Muddied waters The climate of High Mountain Asia is becoming warmer and wetter. Li et al . present data showing that rivers originating in this region have experienced large increases in runoff and sediment fluxes over the past six decades, most dramatically since the mid-1990s. The authors project that sediment flux from those rivers could more than double by 2050 in the case of extreme climate change, with potentially serious impacts on the region’s hydropower capacity, food security, and environment. —HJS

Rapid atmospheric warming since the mid-20th century has increased temperature-dependent erosion and sediment transport processes in cold environments, impacting food, energy and water security. In this Review, we summarize landscape changes in cold environments and provide a global inventory of cryosphere degradation-driven increases in erosion and sediment yield. Anthropogenic climate change, deglaciation, and thermokarst disturbances are causing increased sediment mobilization and transport processes in glacierized and peri-glacierized basins. With continuous cryosphere degradation, sediment transport will continue to increase until reaching a maximum (peak sediment). Thereafter, transport will likely shift from a temperaturedependent regime toward a rainfall-dependent regime roughly between 2100-2200. The timing of the regime shift would be regulated by changes in meltwater, erosive rainfall and landscape erodibility, and complicated by geomorphic feedbacks and connectivity. Further progress in integrating multi-source sediment observations, developing physics-based sediment transport models, and enhancing interdisciplinary and international scientific collaboration are needed to predict sediment dynamics in a warming world. Key points1. A global inventory of cryosphere degradation-driven increases in erosion and sediment yield is presented, with 76 locations from the high Arctic, European mountains, High Mountain Asia and Andes, and 18 Arctic permafrost-coastal sites.2. Sediment mobilization from glacierized basins is dominated by glacial and paraglacial erosion; transport efficiency is controlled by glacio-hydrology and modulated by sub-, pro-, supra-glacial storage and release but is interrupted by glacial lakes and moraines.3. Degraded permafrost mainly mobilizes sediment by eroding thermokarst landscapes in high-latitude terrain and unstable rocky slopes in high-altitude terrain, which is sustained by exposing and melting ground ice and sufficient water supply; transport efficiency is enhanced by hillslope-channel connectivity.4. The sediment transport regime will shift in three stages, from a thermal-controlled regime to one jointly control by thermal and pluvial processes, and finally to a regime controlled by pluvial processes. 5. Peak sediment yield will be reached with or after peak meltwater.2 / 37 6. Between the 1950s and 2010s, sediment fluxes have increased by 2-8 folds in many cold regions and coastal erosion rates have more than doubled along many parts of Arctic permafrost coastlines.

Approximately 40% of the Tibetan Plateau (TP) is underlain by continuous permafrost, yet its impact on fluvial water and sediment dynamics remains poorly investigated. Here we show that water and sediment dynamics in the permafrost‐dominated Tuotuohe basin on the TP are driven by air temperature and permafrost thaw, based on 33‐year daily in situ observations (1985–2017). Air temperature regulates the seasonal patterns of discharge and suspended sediment concentration (SSC) by controlling the changes in active contributing drainage area (ACDA, the unfrozen erodible landscape that contributes hydrogeomorphic processes within a catchment) and governing multiple thermal processes such as glacier‐snow melt and permafrost thaw. Rainstorms determine the short‐lived fluvial extreme events by intensifying slope processes and channel erosion and likely also by enhancing thaw slumps. Furthermore, the SSCs at equal levels of discharges are lower in autumn (September–October) than in spring (May–June) and summer (July–August). This reduced sediment availability in autumn can possibly be attributed to the increased supra‐permafrost groundwater runoff and the reduced surface runoff and erosion. Due to rapid climate warming, the ACDA has increased significantly from 1985 to 2017, implying expanding erodible landscapes for hydrogeomorphic processes. As a result, the fluvial water and sediment fluxes have substantially increased. In a warmer and wetter future for the TP, the fluvial sediment fluxes of similar permafrost‐underlain basins will continue to increase with expanding erodible landscapes and intensifying thermal and pluvial‐driven geomorphic processes. Thus, permafrost thaw should be considered as an important driver of past and future water and sediment changes for the TP.

No abstract