Key evidence of the role of desertification in protecting the underlying permafrost in the Qinghai-Tibet Plateau

Blown sand environment characteristics and bridge sand hazard mechanisms of expressways in the South Tibet Valley

South Tibet expressways are built in the Yalu Tsangpo valley along the route to high-altitude and cold-temperature valleys, which are the main environmental characteristics of the region. Given that the blown sand environment and its potential harm are unclear, targeted prevention and control are not conducive, but its absence would result in severe wind-sand hazards on these expressways. In this research, the blown sand environment of the South Tibet Valley and the mechanism of sand damage to expressway bridges were studied via field observations, wind tunnel experiments and numerical simulations. The sand-moving wind is mainly west wind. The sand-moving wind frequency, sand drift potential, and maximum possible sand transport quantity are high in winter and spring (November-April) and low in summer and autumn (May-October). The sand drift direction is in the east direction. Expressway bridges have a considerable impact on the near-surface blown sand environment, forming a zone for increasing the wind speed between the top of the slope shoulder of the windward side of the bridge to the center of the bridge abutment, forming a zone for weakening the wind speed between the - 3 H distances of the upwind direction to the bottom of the slope middle of the windward side of the bridge, further forming a zone for weakening the wind speed centered in the slope shoulder of the leeward side of the bridge. The wind speed within a distance of 40 H downwind direction of the bridge generally did not recover. The wind-sand flow is partially blocked when it passes through the bridge and accumulates near the bridge, which cause harm. The impact of the bridge on the blown sand environment in the downwind is greater than that in the upwind. The results of this study can provide a reference for preventing and controlling wind-sand hazards on South Tibet expressways.

The changing permafrost environment under desertification and the heat transfer mechanism in the Qinghai-Tibetan Plateau

With the development of desertification in the Qinghai-Tibet Plateau (QTP), aeolian sand becomes the remarkable local factor affecting the thermal state of permafrost along the Qinghai-Tibet Engineering Corridor (QTEC). In this study, a model experiment was conducted to analyze the impact of thickness and water content of aeolian sand on its thermal effect, and a hydro-thermo-vapor coupling model of frozen soil was carried out to reveal the heat transfer mechanism of the aeolian sand layer (ASL) with different thicknesses and its hydrothermal effect on permafrost. The results indicate that: (1) ASL with the thickness larger than 80 cm has the property of converting precipitation into soil water. The thicker the ASL, the more precipitation infiltrates and accumulates in the soil layer. (2) The cooling effect of ASL on permafrost results from the lower net surface radiation, causing the annual average surface heat flux shifting from heat inflow to heat outflow. The warming effect of ASL on permafrost results from the increasing convective heat accompanying the infiltrated precipitation. (3) As the ASL thickens, the thermal effect of ASL on permafrost gradually shifts from the cooling effect dominated by heat radiation and heat conduction to the warming effect dominated by precipitation infiltration and heat convection. The warming effect of thick ASL on permafrost requires a certain amount of years to manifest, and the critical thickness is suggested to be larger than 120 cm.

Effects of desertification on permafrost environment in Qinghai-Tibetan Plateau

Soil thermal state exerts an important role in soil physicochemical properties, nutrient content, soil carbon losses, and hydrological processes in cold regions. In the Qinghai-Tibet Plateau, desertification and aeolian sand accumulation greatly change the surface cover types and simultaneously alter the surface energy budget. However, the quantification of their impacts on the soil thermal state hasn't been studied methodically. Here, a laboratory experiment was conducted to investigate the impact of surface cover types, including bare surface, grass-coved surface, dry and wet (3%) aeolian sand-covered surface, on underlying soil thermal state. Our results demonstrate that there is a reciprocal relationship between environment change and permafrost degradation. The amount of heat entering the active layer was determined by the surface cover types and soil water content. Using the bare surface case as a reference, vegetation layer acted as a buffer to reduce the amount of heat propagation downwards the ground by 20% and to lower the near surface temperature by 0.7 °C. In contrast, dry aeolian sand acted as an insulation layer and warmed the ground by about 2 °C. Also, wet aeolian sand with high thermal conductivity facilitated the heat exchange with the atmosphere and warmed the ground about 1.5 °C. Our results have implications for thermal and hydrological processes in the atmosphere-ground-permafrost system and thermal stability of infrastructure under the effect of the desertification and aeolian sand accumulation. The hydrothermal interaction of desertification and permafrost needs to be quantified in the further study through long-term field observations and a fully-coupled water flow and heat transport model under a changing climate.

Permafrost Thaw with Thermokarst Wetland-Lake and Societal-Health Risks: Dependence on Local Soil Conditions under Large-Scale Warming

A key question for the evolution of thermokarst wetlands and lakes in Arctic and sub-Arctic permafrost regions is how large-scale warming interacts with local landscape conditions in driving permafrost thaw and its spatial variability. To answer this question, which also relates to risks for ecology, society, and health, we perform systematic model simulations of various soil-permafrost cases combined with different surface-warming trends. Results show that both the prevalence and the thaw of permafrost depended strongly on local soil conditions and varied greatly with these for the same temperature conditions at the surface. Greater ice contents and depth extents, but also greater subsurface volumes thawing at depth under warming, are found for peat soils than other studied soil/rock formations. As such, more thaw-driven regime shifts in wetland/lake ecosystems, and associated releases of previously frozen carbon and pathogens, may be expected under the same surface warming for peatlands than other soil conditions. Such risks may also increase in fast permafrost thaw in mineral soils, with only small thaw-protection effects indicated in the present simulations for possible desertification enhancement of mineral soil covers.

No protection of permafrost due to desertification on the Qinghai-Tibet Plateau

Desertification of tundra regions may form an escalating cycle with permafrost degradation where more permafrost thaw leads to continued desertification. This traditional viewpoint has been challenged in recent reports that state desertification protects the underlying permafrost. However, our measurements of soil temperature from nine sites in the Honglianghe River Basin, interior Qinghai-Tibet Plateau, show that desertification can degrade permafrost. If one compares the permafrost temperatures at sites with thin sand covers (e.g. site Yu-7, permafrost temperature of −0.64 °C; site Yu-6, permafrost temperature of −1.15 °C) with that of site Xie-1 (−0.65 °C, with a 120-cm-thick sand cover), the permafrost temperature is not significantly different. It is clear that a thick sand cover does not influence the underlying permafrost temperature. Our observations support traditional geocryological knowledge which states that, under most circumstances, desertification does not protect, but rather degrades, permafrost.