Abstract
ABSTRACTQuantifying historic changes in glacier size and mass balance is important for understanding how the cryosphere responds to climate variability and change. Airborne photogrammetry enables glacier extent and equilibrium line altitudes (ELAs) to be monitored for more glaciers at lower cost than traditional mass-balance programs and other remote-sensing techniques. Since 1977, end-of-summer-snowlines, which are a proxy for annual ELAs, have been recorded for 50 glaciers in the Southern Alps of New Zealand using oblique aerial photographs. In this study, we use structure from motion photogrammetry to estimate the camera parameters, including position, for historic photographs, which we then use to measure glacier change. We apply this method to a small maritime New Zealand glacier (Brewster Glacier, 1670–2400 m a.s.l.) to derive annual ELA and length records between 1981 and 2017, and quantify the uncertainties associated with the method. Our length reconstruction shows largely continuous terminus retreat of 365 ± 12 m for Brewster Glacier since 1981. The ELA record, which compares well with glaciological mass-balance data measured between 2005 and 2015, shows pronounced interannual variability. Mean ELAs range from 1707 ± 6 to 2303 ± 5 m a.s.l., with the highest ELAs occurring in the last decade.
Highlights
Establishing records of glacier change is important for understanding how the cryosphere responds to natural and anthropogenic climate change
We find that for all except 4 years, equilibrium line altitudes (ELAs) calculated with annual digital elevation models (DEMs) are higher in elevation than ELAs calculated with the 2017 DEM alone
We present a new method for quantitatively measuring glacier fluctuations from historic images, and detail the associated uncertainties
Summary
Establishing records of glacier change is important for understanding how the cryosphere responds to natural and anthropogenic climate change. Chronologies of past glacier volume, length, equilibrium line altitude (ELA), and mass balance provide a valuable means for developing and testing predictive models, as well as environmental evidence that help us understand how past climate variability and change impacted glacial systems. From this understanding, we can better evaluate how glaciers will change in the future. New Zealand glaciers make up a small percentage Earth’s glaciers and contain only 0.21 mm of global sea level equivalent (Radić and Hock, 2010) Their response to South Pacific climate variations is important for understanding the relationship between glaciers and regional climate variability (Mackintosh and others, 2017). Their high sensitivity to climate makes them excellent indicators of climatic changes (Oerlemans, 1994), and their location provides one of the few Southern Hemisphere records of maritime glacier variability (e.g. Oerlemans, 2005; Schaefer and others, 2009)
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