Using new regional geodetic mass balance data to reassess whether cumulative mass change at the South Cascade benchmark glacier represents regional mass loss

Since 1959 the U.S. Geological Survey (USGS) has measured the annual and seasonal mass balance at South Cascade Glacier. The USGS chose South Cascade with the intention that this mid-latitude alpine glacier represented the mountain glaciers within the maritime climate of the North Cascade Mountain Range in Washington State. Recent efforts to quantify the long-term (30 to 60-year) geodetic mass balance across all glaciers in the region provide the first comprehensive dataset to evaluate whether cumulative mass change at South Cascade is truly representative. This geodetic mass balance dataset differences digital elevation models (DEMs) derived from sub-meter WorldView/GeoEye satellite imagery with a "pre-lidar" version of the National Elevation Dataset (NED). Previous efforts by Fountain et al. (2009) showed that despite strong correlation of annual mass balance variation between South Cascade and four nearby glaciers, scaling South Cascade cumulative mass loss to estimate regional values resulted in a 300% overestimate. Updated geodetic balance results are similar and show over the past half century, mass loss at South Cascade was 2 to 3 times the rate observed for glaciers in the North Cascades. From 1958 to 2015, South Cascade experienced surface lowering and mass loss over its entire elevation range (1600 to 2100 m.a.s.l) and lost approximately 34 meters of water equivalent thickness averaged over its surface. These results reinforce that traditional glaciological field methods capture inter-annual variability but regional geodetic mass balance studies are needed to derive more accurate long-term, regional mass loss estimates. Re-evaluating the benchmark glacier premise with this modern dataset is critical for reducing uncertainty surrounding regional estimates of mountain glacier contribution to sea-level rise and water resources.