Quantifying retrogressive thaw slump mass wasting and carbon mobilisation on the Qinghai-Tibet Plateau using multi-modal remote sensing

Abstract. Retrogressive Thaw Slumps (RTS) are slope failures triggered by permafrost thaw that occur in ground-ice-rich regions of the Arctic and the Qinghai-Tibet Plateau (QTP). A strong warming trend has amplified RTS activity on the QTP in recent years. Although the region currently acts as a carbon sink, its permafrost-covered area (40 %) contains substantial soil organic carbon (SOC) stocks. Intensifying thaw-driven mass wasting may transform the QTP into a net carbon source by mobilising previously frozen SOC and enhancing decomposition. Yet, regional remote sensing studies have not yet quantified RTS mass wasting, including material erosion volumes and associated SOC mobilisation. Analysing time-series data from digital elevation models (DEMs) enables direct observation of RTS activity by measuring changes in active area, eroded material volume, and the overall magnitude of surface change. However, most available DEM sources lack the spatial resolution and temporal frequency required for comprehensive RTS monitoring. In contrast, optical data provide higher spatial resolution and more frequent observations, but lack elevation information. Here, we evaluated RTS mass wasting across the QTP from 2011 to 2020 by combining DEMs derived from bistatic Interferometric Synthetic Aperture Radar (InSAR) observations of the TanDEM-X mission with annual RTS inventories generated from high-resolution optical satellite imagery and geophysical soil property data to estimate erosion volume, ground ice loss, and SOC mobilisation. We estimated that RTS activity on the QTP during 2011–2020 relocated 5.020.7525.35×107 m3 previously frozen material, resulting in a loss of 3.580.2828.20×106 m3 of ground ice, and mobilised 2.780.117.98×108 kg C of organic carbon. We found a reliable power-law scaling relationship between RTS area in the optical RTS inventory and calculated volume change, with α values ranging from 1.20±0.01 to 1.30±0.01 (R2=0.87, p<0.001) depending on the regression model used, which may readily transform planimetric RTS area into volume estimates at scale on the QTP. Despite the relatively recent initiation and smaller size of RTSs on the QTP, material erosion and SOC mobilisation over the past decade exceeded levels in some Siberian Arctic regions, but remained up to 10 times lower than hotspots in the Canadian High Arctic. While current RTS impacts on the QTP are relatively modest, affecting < 0.01 % of the total permafrost area and contributing approximately 0.1 % to the regional carbon budget, the accelerating rates of RTS activity indicate that this phenomenon could become increasingly significant in the future. Our findings highlight the importance of regional studies in advancing our understanding of permafrost thaw-driven changes to the carbon dynamics of rapidly changing permafrost ecosystems.