Effects of changes in glacier geometry on mass balance: a case study of the Aldegondabreen and Vestre Grønfjordbreen glaciers (Spitsbergen)

Abstract

Glacier mass balance is widely used as an indicator of climate change; however, long-term mass-balance records may reflect not only climatic forcing but also a “geometric” signal associated with changes in glacier hypsometry and spatial configuration. In view of the accelerated glacier retreat observed over recent decades, this effect becomes increasingly important. This study quantifies the contribution of geometric changes to the mass balance of two valley glaciers on the Svalbard archipelago — Aldegondabreen and Vestre Grønfjordbreen — where continuous glaciological monitoring has been conducted since the early 21st century. Using elevation data combined with in situ mass-balance measurements, specific mass balance was calculated using two approaches: with a fixed reference surface and with annually updated area–altitudinal distributions. Geometric changes lead to additional mass losses of 0.02–0.04 m w. e. yr⁻¹ for Aldegondabreen, and only of 0.01–0.02 for Vestre Grønfjordbreen. This effect is positive geometric feedback: as the glacier loses mass, its surface becomes less elevated, thus enhancing future mass loss. For both glaciers, the “purely climatic” mass-balance trend does not differ significantly from the observed trend, equaling –0.09 ± 0.03 m w. e. yr⁻¹ for Aldegondabreen and –0.13 ± 0.04 m w. e. for Vestre Grønfjordbreen (α = 0.05). These findings indicate that neglecting glacier geometry evolution can bias interpretations of mass-balance time series and associated climate signals. Repeating similar analyses across different glacier types and regions is essential for assessing the magnitude and significance of the geometric signal in diverse glaciated environments.