. 30 less than 5% of Jackson and ~10% of Blackfoot had retained snowcover. By September 6, 2021 Jackson Glacier had no significant snowcover and Blackfoot Glacier had less than 10% remaining. Both of these are minimum values indicating large mass balance losses for both glaciers in 2021, likely over 2 m as has been observed to the north on Kokanee Glacier, BC and to the west on Easton Glacier, Mount Baker, WA. There is considerable exposed firn on both glaciers, snow that was retained in recent years, indicative that the glacier had been stripped of more snowcover than other years. The gray color of the firn indicates it is dirty, which will enhance melting.
Jackson and Blackfoot Glacier in late early September of 2019 and 2020 Sentinel 2 false color images. Point A indicates areas of retained accumulation. layers. Point F indicates exposed firn that had been retained through previous summers.
Blackfoot Glacier (B) and Jackson (J) Glacier snowpack in 2005 and 2015.