## 6 Conclusions  
So far, InSAR studies in permafrost environments have focused on areas with little vegetation, with shallow active-layer thickness in the Arctic or the Tibet plateau where limited amount of organic material is present. The active-layer thickness on central Yamal exceeds 100 cm and different types of land cover occur, largely with tundra shrubs. We demonstrate the applicability and added value of InSAR-derived summer displacements for permafrost monitoring in such environments. An important aspect is the scarcity of specific in situ activities tailored to validate the InSAR results. We demonstrate the utility of a simple, but distributed and multi-year measurement setup.  
The impact of warmer years on subsidence has been demonstrated before with in situ measurements in the Lena Delta [12]. Our results confirm increased subsidence also with satellite retrievals. The joint analyses with land cover reveal impact differences with respect to ground properties. Especially the dense time series available from COSMO-Skymed due to its special constellation allows for detailed assessment of thaw progression and impact assessment of extreme years. Results indicate presence and melt of ground ice at the base of the active layer in case of above average air temperature (and subsequent higher degree-days of thawing) conditions. Our results exemplify the impact of higher temperatures and the role of near surface soil properties. Knowledge regarding presence of ground ice is crucial for climate change impact assessment and its mapping one of the present key issues in permafrost research [69].  
The comparison of the X-band records with C-band based on degree-days of thawing allows for evaluation of displacement retrievals. The agreement between the two sensors and the results from evaluation with in situ data suggest that InSAR-based seasonal subsidence retrievals in shrub dominated tundra are reasonable. The agreement between the wavelengths also suggests that changes of near surface soil water content and subsequent change of signal penetration depth are only of minor importance in such environments. The typical underestimation of displacement by InSAR may result from retrieval issues at the very beginning of thaw since rates agree later on.  
We provide a strategy for comparing subsidence observations between years that allows for enhanced understanding of permafrost thaw. However, only L-band preserves long-time coherence over natural terrain, which is required to study long-term evolution of subsidence. Further assessment over similar monitoring sites as on central Yamal, with dense observations of active-layer thickness,are required to develop a scheme for fully exploiting the capabilities of InSAR-derived seasonal subsidence across the entire Arctic.  
Conceptualization A.B. and M.L.; methodology, A.B., T.S. and M.L.; data curation, A.K., E.B. and D.M.; validation A.B., H.B. and K.E.; formal analyzes A.B., T.S. and B.W.; investigation, A.B.; writing--original draft preparation, A.B., T.S. and M.L.; visualization, C.K. and B.W.  
This work was supported by the European Space Agency project DUE GlobPermafrost (contract number 4000116196/15/I-NB), the Austrian Science Fund under grant [I 1401] and the Russian Foundation for Basic Research grant 13-05-91001-ANF-a (joint Russian-Austrian project COLD-Yamal) and the Russian Foundation for Basic Research Grant 18-04-01010-A for the Westerns Siberian botanical survey and the Austrian Science Fund (Fonds zur Forderung der wissenschaftlichen Forschung, FWF) through the Doctoral College GIScience (DK W1237-N23).  
TanDEM-X data were made available by DLR through PI agreement HYDR0226 and Cosmo-Skymed data are courtesy of ID 656, copyright ASI. Results are partially based on modified Copernicus data from 2016 to 2018.  
The authors declare no conflict of interest.