Decadal variations of active‐layer thickness in moisture‐controlled landscapes, Barrow, Alaska

Abstract

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.

Shifted energy fluxes, increased Bowen ratios, and reduced thaw depths linked with drainage-induced changes in permafrost ecosystem structure

“…Direct effects comprise the decrease in both heat capacity as well as thermal conductivity when drying out organic soils, and our results show indications of reduced soil heat fluxes across the seasons. The observed patterns between soil water content and progress of thaw depth over the course of the growing season agree well with findings previously reported for Arctic ecosystems in Alaska (Hinzman et al, 1991;Shiklomanov et al, 2010;Sturtevant et al, 2012). A second direct effect of drier conditions in shallow soil layers is a shift in energy partitioning towards higher Bowen ratios, with a larger portion attributed to sensible heat fluxes.…”

The importance of a surface organic layer in simulating permafrost thermal and carbon dynamics

“…The CALM network is a part of the Global Terrestrial Network for Permafrost (Burgess et al, 2000). The monitoring network measures ALT either using a mechanical probe or a vertical array of temperature sensors Shiklomanov et al, 2010). After matching up the CALM coordinates with the coordinates of previously simulated ALT (Schaefer et al, 2011), we excluded sites with no measurements or ALT greater than 3 m depth, ending up with 76 CALM stations.…”

Uncertainties in the global temperature change caused by carbon release from permafrost thawing

“…Recent observations show that permafrost has typically warmed by 0.5 to 2 • C, depending on location (Solomon et al, 2007;Etzelmüller et al, 2011;Osterkamp, 2007). In addition, measurements suggest there is an observable deepening of the permafrost active layer (Shiklomanov et al, 2010;Frauenfeld et al, 2004;Wu and Zhang, 2010;Callaghan et al, 2010;Isaksen et al, 2007). Future climate change projections suggest a marked warming at northern high latitudes of between 2.8 and 7.8 degrees (A1B scenario) by the end of the century (Solomon et al, 2007), which will result in further degradation of permafrost.…”