Thermal conduction control is important for retarding permafrost degradation and mitigating of frost geohazards. Similar to a thermodiode, high thermal conductivity contrast (HTCC) materials can serve as good thermal insulators. A preferred HTCC material for ground cooling is larger in thermal resistance in summer and smaller in winter. Because of contrasting thermal conductivity under frozen and thawed states, organic soil is blessed with such a property. This study quantified and reported the HTCC effects on a range of soil organic matter concentrations (SOMC) and soil moisture saturation degree (SMSD). Using the COMSOL, influences of different SOMC and SMSD on ground temperatures were simulated and compared with laboratory-measured properties. Simulation results demonstrated that with constant SMSD at 20% throughout the year, the thermal insulation effect was strengthened with increasing SOMC. A better insulating effect was judged by lower annual amplitudes and smaller depths of zero annual amplitude of ground temperatures. In case of low SMSD in summer (20%) and high SMSD in winter (60-80%), the HTCC effect of soil is enhanced with increasing SOMC. This enhancement was evidenced by increased thermal offsets and decreased maximum summer and average nearsurface soil temperatures. With constant SOMC and increasing SMSD, the rising HTCC effect gradually cools the ground. An integral analysis indicates that the higher the SOMC and SMSD in winter, the larger the thermal offset and the lower the ground temperature, i.e., the greater the HTCC effect of organic soil. This study may provide geocryological bases for engineering and environmental applications in cold regions.

The Nanwenghe Wetlands Reserve in the Yile'huli Mountains is a representative region of the Xing'an permafrost. The response of permafrost to climate change remains unclear due to limited field investigations. Thus, longer-term responses of the ground thermal state to climate change since 2011 have been monitored at four sites with varied surface characteristics: Carex tato wetland (P1) and shrub-C. tato wetland (P2) with a multi-year average temperatures at the depth of zero annual amplitude (T-ZAA) of -0.52 and -1.19 degrees C, respectively; Betula platyphylla-Larix gmelinii (Rupr.) Kuzen mixed forest (P3) with T-ZAA of 0.17 degrees C, and; the forest of L. gmelinii (Rupr.) Kuzen (P4) with T-ZAA of 1.65 degrees C. Continuous observations demonstrate that the ecosystem-protected Xing'an permafrost experienced a cooling under a warming climate. The temperature at the top of permafrost (TTOP) rose (1.8 degrees C per decade) but the T-ZAA declined (-0.14 degrees C per decade), while the active layer thickness (ALT) thinned from 0.9 m in 2012 to 0.8 m in 2014 at P1. Both the TTOP and T-ZAA increased (0.89 and 0.06 degrees C per decade, respectively), but the ALT thinned from 1.4 m in 2012 to 0.7 m in 2016 at P2. Vertically detached permafrost at P3 disappeared in summer 2012, with warming rates of +0.42 and + 0.17 degrees C per decade for TTOP and T-ZAA, respectively. However, up to date, the ground thermal state has remained stable at P4. We conclude that the thermal offset is crucial for the preservation and persistence of the Xing'an permafrost at the southern fringe.

Precise temperature data from four Alaskan permafrost sites (Prudhoe Bay, Barrow and two sites near Fairbanks) combined with computer modelling provide quantitative measures of the existence and dynamics of unfrozen water in the active layer and permafrost. Unfrozen water contents are negligible for living and dead moss layers, small in the peat layers and larger in the silts, and show significant site-to-site variation. The effect of unfrozen water on the ground thermal regime is largest immediately after freeze-up and during cooling of the active layer. It is less important during warming and thawing of the active layer and during freezing and thawing of seasonally frozen ground. The effects last less than a month in cold permafrost and throughout most of the freeze-up period in warm permafrost. Physically, unfrozen water introduces a spatially distributed latent heat and changes thermal properties which retards the thermal response of an active layer or permafrost. Unfrozen water in the freezing and frozen active layer and nearsurface permafrost also protects the ground from rapid cooling and creates a strong thermal gradient at the ground surface that increases the heat flux out of the ground. This enlarged heat flux also enhances the insulating effect of the snow cover. There do not appear to be any inherent difficulties in using conductive heat modelling for the active layer during the period when the zero curtain exists. Copyright (C) 2000 John Wiley & Sons, Ltd.