Permafrost carbon could produce a positive climate feedback. Until now, the ecosystem carbon budgets in the permafrost regions remain uncertain. Moreover, the frequently used models have some limitations especially regarding to the freeze-thaw process. Herein, we improved the IBIS model by incorporating an unfrozen water scheme and by specifying the parameters to estimate the present and future carbon budget of different land cover types (desert steppe, steppe, meadow, and wet meadow) in the permafrost regions. Incorporating an unfrozen water scheme reduced the mean errors in the soil temperature and soil water content by 25.2%, and the specifying leaf area parameters reduced the errors in the net primary productivity (NPP) by 79.9%. Further, the simulation results showed that steppes are carbon sources (39.16 gC/m(2)/a) and the meadows are carbon sinks (-63.42 gC/m(2)/a ). Under the climate warming scenarios of RCP 2.6, RCP 6.0, and RCP 8.5, the desert steppe and alpine steppe would assimilated more carbon, while the meadow and wet meadow were projected to shift from carbon sinks to carbon sources in 2071-2100, implying that the land cover type plays an important role in simulating the source/sink effects of permafrost ecosystem carbon in the IBIS model. The results highlight the importance of unfrozen water to the soil hydrothermal regime and specific leaf area for the growth of alpine vegetation, and present new insights on the difference of the responses of various permafrost ecosystems to climate warming.

Permafrost is ground that remains at or below 0 degrees C for two or more consecutive years. It is overlain by an active layer which thaws and freezes annually. The difference between these definitions - the active layer based on pore water phase and permafrost based on soil temperature - leads to challenges when monitoring and modelling permafrost environments. Contrary to its definition, the key properties of permafrost including hardness, bearing capacity, permeability, unfrozen water content, and energy content, depend primarily on the ice content of permafrost and not its temperature. Temperature-based measurements in permafrost systems often overlook key features, e.g. taliks and cryopegs, and comparisons between measured and modelled systems can differ energetically by up to 90 % while reporting the same temperature. Due to the shortcomings of the temperature-based definition, it is recommended that an estimate of ice content be reported alongside temperature in permafrost systems for both in-situ measurements and modelling applications.Plain language summary: Permafrost is ground that remains at or below 0 degrees C for two or more consecutive years. Above it sits an active layer which thaws and freezes annually (meaning that the water in the ground changes to ice each winter). The difference between these definitions - the active layer based on the state or water in the ground and permafrost based on ground temperature - leads to challenges when measuring (in the field) and modelling (using computers) permafrost environments. In addition to these challenges, the key properties of permafrost including its ability to support infrastructure, convey water, and absorb energy depend more on its ice content than its temperature. Due to the shortcomings of the temperature-based definition, it is recommended that an estimate of ice content be reported alongside temperature in permafrost systems for both field measurements and modelling applications.

The temperature and thermal properties of shelf sediments from the East Siberian, Laptev, and Kara Seas were determined from field investigations. The sediments were in an unfrozen cryotic state (ice-free) and showed negative temperatures, ranging from-1.0 to-1.4 degrees C. These temperatures imply the presence of widespread subsea permafrost from the shelf to the continental slope of the East Siberian Arctic Seas, reaching-1000-1500 km off the coast. The thermal conductivity and heat capacity of sediments (up to a depth of 0.5 m) from the Eastern Arctic Seas averaged 0.95 W/(m.K) and 3010 kJ/(m(3).K), respectively. We also conducted temperature and thermal conductivity measurements of the upper sediment horizons of the permafrost in the Laptev Sea shelf (drilling depth of 57 m). The analysis of sediment cores ensured the determination of thermal conductivity with depth. We also analyzed the influence of moisture content, density, particle size distribution, salinity, and thermal state on sediment thermal conductivity. The thermal conductivity of unfrozen cryotic (ice-free) sediments was predominantly dependent on the contents of silt and clay. In general, unfrozen cryotic sandy sediments had a thermal conductivity range 1.7-2.0 W/(m.K), a moisture content of-20%, and a density of 2.0-2.2 g/cm(3). Frozen (ice-containing) sediments showed higher thermal conductivities of 2.5-3.0 W/(m.K), with a density of 1.9-2.0 g/cm(3) and a moisture content exceeding 25-30%. The high thermal conductivity of sand was associated with low salinity (0.1-0.2%), high ice content, and moderate unfrozen water content.

Layers of volcanic ash and Andosol soils derived from the ash may play an important role in preserving snow and ice as well as in the development of permafrost conditions in (a) the immediate vicinity of volcanoes at high elevations or at high latitudes and (b) land areas that are often distant from volcanic activity and are either prone to permafrost or covered by snow and ice, but have been affected by subaerial ash deposition. The special properties of volcanic ash are critically reviewed, particularly in relation to recent research in Kamchatka in the Far East of Russia. Of special importance are the thermal properties, the unfrozen water contents of ash layers, and the rate of volcanic glass weathering. Weathering of volcanic glass results in the development of amorphous clay minerals (e.g. allophane, opal, palagonite), but occurs at a much slower rate under cold compared to warm climate conditions. Existing data reveal (1) a strong correlation between the thermal conductivity, the water/ice content, and the mineralogy of the weathered part of the volcanic ash, (2) that an increase in the amounts of amorphous clay minerals (allophane, palagonite) increases the proportion of unfrozen water and decreases the thermal conductivity, and (3) that amorphous silica does not alter to halloysite or other clay minerals, even in the Early Pleistocene age (Kamchatka) volcanic ashes or in the Miocene and Pliocene deposits of Antarctica due to the cold temperatures. The significance of these findings are discussed in relation to past climate reconstruction and the influence of volcanic ash on permafrost aggradation and degradation, snow and ice ablation, and the development of glaciers.

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.