Thoughts on Mount Rainier's glaciers

This post is a bit later than I would've liked, but nevertheless seems an appropriate one to wind up this hot month of July.

elevation change on Mount Rainier's glaciersWhen we received our hard copy of the May 2019 issue of National Geographic, I wasn't home. But I arrived home that evening to see the pages spread to a stunning graphic depicting the change in elevation of Mount Rainier's glaciers since 1970. The story is similar to that told by Sisson and others in this 2011 Geology paper. Glaciers on Mount Rainier are, for the most part, losing mass (i.e., ice surface elevation has lowered). Many have retreated. Some of the greatest mass loss is in the lower-elevation glacier termini, while some areas of mass gain are mostly limited to higher elevations. There are two oddball spots on the map, however, and they jump out to me, begging for explanation. On the north and east flanks of the mountain, Winthrop and Emmons glaciers both have blue zones, indicating thickening in their termini. What explains the anomalous thickening there? One thing that these two glaciers have in common is an extensive blanket of supraglacial (on top of the ice) rock debris near their termini. This could certainly have a significant impact, but can the presence or absence of supraglacial debris alone explain how these two glaciers differ from the others in their response to 20th and 21st century climate changes? Nope! Turns out that Carbon Glacier, the next one to the west from Winthrop Glacier, has extensive debris cover as well, and it has experienced more thinning than most others. So the presence of supraglacial debris alone cannot fully explain why Emmons and Winthrop Glaciers are different.

The Applied Geomorphology Lab has recently contributed to this conversation with a paper in Arctic, Antarctic, and Alpine Research examining the role of supraglacial debris in melt rate on these three glaciers. Using debris-surface temperatures from satellite imagery and a simple energy-balance model, we found that the supraglacial debris in the thickening zones of Emmons and Winthrop Glaciers is likely thicker or less conductive than debris elsewhere. This tells part of the story, but not all of the story. Reduced ablation rates under thick debris may be important, but if the ice has thickened in these blanketed termini, there must also have been either an increase in debris thickness in these areas since 1970 or a change in the ice flux to these areas. More research will be needed to sort out these remaining details!