# 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  
The proportion of 10% of total sea level rise related to mountain glaciers being associated with the South American glaciers [8] emphasizes the importance of a comprehensive discussion of glacier variations beyond our studied area and time period. Since last peak extents during Little Ice Age (LIA), glaciers of South America are reported to show a pattern of overall recession, synchronous along latitudes and with slight local fluctuations [37; 38]. The depletion pattern we found all over the SPI with highest change rates at the glacier tongues reflects these findings [PERSON] and others [38] attributed to climate forcing for the recent study period. Nevertheless, the magnitudes differ remarkably comparing the period between the LIA to date to the last two decades: Table 3 displays results of several studies on mass balance in the region that have been carried out by different teams with different methods. The volume loss estimated by [PERSON] and others [39] since the LIA peak extents which we downscaled to annual rates is a magnitude lower than other listed studies. The overview accentuates the increase in volume/mass loss rates accompanying the accelerated retreat of over 90% of the glaciers over the last century [9] as well as it stresses the dominant role of the SPI in the region, reacting to recent forcing. To improve the comparability of the various studies, we have generated specific mass balances (where possible), by dividing the mass change rates by the investigated area. Our results compare well with previous studies covering a similar time interval as e.g., [7; 8; 40]. We cannot confirm the magnitude of the acceleration of mass change estimates for the 5-year period 1995-2000 as compared to the period 1975-2000. This acceleration was stated by [PERSON] and others [8] and estimated on a small subset of glaciers only. Nonetheless, it has to be considered, that those computations were done only on a comparable small sub-sample and extrapolated to the entire icefield. Our results differ from the study by [PERSON] and others [7] since we use no penetration depth correction for the SRTM C-band data. In contrast to those authors and in line with [PERSON] [40; 41] we consider surface melt conditions for almost the entire SPI during SRTM acquisition and hence no signal penetration. Our TDX data from the austral summer 2015/16 show similar surface conditions as the careful inspection of the SAR intensity imagery revealed. In contrast to [PERSON] [41] our analysis of entire SPI covers a larger time span and uses the SRTM C-Band 30 m dataset as reference in contrast to the 90 m product used in those earlier studies. We used a different processing approach as the operational TanDEM-X DEM processor of the German Aerospace Center. This, together with slight differences in glacier outlines and upscaling in unmeasured areas, explains the differences between these two studies. We want to stress that our estimate is a lower bound since we did not consider mass loss due to subaqueous calving, which is larger than indicated above the sea/lake level, since calving glaciers often occupy over deepened valleys. However, only very limited information is available on the ice thickness and bathymetry in front of the glaciers. Nevertheless, this may explain partly the differences to the data retrieved from gravimetric time series of the Gravity and Climate Experiment (GRACE). Those measurements are additionally influenced by uncertainties in the estimation of the glacio-isostatic uplift (GIA) for which rates are known to be one of the highest in Patagonia in a global comparison. Recent studies report maximum uplift rates of up to 41 mm-a\({}^{-1}\) at the margin of the icefield [15; 16; 42]. Moreover, the GRACE measurements cover both icefield plus ice bodies to the north and south of the NPI and SPI. The observed spatial pattern in our results is comparable to the work of [PERSON] and others [7], although our dh/dt map is smoother.