Vulnerable top-of-permafrost ground ice indicated by remotely sensed late-season subsidence

• Main Authors: Zwieback, Simon, Meyer, Franz J.
• Format: Text
• Language: English
• Published: 2020
• Subjects: Arctic; Ice; permafrost; Alaska
• Online Access: https://doi.org/10.5194/tc-2020-312; https://tc.copernicus.org/preprints/tc-2020-312/

Ground ice is foundational to the integrity of Arctic ecosystems and infrastructure. However, we lack fine-scale ground ice maps across almost the entire Arctic, chiefly because ground ice cannot be observed directly from space. Focusing on northwestern Alaska, we assess the suitability of late-season subsidence from Sentinel-1 satellite observations as a direct indicator of vulnerable excess ground ice at the top of permafrost. The idea is that, towards the end of an exceptionally warm summer, the thaw front can penetrate materials that were previously perennially frozen, triggering increased subsidence if they are ice rich. For locations independently determined to be ice rich, the late-season subsidence in an exceptionally warm summer was 4–8 cm (5th–95th percentile), while it was lower for ice-poor areas (−1–2 cm). The distributions overlapped by 2 %, demonstrating high sensitivity and specificity for identifying top-of-permafrost excess ground ice. The strengths of late-season subsidence include the ease of automation and its applicability to areas that lack conspicuous manifestations of ground ice, as often occurs on hillslopes. One limitation is that it is not sensitive to excess ground ice below the thaw front and thus the total ice content. Late-season subsidence can enhance the automated mapping of vulnerable permafrost ground ice, complementing existing (predominantly non-automated) approaches based on largely indirect associations with vegetation cover and periglacial landforms. Improved ground ice maps will prove indispensable for anticipating terrain instability in the Arctic and sustainably stewarding its ecosystems.