Observation of mass balance and flow of very small glaciers in northern Japanese Alps.

Nine very small glaciers (VSGs) have been identified in the northern Japanese Alps (e.g., Fukui et al., 2018; Arie et al., 2025). These VSGs persist under relatively warm conditions at elevations around 2,000 m, where the mean annual air temperature is approximately 2–3  ℃. Understanding the mechanism that enables their persistence is therefore an important scientific issue. However, long-term mass-balance observations and understanding of the flow mechanisms remain limited.

In this study, the mass balance of eight glaciers and perennial snow patch was observed from the 1960s to 2025, and interannual and seasonal variations in flow velocity were investigated at the Shakushizawa Glacier. The studied glaciers and snow patches―Hakubazawa, Shakushizawa, Kaerazuzawa, Karamatsuzawa, Kakunezato, Komado, Sannomado, Gozenzawa―are located at elevations of 1,700-2,700 m and are characterized by heavy snow accumulation (snow depth: 20-30 m) due to the northwesterly winter monsoon and frequent avalanches.

Cumulative mass balance from the 1960s to the mid-2010s showed only slight gains or losses; however, following the low-snowfall year of 2016, all glaciers and perennial snow patches experienced substantial mass loss by 2025. Meteorological observations (Hakuba AMeDAS) and ERA5 reanalysis data indicate no significant long-term decrease in snowfall since the 1950s, whereas melt amounts estimated using both an energy-balanzsce and a degree-day methods show an increasing trend. These results suggest that the recent acceleration of mass loss is mainly driven by enhanced summer melting associated with rising air temperatures.

Flow observations at the Shakushizawa Glacier show that annual flow velocities exceed those observed at the end of melt season, implying either accelerated internal deformation under heavy winter snow load or enhanced basal sliding during the early melt season.