Incoming shortwave radiation in the Barentsburg area, Spitzbergen, 2015–2025.

We present an analysis of ten-year (2015–2025) in situ measurements of shortwave downward radiation (SWD) near Barentsburg, Spitzbergen. The measurements were performed using an automatic weather station near the Aldegondabreen Glacier at 180 m a. s. l., equipped with a Hobo silicon pyranometer (300–1100 nm, ±5 % accuracy). The actual maximum radiation occurs between 15 May and 25 June, preceding the theoretical peak of the astronomical cycle, with a mean daily flux of 204 W m– 2 , then gradually decreasing after late June. The data collected show good agreement with Ny-Ålesund measurements (R 2 = 0.83) and ERA5 reanalysis data (R 2 = 0.89). According to the latter, during the melt season a negative trend in shortwave flux has been observed since at least 1960, with –3.3 W m– 2 per decade over 1976–2024, and a sudden decrease occurred in the late 1970s, likely linked to increased cloudiness from reduced sea ice. A comparison of the two climatic normals shows that the decrease in incoming shortwave radiation is seasonally uneven. It is limited to late summer, when radiation levels are already low, whereas in May and early June — during the seasonal maximum — no reduction is evident. Consequently, the timing of snow cover disappearance is a key control on glacier melt as maintaining a high surface albedo in early summer is critical for limiting melt. Quantitative assessment shows that a two-week shift in snowmelt timing changes the solar radiation absorbed by glaciers by ~111 MJ m– 2 , which is an equivalent of 0.36 m w. e. of glacier melt.