Spatio-temporal variability in Southern Hemisphere glacier snowline altitudes from 2000-2020

The glacierised Southern Hemisphere is vulnerable to continued shrinkage under climate change, but representation of these mountainous regions in climate research is limited by hemispheric and altitudinal scarcity of meteorological observations. End-of-summer snowline altitude (SLAEOS) indicates glacier response to climatic forcing, though has been estimated with low spatio-temporal coverage for the Southern Hemisphere. This study presents the first Southern Hemisphere-wide quantification of SLAEOS, with analysis of regional and intra-regional trends. An automated approach was implemented in Google Earth Engine, in which glacier snow cover was classified in Landsat scenes using Otsu image segmentation and SLAEOS was estimated as the lowest altitude from which snow cover ratio was continuously > 0.5. Results encompassed 6485 glaciers of the Southern Alps, Andes, and Antarctic Peninsula, with trends calculated from 2000-2020. Snowlines underwent widespread retreat in this period; mean rates of SLAEOS rise were between 2.19 and 6.28 m yr-1 for regions, between 1.63 and 7.55 m yr-1 for east/west sub-regions, and were mostly accelerated for the recent decade (2010-2020). Mean SLAEOS lowering (-30 to -1 m yr-1) indicated stability in the southernmost Andes, contrasting to rapid SLAEOS rise (10 to 30 m yr-1) in the southern Central Chilean Andes, and eastern slopes generally experienced increased rates of SLAEOS rise compared to western slopes. SLAEOS variability was reflected in periods of summer warming and reductions in summer snowfall, though correlation with these variables was not consistently identified. East-west and north-south disparities in absolute SLAEOS and rates of SLAEOS change were linked to spatial variability in terrain elevation and prevailing moisture transport, with the latter evidencing the variability and impact of large-scale climatic modes. Given implications of observed trends for glacier mass loss, continued research may involve developing an annually-updated global dataset, investigating additional drivers of SLAEOS variability, and estimating glacier response times.