Decadal Topographic Change in the McMurdo Dry Valleys: Thermokarst Subsidence and Glacier Thinning Indicate Transfer of Water Storage from the Cryosphere to the Hydrosphere in the Terrestrial Antarctic

Recent ground-based observations of glaciers, streams, and soil surfaces in the McMurdo Dry Valleys (MDV), the largest ice-sheet-free area in Antarctica, have documented evidence for rapid ground ice loss, glacial thinning, and ground surface subsidence. To evaluate the extent, magnitude, and location of decadal-scale landscape change in the MDV we collected airborne LiDAR elevation data over 3,300 km² of the MDV in 2014-2015 and compared this data to a 2001-2002 airborne LiDAR campaign. This regional assessment of elevation change spans the recent acceleration of warming and melting observed by long term meteorological and ecosystem response experiments, allowing us to assess the response of MDV surfaces to warming and potential thawing feedbacks. We find that locations of thermokarst subsidence are strongly associated with the presence of excess ground ice and with proximity to surface or shallow subsurface (active layer) water. Subsidence occurs across soil types and landforms, in both low-lying, low-slope areas with impeded drainage and also high on steep valley walls. Glacier thinning is widespread and is associated with the growth of fine-scale roughness on glacier toes. Pond levels are rising in most closed-basin lakes in the MDV, across all microclimate zones, from the coastal thaw zone to the upland frozen zone. These observations highlight the continued importance of insolation-driven melting in the MDV. The regional melt pattern is consistent with an overall transition of water storage from the local cryosphere (glaciers, permafrost) to the hydrosphere (closed basin lakes and ponds as well as the Ross Sea). We interpret this regional melting pattern to reflect the potential for a transition to Arctic and alpine-style, hydrologically mediated permafrost and glacial melt in the Antarctic cold desert.