Permafrost and Frozen Ground Assessments


The Recent State of Permafrost, 2015


Aaron Letterly
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison, WI, USA
15 September 2016

Alaskan Active Layer Thickness Trends


In 2015, Alaskan active layer thicknesses (ALT)* increased relative to 2014 and were higher than their local site long-term average. A trend of increasing temperatures over the Beaufort Sea since 1990 (link to most recent atmospheric assessment) also affects the Alaskan North Slope region, where hotter summers and shorter, less-cold winters allow warmer surface temperatures to penetrate more deeply into the frozen soil. Figure 1 shows end-of-season active layer thicknesses by Alaskan site, illustrating the slow increase in active layer depths in all but a few locations. The Westdock and Deadhorse sites experienced their thickest active layers ever recorded, while sites in Happy Valley saw little change or even small decreases in active layer depth in 2015. This network of sites provides strong evidence that changes in terrestrial Arctic surface parameters are a main driver of permafrost thicknesses and temperatures on decadal time scales (Romanovsky et al., 2014). Figure 2 shows the locations of some of the Alaskan sites.

Figure 1
Figure 1: Time series of thaw depths measured in Alaska’s North Slope and Brooks Range regions. All sites are located in the continuous permafrost zone. Data are from the Circumpolar Active Layer Monitoring (CALM) website.
Figure 2
Figure 2: Site locations of permafrost measurement sites in Alaska. Sites south of Old Man are located outside of the continuous permafrost zone, as defined by Jorgensson et al, 2008. Image taken from the 2014 Arctic Report Card (http://www.arctic.noaa.gov/report11/permafrost.html).

Active Layer Thickness Changes by Region


Long-term measurements of active layer thicknesses in different regions of the Arctic are in general agreement, showing an increasing 15-year ALT trend (Figure 3). However, thaw depth observations exhibit significant inter-annual fluctuations, forced substantially by variations in summer air temperatures (e.g., Smith et al. 2009, Popova and Shmakin, 2009). The variability in soil types, snow depth, atmospheric behavior, and time of measurement across the Arctic all contribute to variability of ALT at different locations (Shiklomanov et al. 2012). Trends in permafrost changes occur on time scales ranging from shorter than 10 years to greater than 15 years (Christiansen et al. 2010, Burn and Kokelj 2009, Streletskiy et al. 2008). In 2015, the average thaw depths of all representative Arctic regions were at or above their 25-year average.

Active layer trends in North America and Greenland show increases that are connected to strong positive Arctic surface temperature anomalies from 2012-2015. The anomalously warm Beaufort Sea and Canadian Archipelago in 2015 likely contributed to the increased ALTs in northern and central Alaska. Although the thaw depth in the Alaskan Interior is less than it was in 2014, it still almost 50% greater than the long-term average for that region. Thaw depths in Greenland and Canada may remain above the long-term average as of 2015, but positive anomalies in surface temperature in these regions have been far less extreme than elsewhere in the North American Arctic (link to figure 3 of 2016 atmospheric assessment).

Thaw depths in Russia and Siberia show far more inter-annual variation and less adherence to the trends in North America. Figure 4 shows that far fewer of these sites than in North America provide observations for a much larger spatial area, and may be attributable to some of its variability. An approximately 40% thinning of active layer thickness in Northern Europe/Russia could be the result of a relatively warm winter period in 2015, especially over the Baltic and Barents Seas. Eastern Siberia saw a 20% decrease in ALT, while Western Siberia experiences little change in thaw depths compared to 2014, which was also average.

Figure 3
Figure 3: Percent change in active layer thicknesses (ALT) relative to the 1991-2015 average for 7 different Arctic regions. The legend in the lower left details the number of observation sites in a region that were used to make the corresponding time series.
Figure 4
Figure 4: Location of CALM sites around the Arctic categorized by measurement type. The network of sites in Alaska and the Canadian Archipelago is far more dense than sites in Northern Europe and Siberia. Image is taken from the CALM website (https://www.gwu.edu/~calm/data/data-links.html).

*Active layer thickness (ALT), or thaw depth, refers to the depth of the top layer of soil or rock that thaws during the Arctic summer before freezing again in the fall. Changes in temperature near the surface affect ALT, meaning that changes in ALT indicate a changing permafrost state for a given region. The depth of the ALT can range from a few meters in warmer, ice-rich environments to 20 m or greater in bedrock and the coldest permafrost regions. To best observe long-term change in the Arctic, continuous year-round ground temperature measurements within the upper 15 m can be analyzed. Active layer thicknesses are typically measured by mechanical probing at regular intervals, thaw-tube measurements, or inferring thaw depth based on ground temperature measurements.


References


Burn C. R. and S. V. Kokelj. 2009. The environment and permafrost of the Mackenzie Delta area. Permafr. Periglac. Process., 20(2), 83-105, doi: 10.1002/ppp.655.

Christiansen, H. H., B. Etzelmüller, K. Isaksen, H. Juliussen, H. Farbrot, O. Humlum, M. Johansson, T. Ingeman-Nielsen, L. Kristensen, J. Hjort, P. Holmlund, A. B. K. Sannel, C. Sigsgaard, H. J. Åkerman, N. Foged, L. H. Blikra, M. A. Pernosky and R. Ødegård. 2010. The Thermal State of Permafrost in the Nordic area during the International Polar Year. Permafr. Periglac. Process., 21, 156-181, doi: 10.1002/ppp.687.

Popova, V. V. and A. B. Shmakin. 2009. The influence of seasonal climatic parameters on the permafrost thermal regime, West Siberia, Russia. Permafr. Periglac. Process., 20, 41-56, doi:10.1002/ppp.640.

Romanovsky, V.E., W.L. Cable, A.L.Kholodov, S.S. Marchenko, S.K. Panda, N.I. Shiklomanov, and Walker, D.A., 2014, Changes in permafrost and active-layer thickness due to climate in Prudhoe Bayregion and North Slope, AK. Poster. http://www.geobotany.uaf.edu/library/posters/Romanovsky2014_OttawaAC2014_pos20141205.pdf

Shiklomanov N. I., D. A. Streletskiy and F. E. Nelson. 2012. Northern Hemisphere component of the global Circumpolar Active Layer Monitoring (CALM) Program. Proceedings of the 10th International Conference on Permafrost, K. M. Hinkel (ed.), Salekhard, Yamal-Nenets Autonomous District, Russia. The Northern Publisher Salekhard, vol. 1, 377-382.

Smith, S. L., S. A. Wolfe, D. W. Riseborough and F. M. Nixon. 2009. Active-layer characteristics and summer climatic indices, Mackenzie Valley, Northwest Territories, Canada. Permafr. Periglac. Process., 20, 201-220, doi:10.1002/ppp.651.

Streletskiy D. A, N. I. Shiklomanov, F. E. Nelson and A. E. Klene. 2008. 13 years of observations at Alaskan CALM sites: Long-term active layer and ground surface temperature trends. Proceedings of the 9th International Conference on Permafrost, D. L. Kane and K. M. Hinkel (eds.), June 29-July 3, Fairbanks, Alaska, Institute of Northern Engineering, University of Alaska Fairbanks, vol. 2, 1727-1732.

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