Surface area of glaciers and perennial snow within Mount Rainier National Park were delineated based on 2021 aerial Structure-from-Motion (SfM) and satellite imagery to document changes to glaciers over the last 125 years. These extents were compared with previously completed databases from 1896, 1913, 1971, 1994, 2009, and 2015. In addition to the glacial features mapped at the Park, any snow patches noted in satellite- and fixed-wing- acquired aerial images in September 2021 were mapped as perennial snowfields. In 2021, Mount Rainier National Park contained a total of 28 named glaciers which covered a total of 75.496 ± 4.109 km2 (29.149 ± 1.587 mi2). Perennial snowfields added another 1.938 ± 0.112 km2 (0.748 ± 0.043 mi2), bringing the total perennial snow and glacier cover within the Park in 2021 to 77.434 ± 4.221 km2 (29.897 ± 1.630 mi2). The largest glacier at Mount Rainier was the Emmons Glacier, which encompasses 10.959 ± 0.575 km2 (4.231 ± 0.222 mi2). The change in glacial area from 1896 to 2021 was -53.812 km2 (-20.777 mi2), a total reduction of 41.6%. This corresponds to an average rate of -0.430 km2 per year (-0.166 mi2 × yr-1) during the 125 year period. Recent changes (between the 6-year period of 2015 to 2021) showed a reduction of 3.262 km2 (-1.260 mi2) of glacial area, or a 4.14% reduction at a rate of -0.544 km2 per year ( 0.210 mi2 × yr-1). This rate is 2.23 times that estimated in 2015 (2009-2015) of -0.244 km2 per year (-0.094 mi2 × yr-1). Changes in ice volume at Mount Rainier and estimates of total volumes were calculated for 1896, 1913, 1971, 1994, 2009, 2015, and 2021. Volume change between 1971 and 2007/8 was -0.65 km3 ( 0.16 mi3; Sisson et al., 2011). We used the 2007/8 LiDAR digital elevation model and our 2021 SfM digital surface model to estimate a further loss of -0.404 km3 (-0.097 mi3). In the 50-year period between 1971 and 2021, the glaciers and perennial snowfields of Mount Rainier lost a total of -1.058 km3 (-0.254 mi3) at a rate of -0.021 km3 per year (-0.005 mi3 × yr-1). The calculation of the total volume of the glaciers during various glacier extent inventories at Mount Rainier is not straightforward and various methods are explored in this paper. Using back calculated scaling parameters derived from a single volume measurement in 1971 and estimates completed by other authors, we have developed an estimate of glacial mass during the last 125-years at Mount Rainier that mostly agree with volumetric changes observed in the last 50 years. Because of the high uncertainty with these methods, a relatively modest 35% error is chosen. In 2021, Mount Rainier’s 28 glaciers contain about 3.516 ± 1.231 km3 (0.844 ± 0.295 mi3) of glacial ice, snow, and firn. The change in glacial mass over the 125-year period from 1896 to 2021 was 3.742 km3 (-0.898 mi3), a total reduction of 51.6%, at an average rate of -0.030 km3 per year ( 0.007 mi3 × yr-1). Volume change over the 6-year period of 2015 to 2021 was 0.175 km3 (-0.042 mi3), or a 4.75% reduction, at a rate of -0.029 km3 per year (-0.007 mi3 × yr-1). This survey officially removes one glacier from the Park’s inventory and highlights several other glaciers in a critical state. The Stevens Glacier, an offshoot of the Paradise Glacier on the Park’s south face, was removed due to its lack of features indicating flow, and therefore is no longer a glacier but instead a perennial snowfield. Two other south facing glaciers – the Pyramid and Van Trump glaciers – are in serious peril. In the six-year period between 2015 and 2021, these two glaciers lost 32.9% and 33.6% of their area and 42.0% and 42.9% of their volume, respectively. These glaciers are also becoming exceedingly fragmented and no longer possess what can be called a main body of ice. Continued losses will quickly lead to the demise of these glaciers in the coming decades. Overall, the glaciers on the south face of the mountain have been rapidly shrinking over the last 125 years. Our data shows a continuation of gradual yet accelerating loss of glacial ice at Mount Rainier, resulting in significant changes in regional ice volume over the last century. The long-term impacts of this loss will be widespread and impact many facets of the Park ecosystem. Additionally, rapidly retreating south-facing glaciers are exposing large areas of loose sediment that can be mobilized to proglacial rivers during rainstorms, outburst floods, and debris flows. Regional climate change is affecting all glaciers at Mount Rainier, but especially those smaller cirque glaciers and discontinuous glaciers on the south side of the volcano. If the regional climate trend continues, further loss in glacial area and volume parkwide is anticipated, as well as the complete loss of small glaciers at lower elevations with surface areas less than 0.2 km2 (0.08 mi2) in the next few decades.
