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[1] Observations in undisturbed terrain within some regions of the Arctic reveal limited correlation between increasing air temperature and the thickness of the seasonally thawed layer above ice-rich permafrost. Here we describe landscape-scale, thaw-induced subsidence lacking the topographic contrasts associated with thermokarst terrain. A high-resolution, 11 year record of temperature and vertical movement at the ground surface from contrasting physiographic regions of northern Alaska, obtained with differential global positioning systems technology, indicates that thaw of an ice-rich layer at the top of permafrost has produced decimeter-scale subsidence extending over the entire landscapes. Without specialized observation techniques the subsidence is not apparent to observers at the surface. This “isotropic thaw subsidence” explains the apparent stability of active layer thickness records from some landscapes of northern Alaska, despite warming near-surface air temperatures. Integrated over extensive regions, it may be responsible for thawing large volumes of carbon-rich substrate and could have negative impacts on infrastructure.

Active-layer thickness was determined in late August 1995 and 1996 at 100 m intervals over seven 1 km 2 grids in the Arctic Coastal Plain and Arctic Foothills physiographic provinces of northern Alaska. Collectively, the sampled areas integrate the range of regional terrain, soil, and vegetation characteristics in this region. Spatial autocorrelation analysis indicates that patterns of active-layer thickness are governed closely by topographic detail, acting through near-surface hydrology. On the coastal plain, maximum variability occurs at scales involving hundreds of meters, and patterns were similar in the two years. Substantially less spatial structure and interannual correspondence were found within the foothill sites, where high variability occurs over smaller distances. The divergence in patterns of thaw depth between the two provinces reflects the scale of local terrain features, which predetermines the effectiveness of fixed sampling intervals. Exploratory analysis should be performed to ascertain the scale(s) of maximum variability within representative areas prior to selection of sampling intervals and development of long-term monitoring programs.

[1] A continuous time series of annual soil thaw records, extending from 1994 to 2009, is available for comparison with the records of thaw obtained from the Biocomplexity Experiment (BE) for the period 2006–2009. Discontinuous records of thaw at Barrow from wet tundra sites date back to the 1960s. Comparisons between the longer records with the BE observations reveal strong similarities. Records of permafrost temperature, reflecting changes in the annual surface energy exchange, are available from the 1950s for comparison with results from measurement programs begun in 2002. The long-term systematic geocryological investigations at Barrow indicate an increase in permafrost temperature, especially during the last several years. The increase in near-surface permafrost temperature is most pronounced in winter. Marked trends are not apparent in the active-layer record, although subsidence measurements on the North Slope indicate that penetration into the ice-rich layer at the top of permafrost has occurred over the past decade. Active-layer thickness values from the 1960s are generally higher than those from the 1990s, and are very similar to those of the 2000s. Analysis of spatial active-layer observations at representative locations demonstrates significant variations in active-layer thickness between different landscape types, reflecting the influence of vegetation, substrate, microtopography, and, especially, soil moisture. Landscape-specific differences exist in the response of active-layer thickness to climatic forcing. These differences are attributable to the existence of localized controls related to combinations of surface and subsurface characteristics. The geocryological records at Barrow illustrate the importance and effectiveness of sustained, well organized monitoring efforts to document long-term trends.

[1] Several model-based assessments predict a discernible increase in the depth of seasonal thawing and circumpolar-scale warming of permafrost by the mid-21st century. Quantitative estimates of the environmental and socioeconomic impacts of changing climate in northern regions require robust projection of changes in permafrost, which in turn depend on the availability of appropriate models and forcing data. We examined four high-resolution, hemispheric-scale gridded sets of monthly temperature and precipitation constructed using different interpolation routines and reanalysis of data from a large number of weather stations. At many of 455 Russian weather stations, the four data sets depart from empirical mean annual air temperatures averaged over the 15-year period by 1–2°C and in cumulative daily positive temperature sums (degree days of thawing) by more than 200°C days. A permafrost model, forced with the gridded climatic data sets, was used to calculate the large-scale characteristics of permafrost in northern Eurasia. We analyzed zonal-mean air and ground temperatures, depth of seasonal thawing, and area occupied by near-surface permafrost in Eurasia north of 45°N. The 0.5–1.0 °C difference in zonal-mean air temperature between the data sets translates into a range of uncertainty of 10–20% in estimates of near-surface permafrost area, which is comparable to the extent of changes projected for the following several decades. We conclude that more observations and theoretical studies are needed to improve characterization of baseline climatic conditions and to narrow the range of uncertainties in model-based permafrost projections.

[1] The Hovsgol mountain region (49°–52°N, 98°–102°E) of northern Mongolia contains widespread mountain permafrost and comprises the southern fringe of the Siberian continuous permafrost zone. In this paper, we report our initial monitoring of permafrost by means of measuring ground temperatures and active layer thickness in boreholes and some cryogenic processes under the influence of climate warming and human activities in the region. The average rate of increase in mean annual permafrost temperatures is from 0.2°C to 0.4°C per decade. Permafrost has been degrading more intensively during the last 15 years (since 1990s) than during the previous 15–20 years (1970s and 1980s). Recent degradation of permafrost under climate warming in the Hovsgol mountain region is generally more intensive than in the Hentei and Hangai Mountain regions. Moreover, livestock grazing in some local areas accelerates degradation of permafrost due to loss of vegetation cover. Year-round temperature recordings by data loggers placed beneath different vegetation covers showed marked differences in active layer thickness and ground temperature.

[1] In this study we compare gridded active layer thickness (ALT) fields representing northern Alaska produced by the State Hydrological Institute (SHI), National Snow and Ice Data Center (NSIDC), and Geophysical Institute–University of Alaska Fairbanks (UAF-GIPL 2.0) spatial permafrost models. Comparison of model-produced ALT fields representing northern Alaska revealed substantial differences. The largest difference in modeled ALT was found in the interior of Alaska. However, the spatial distribution of observational sites, most of which are on the North Slope, precludes definitive conclusions about the accuracy of model prediction for the Alaskan interior. Accuracy of the model-produced ALT fields was assessed at a series of monitoring sites and over a 27,000 km2 region in north-central Alaska with known spatial ALT distribution. The NSIDC model is very accurate over the extent of the topographically homogeneous Coastal Plain but overestimates ALT in more complex terrain of the Brooks Range Foothills. The UAF-GIPL 2.0 model reproduced site-specific active layer values well but overestimated ALT on the Coastal Plain. Although the SHI model provided relatively accurate estimates, it was unable to reproduce interannual active layer dynamics. Large differences in ALT fields have primarily resulted from differences in approaches adopted in each model for characterization of largely unknown spatial distribution of surface (vegetation, snow) and subsurface (soil properties, soil moisture) conditions. Results from this study identify the nature and magnitude of error in ALT fields produced by permafrost models.