## 5 Discussion  
We find that the backscatter in the two profiles in the main trunk of the glacier has its lowest values in the low elevated part in which also the optical reflectance is low. In addition to that, the drop of backscatter in wet, summer, conditions is in the order of 10 dB, whereas the drop in the areas with low reflectance is only about 5 dB (Figures 4 and 5). This means that the water content does not affect the absorption in the same way in areas of high or low impurity content at the surface. This needs to be interpreted in relation to the change in backscatter from low to high impurity content of about 10 dB. Once the impurities at the surface reach a significant level, the backscatter is more dominated by impurity content than by moisture. This is further supported by the 2 m air temperature (see Figure S5), which has the same range at the lower elevated floating tongue than in the upper areas (km 70-140 in Figures 4 and 5).  
Non local sources annual rate of change is currently stabilized or only slightly increasing compared to past decades. For long term trends in BC, global emissions were estimated to grow by about 15% by 2010 compared to 1990 [50]. The aerosol light absorption levels do not show a significant trend in the Arctic as recently displayed in the analysis by [PERSON] et al. [22] for long term data from Arctic stations Alert, Summit and Zeppelin-Ny Alesund.  
How will this develop in future? With rising temperatures the area that may undergo melt in future is likely to increase. Given that the floating tongue is experiencing now already strong melt during summer, we consider two scenarios: (i) with increasing melt the number of ponds on the floating tongue is increasing and/or the size of the ponds is increasing. In both cases the albedo becomes closer to that of open ocean. (ii) Increasing melt will lead to more surface water streams that flush the surface around the melt and sweep more dirt into the lakes. The consequence may be that the ice surface is coming to an increased albedo that is predominantly driven by coarsening of the upper layer. Also the area upstream the floating tongue that is currently prone to in-situ deposition from nearby mountains may undergo change. With increasing melt the cycle of lake filling/drainage can increase and lake size may increase, as well as rivers may increase in size and number. All that would lead to a more regular cleaning of the surface and consequently increase in albedo.  
Figure 8: Evolution of the snowline (limit between persistent snow cover and ice) and of the extension of dark ice, defined by [PERSON] et al. [13] from 2016 to 2018.  
Taking the findings of [PERSON] and [PERSON] [3] into account, we recognise that there is a need of representation of the scavenging effect in firm hydrology modelling, which itself requires an evolution equation of grain size in wet conditions. Similar to that, using an ice sheet model tracer for dust concentration for accounting for albedo of early Holocene and Pleistocene ice in the ablations zones may become an important factor for projections of Greenland's contribution to sea level rise.  
Satellite remote sensing approaches to detect albedo in ablation zones may provide valuable information for assessing the influence over time span of decades and to validate simulated albedo. Approaches like the one by [PERSON] et al. [51] that employ a sophisticated atmospheric correction to Sentinel-3's OLCI instrument obtaining an albedo estimation of high accuracy can potentially be combined with a debris cover ratio, as proposed by [PERSON] et al. [52] allowing for separating in future better between debris/dust influence and grain size.