Seasonal to decadal evolution of firn properties and impacts on hydrology of the Juneau Icefield

Alpine glaciers of Alaska are significant contributors to global sea-level rise. Most Alaskan glaciers lose mass through surface melt due to increasing atmospheric temperatures. Surface melt may be retained or run off in the snow and firn, potentially altering regional glacial hydrology. Here, we used field observations and firn modeling to investigate seasonal to decadal changes in thermal and physical properties of firn on the Juneau Icefield, Alaska, and assessed the resulting impacts on meltwater retention and runoff. First, we found that measured mean density and liquid-water content generally increased up to 5 % and 71 %, respectively, during the 2024 summer season relative to our initial summer measurements. This indicates meltwater retention in the snow and firn, and reveals the potential for local or regional aquifers. Second, from 1980 to 2019, modeled firn thickness decreased at a rate of 1.2–3.3 m per decade and the firn-air content decreased at a rate of 0.43–0.83 m per decade. Third, we found that a reduction in modeled firn cold content was a primary driver of increased meltwater runoff from the firn (0.11–0.16 m i.e. per decade), decreased meltwater refreeze (0.37–0.44 cm i.e. per decade), and onset of seasonal runoff shifted earlier by 4 to 6 d per decade from 1980 to 2019. Our results suggest that firn on the Juneau Icefield and other similar temperate Alaskan alpine glaciers will continue to lose meltwater refreezing and storage capacity, routing increased melt directly into runoff. In addition, inter-seasonal shifts in liquid-water retention may introduce critical uncertainties in mass-balance calculations used for sea-level rise estimates.