Jin Hui-jun

Permafrost Degradation on the Qinghai-Tibet Plateau and its Environmental Impacts

An increase of mean annual air temperature (MAAT) of about 0.2‐0.48C on the Qinghai‐Tibet Plateau as compared with the 1970s, and especially winter warming, has resulted in extensive permafrost degradation. An increase of 0.1‐0.58C in the mean annual ground temperature (MAGT) has been observed. Discontinuous permafrost bodies and thawed nuclei have been widely detected. The lower altitudinal limit of permafrost has risen 40‐80 m on the Qinghai‐ Tibet Plateau. The total permafrost area on the Plateau has shrunk about 10 5 km 2 . Permafrost degradation has caused environmental deterioration, including the destabilization of buildings, impacted upon cold regions hydrology and water resources, and accelerated desertification. Copyright # 2000 John Wiley & Sons, Ltd.

Thermal regimes and degradation modes of permafrost along the Qinghai-Tibet Highway

Permafrost on the Qinghai-Tibet Plateau (QTP) is widespread, thin, and thermally unstable. Under a warming climate during the past few decades, it has been degrading extensively with generally rising ground temperatures, the deepening of the maximum summer thaw, and with lessening of the winter frost penetration. The permafrost has degraded downward, upward and laterally. Permafrost has thinned or, in some areas, has totally disappeared. The modes of permafrost degradation have great significance in geocryology, in cold regions engineering and in cold regions environmental management. Permafrost in the interior of the QTP is well represented along the Qinghai-Tibet Highway (QTH), which crosses the Plateau through north to south and traverses 560 km of permafrost-impacted ground. Horizontally, the degradation of permafrost occurs more visibly in the sporadic permafrost zone in the vicinity of the lower limit of permafrost (LLP), along the margins of taliks, and around permafrost islands. Downward degradation develops when the maximum depth of seasonal thaw exceeds the maximum depth of seasonal frost, and it generally results in the formation of a layered talik disconnecting the permafrost from the seasonal frost layer. The downward degradation is divided into four stages: 1) initial degradation, 2) accelerated degradation, 3) layered talik and 4) finally the conversion of permafrost to seasonally frozen ground (SFG). The upward degradation occurs when the geothermal gradient in permafrost drops to less than the geothermal gradients in the underlying thawed soil layers. Three types of permafrost temperature curves (stable, degrading, and phase-changing transitory permafrost) illustrate these modes. Although strong differentiations in local conditions and permafrost types exist, the various combinations of the three degradation modes will ultimately transform permafrost into SFG. Along the QTH, the downward degradation has been proceeding at annual rates of 6 to 25 cm, upward degradation at 12 to 30 cm, and lateral degradation in the sporadic permafrost zone at 62 to 94 cm during the last quarter century. These rates exceed the 4 cm per year for the past 20 years reported for the discontinuous permafrost zone in subarctic Alaska, the 3 to 7 cm per year reported in Mongolia, and that of the thaw-stable permafrost in subarctic Yakutia and Arctic Alaska.

Distribution and changes of active layer thickness (ALT) and soil temperature (TTOP) in the source area of the Yellow River using the GIPL model

Active layer thickness (ALT) is critical to the understanding of the surface energy balance, hydrological cycles, plant growth, and cold region engineering projects in permafrost regions. The temperature at the bottom of the active layer, a boundary layer between the equilibrium thermal state (in permafrost below) and transient thermal state (in the atmosphere and surface canopies above), is an important parameter to reflect the existence and thermal stability of permafrost. In this study, the Geophysical Institute Permafrost Model (GIPL) was used to model the spatial distribution of and changes in ALT and soil temperature in the Source Area of the Yellow River (SAYR), where continuous, discontinuous, and sporadic permafrost coexists with seasonally frozen ground. Monthly air temperatures downscaled from the CRU TS3.0 datasets, monthly snow depth derived from the passive microwave remote-sensing data SMMR and SSM/I, and vegetation patterns and soil properties at scale of 1:1000000 were used as input data after modified with GIS techniques. The model validation was carried out carefully with ALT in the SAYR has significantly increased from 1.8 m in 1980 to 2.4 m in 2006 at an average rate of 2.2 cm yr−1. The mean annual temperature at the bottom of the active layer, or temperature at the top of permafrost (TTOP) rose substantially from −1.1°C in 1980 to −0.6°C in 2006 at an average rate of 0.018°C yr−1. The increasing rate of the ALT and TTOP has accelerated since 2000. Regional warming and degradation of permafrost has also occurred, and the changes in the areal extent of regions with a sub-zero TTOP shrank from 2.4×104 to 2.2×104 km2 at an average rate of 74 km2 yr−1. Changes of ALT and temperature have adversely affected the environmental stability in the SAYR.

Active layer seasonal freeze-thaw processesand influencing factors in the alpine permafrost regions in the upperreaches of the Heihe River in Qilian Mountains

Observational data on permafrost and active layer soil hydrothermal processes are extremely limited in the upper reaches of the Heihe River (URHHR) in Qilian Mountains. This lack acts as a bottleneck, restricting the research on the hydrological functions of different landscapes and the hydrological effects of the changes in the permafrost and active layer in the alpine permafrost regions of the Heihe River Basin. The active layer seasonal freeze-thaw processes and the soil hydrothermal dynamics and influencing factors were analyzed using soil temperature and water content observation data of the active layer established on the north slope of Ebo Mountains (NSEBM) in the east branch and in the alluvial plain in the west branch in alpine permafrost regions in the URHHR from 2013 to 2014. The results showed that climatic conditions in the alpine permafrost regions and local factors, such as topography, geomorphology, vegetation, lithology, and soil water content, evidently affected the active layer seasonal freeze-thaw processes and soil hydrothermal dynamics, and the main influence factors also contained snow, water, and winter temperature inversion. The annual surface temperature range (ASTR), mean annual ground temperatures (MAGTs) of the active layer, ground temperature at the bottom of the active layer (TTOP), and MAGT at the depth of zero annual amplitude were lower by 8.8, 0.6 to 1.7 (at depths of 5–77 cm), 0.7, and 0.7°C, respectively, on the NSEBM than at the same elevation in the west branch. Compared with the active layer at the same elevation in the west branch, the active layer onset date of soil thaw on the NSEBM occurred earlier. Moreover, the date reaching the maximum thaw depth occurred later, the duration of the seasonal thaw process was significantly longer, and the rate of the seasonal thaw process was lower. With the later active layer onset date of soil freeze from the ground surface downward, the duration and the rate of the seasonal freeze process on the NSEBM was longer and lower, respectively. Furthermore, the duration of the completely frozen stage on the NSEBM was longer. The active layer onset date of the thaw-rising stage, the relative completely thaw stage, the freeze-fall stage, and the completely frozen stage on the NSEBM lagged significantly from top to bottom, whereas that of the last three stages were evident at the same elevation in the west branch. The rate of change in the active layer soil water content at depths of 20–60 cm at the thaw-rising stage and the freeze-fall stage was both significantly higher on the NSEBM. In addition, the active layer soil water content at the depths of 20–60 cm from the mid- to late completely frozen stage in the beginning of the year to the early thaw-rising stage was lower on the NSEBM. These results were mainly due to the effects of local factors, such as the soil particle composition, dry density, ice content, and organic matter content on the active layer soil water dynamics. This study provides basic data that identify, simulate, and predict the hydrological functions of different landscapes and the hydrological effects of the changes in the permafrost and active layer of the Heihe River Basin. Our study can also provide a reference for the study of seasonal freeze-thaw processes and influencing factors in permafrost regions with different climate types, such as other alpine areas in Western China, the north-eastern Qinghai-Tibet Plateau, Northeast China, and even in the Arctic or sub-Arctic.