Arctic glacier snowline altitudes rise 150 m over the last 4 decades

The number of Arctic glaciers with direct, long-term measurements of mass balance is limited. Here we used satellite-based observations of the glacier snowline altitude (SLA), the location of the transition between snow cover and ice late in the summer, to approximate the position of the equilibrium-line altitude (ELA), a variable important for mass balance assessment and for understanding the response of glaciers to climate change. We mapped the snowline (SL) on a subset of 269 land-terminating glaciers above 60 degrees N latitude in the latest available summer, clear-sky Landsat satellite image between 1984 and 2022. The mean SLA was extracted using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM). We compared the remotely observed SLA observations with available long-term field-based measurements of ELA and with ERA5-Land reanalysis climate data. Over the last 4 decades, Arctic glacier SLAs have risen an average of similar to 152 m (3.9 +/- 0.4 m yr(-1); R2=0.74, p<0.001), with a corresponding summer (June, July, August) temperature shift of +1.2 degrees C at the glacier locations. This equates to a 127 +/- 5 m shift per 1 degrees C of summer warming. However, we note that the effect of glacier surface thinning could bias our estimates of SLA rise by up to similar to 1 m yr(-1), a significant fraction (similar to 25 %) of the overall rate of change, and thus should be interpreted as a maximum constraint. Along with warming, we observe an overall decrease in snowfall, an increase in rainfall, and a decrease in the total number of days in which the mean daily temperature is less than or equal to 0 degrees C. Glacier SLA is most strongly correlated with the number of freezing days, emphasizing the dual effect of multi-decadal trends in mean annual temperature on both ablation (increasing melt) and accumulation processes (reducing the number of days in which snow can fall). Although we find evidence for a negative morpho-topographic feedback that occurs as glaciers retreat to higher elevations, we show that more than 50 % of the glaciers studied here could be entirely below the late-summer SLA by 2100, assuming the pace of global warming and the mean rate of SLA rise are maintained.