Mongolia is one of the most sensitive regions to climate change, located in the transition of several natural and permafrost zones. Long-term trends in air freezing and thawing indices can therefore enhance our understanding of climate change. This study focuses on changes of the spatiotemporal patterns in air freezing and thawing indices over Mongolia from 1960 to 2020, using observations at 30 meteorological stations. Our results shows that the freezing index ranges from -945.5 to -4,793.6 degrees C day, while the thawing index ranges from 1,164.4 to 4,021.3 degrees C day over Mongolia, and their spatial patterns clearly link to the latitude and altitude. During the study period, the trend in the thawing index (14.4 degrees C-day per year) was larger than the trend in the freezing index (up to -10.1 degrees C-day per year), which results in the net increase of air temperature by 2.4 degrees C across Mongolia. Overall, the increase in the thawing index was larger in the low latitudes and altitudes (e.g., the Gobi-desert, steppes, the Great lake depression and major river valleys) than in high latitudes and altitudes (mountain regions), while it was the opposite for the freezing index. The highest values for both thawing index and freezing index (i.e. the least negative values) have occurred during the last 2 decades. As the trends in the freezing and thawing indices and mean annual air temperature confirm intensive climate warming, increased permafrost degradation and shallower seasonally frozen ground are expected throughout Mongolia.

The acceleration of permafrost thaw due to warming, wetting, and disturbance is altering circumpolar landscapes. The effect of thaw is largely determined by ground ice content in near-surface permafrost, making the characterization and prediction of ground ice content critical. Here we evaluate the spatial and stratigraphic variation of near-surface ground ice characteristics in the dominant forest types in the North Slave region near Yellowknife, Northwest Territories, Canada. Physical variation in the permafrost was assessed through cryostructure, soil properties, and volumetric ice content, and relationships between these parameters were determined. Near-surface ground ice characteristics were contrasted between forest types. In black spruce forests the top of the permafrost was ice-rich and characterized by lenticular and ataxitic cryostructures, indicating the presence of an intermediate layer. Most white spruce/birch forests showed similar patterns; however, an increase in the active layer thickness and permafrost thaw at some sites have eradicated the transition zone, and the large ice lenses encountered at depth reflect segregated ground ice developed during initial downward aggradation of permafrost. Our findings indicate that white spruce/birch terrain will be less sensitive than black spruce forests to near-surface permafrost thaw. However, if permafrost thaws completely, white spruce/birch terrain will probably be transformed into wetland-thaw lake complexes due to high ground ice content at depth.

Research treating permafrost-climate interactions is traditionally based on a two-layer conceptual model involving a seasonally frozen active layer and underlying perennially frozen materials. This conceptualization is inadequate to explain the behaviour of the active-layer/permafrost system over long periods, particularly in ice-rich terrain. Recent research in North America supports earlier Russian conclusions about the existence of a transition Zone that alternates in status between seasonally frozen ground and permafrost over sub-decadal to centennial time scales. The transition zone is ice-enriched, and functions as a buffer between the active layer and long-term permafrost by increasing the latent heat required for thaw. The existence of the transition zone has an impact on the formation of a cryogenic soil structure, and imparts stability to permafrost under low-amplitude or random climatic fluctuations. Despite its importance, the transition zone has been the focus of relatively little research. The impacts of possible global warming in permafrost regions cannot be understood fully without consideration of a more realistic three-layer model. The extensive data set under development within the Circumpolar Active Layer Monitoring (CALM) program will provide a significant source of information about the development, characteristics, behaviour, and extent of the transition zone. This paper is focused on the uppermost part of the transition zone, which joins the active layer at sub-decadal to multi-centennial time scales. This upper part of the transition zone is known as the transient layer. Copyright (c) 2005 John Wiley & Sons, Ltd.