# Elevation and Mass Changes of the Southern Patagonia Icefield Derived from TanDEM-X and SRTM Data
## 4 Discussion
### Elevation and Mass Changes of the Entire SPI
This is caused by the interferometric approach of our study while in the optical ASTER datasets cloudy pixels or correlation mismatches remain in bright areas of the accumulation areas, which might not have been filtered out sufficiently by the time series analysis. The most pronounced surface lowering rates of Jorge Montt and Upsala as well as HPS12 Glacier have been noted by previous studies as well [7; 41]. Those are most likely caused by dynamic adjustments to the fast retreat of their glacier fronts. The retreat and acceleration had been reported by various authors [43; 44; 45; 46; 47]. For Upsala glacier the retreat has accelerated between 2008 and 2011 remaining stablesince then [46; 48; 49; 50]. Those glaciers show also the largest dh/dt rates despite accumulation area ratios (AAR) ranging from 0.65 to 0.7 (Supplementary Materials Table S2). Negative elevation changes are also measured on Occidental, Chico, O'Higgins, Viedma and Ameghino glaciers as well as in the very south of SPI. Occidental Glacier is also considered most sensitive to Equilibrium Line Altitude (ELA) changes by [PERSON] [17], since the bulk area is located below the ELA--addressed as class C in his study. This is supported by our measured mean dh/dt rates of more than 1.5 m\(\cdot\)a\({}^{-1}\) and an AAR of about 0.26. Our observed negative mass balance for the catchment of Chico Glacier is slightly higher than previous observations finding \(-\)0.29 \(\pm\) 0.097 w.e.\(\cdot\)a\({}^{-1}\) between 1975 and 2001 [10], which was attributed to increasing temperatures and reduced precipitation. Grey Glacier has also lost major parts of its northern and southern frontal lobe causing largest lowering rates to occur in those areas. For the main branch lowering rates are comparable to adjacent Tyndall Glacier or the low glacier tongues on the west coast e.g., Amalia Glacier. Our surface lowering rates at the tongue of Tyndall Glacier are higher than the thinning rates of 1.7 m\(\cdot\)a\({}^{-1}\) (1975-1985), 3.3 m\(\cdot\)a\({}^{-1}\) (1985-1993) and 3.6 m\(\cdot\)a\({}^{-1}\) (1993-2002) obtained by markers at the central part of the glaciers reported by [PERSON] and others [11]. Another study by [PERSON] and others [51] reported a similar magnitude of elevation changes, but also states maximum rates of up to 7.6 m\(\cdot\)a\({}^{-1}\) surface lowering at Dickson Glacier. Those patterns can most likely be explained by the hyposometry of the glaciers. First mentioned by [PERSON] and others [52], [PERSON] [17] showed by mass balance computations, that for many of the Patagonian glaciers the Accumulation Area Ratio (AAR) and hypsometry can explain many of the observed glacier changes of SPI despite dynamic adjustments. Several studies [53; 54; 55] also attributed the stability of Perito Moreno Glacier partly to this, since the equilibrium line is located in a steep area and hence a shift does not affect large areas as e.g., on flat areas or glaciers with a primarily low elevation distribution such as Upsala [55]. [PERSON] referred to this as the location of the bulge in the hypsometric profile [17]. For most of the intermediate sensitive glaciers to changes in ELA he observed a bi- or multi-modal hypsometric distribution with the ELA between peaks (class E) or uni-modal distribution with the bulge at the ELA (class D). This also explains the considerable larger surface lowering rates at Ameghino Glacier in close vicinity to Perito Moreno Glacier, since its hypsometry shows a considerably larger proportion below the ELA in addition to differences in the calving and basal melt [56; 57]. A large proportion of the SPI glaciers show almost balanced or slightly positive elevation changes. Pio XI glacier has the second largest positive geodetic mass balances. This is in accordance with [PERSON], who contested for this class of glacier having the bulk area above the ELA (class B) and low sensitivity towards ELA changes [17].