Abstract
New Zealand glaciers were recently identified as having the highest rate of thinning (2015-2019) of any region globally (Hugonnet et al., 2021). However, in many global studies, New Zealand glacier measurements have high uncertainties and lower spatial coverage compared with other regions. More precisely quantifying New Zealand glacier change has implications particularly for water resources, tourism, and model calibrations that future projections are dependent on. Within New Zealand, a subset of glaciers have been photographed annually for over 40 years. This survey measures the end of summer snowline elevation, a proxy for relative glacier mass change. Since 2016, the survey has included structure-from-motion photogrammetry, enabling precise, quantitative measurements of glacier length, area, and volume change. The method uses microsecond timing of camera locations to georeference the photogrammetry-derived models, removing the need for collection of control points at each glacier. Additionally, two New Zealand glaciers have ongoing glaciological mass balance surveys running since 2005 and 2010. Here, we present a new end of summer snowline record (1977-2020), generated as a composite from 41 glaciers. The snowline series shows a strong relationship with warm-season temperatures, and shows above-average snowline elevations in 15 of the 22 years since 1999. We compare the snowline series with existing glaciological (2005/2010-2022) and newly-derived structure-from-motion geodetic (2016-2022) mass balance measurements. We show consistencies in glacier mass change between estimates from the snowline series and direct mass balance measurements. Comparisons also show a bias in glaciological measurements for one for the two glaciers, with melt (2017-2022) underestimated by at least 40% compared to geodetic data. We also compare the snowline series with published geodetic measurements (2000-2019; Hugonnet et al., 2021), both of which show accelerating mass loss of New Zealand glaciers. Finally, using an enhanced temperature index model, we consider how to use these new measurements to reduce uncertainties in modern and future estimates of New Zealand-wide glacier change.