Measuring Annual Geodetic Mass Balance of Brewster Glacier Using 'Structure-from-Motion' Photogrammetry: Implications for Anthropogenic Climate Change Attribution

Abstract

Alpine glaciers are key indicators of climate change due to their high sensitivity to small variations in temperature and precipitation. As global temperatures rise, glaciers have declined rapidly, highlighting one of the negative impacts of anthropogenic climate change. For this reason, monitoring glaciers is essential to understand the extent of ice loss and its broader environmental, cultural, and socioeconomic impacts. Both global and regional studies show that New Zealand glaciers are undergoing accelerating mass loss. Continuing the measuring of glacier change, this thesis quantifies recent mass balance at Brewster Glacier using structure-from-motion (SfM) photogrammetry on oblique photographs collected during end-of-summer snowline surveys in 2023, 2024, and 2025, all together with precise image locations. In doing so, methodological choices were applied by deriving a new SfM workflow. Oblique imagery were processed in Agisoft Metashape to generate co-aligned, georeferenced 3D dense point clouds, digital elevation models (DEMs), and orthomosaics. Surface elevation change was calculated using two independent methods: cloud-to-cloud comparison using the Multiscale Model-to-Model Cloud Comparison (M3C2) algorithm and the DEM differencing method. Volume change was converted to geodetic mass balance using both methods for cross-validation. The uncertainty in vertical change was quantified using stable snow-and ice-free bedrock surrounding the glacier, with the Normalised Median Absolute Deviation (NMAD). The DEM-derived mass balance was applied to an attribution framework to assess the role of anthropogenic climate change. For the period 2023–2024, geodetic mass balance results were −2.07 ±0.14 m w.e. (M3C2) and −2.07 ±0.18 m w.e. (DEM differencing). Event attribution methods show that this negative mass balance was at least 90 times more likely to occur in the current climate than without anthropogenic climate change. For the period 2024–2025, mass balance was −1.08 ±0.15 m w.e. (M3C2) and −1.13 ±0.15 m w.e. (DEM differencing), which was at least 15 times more likely to occur under the current climate. These findings indicate that anthropogenic forcing has shifted the probability distribution of mass balance towards more negative outcomes.