Permafrost and Frozen Ground Assessments

Aaron Letterly
Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison, WI, USA
[5 June 2015]

Alaskan Active Layer Thickness Trends

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.

In 2014, Alaskan ALTs decreased relative to 2013 and were similar to or lower than those in 2012 (Figure 1). Decadal trends of increasing surface temperatures as well as high temperatures over the Beaufort Sea in 2012 led to record-breaking ALT thicknesses in the region during 2012 and 2013, which decreased through 2014. The majority of Alaskan regions, however, still report large active-layer thicknesses relative to the 1995-2012 average. Observations of the air temperature and snow depth from North Slope of Alaska sites provide conclusive evidence that changes in these parameters are a primary driver of permafrost thicknesses and temperatures on decadal time scales (Romanovsky et al 2014).

Figure 1: Time series of annual permafrost temperatures (top) measured in Alaska's North Slope and Brooks Range regions. A characteristic permafrost map of Alaska (bottom) shows sites location within Alaska. Sites in the top figure are all located in the continuous permafrost zone.

Active Layer Thickness Changes by Region

Long-term observations of active layer thicknesses in different regions of the Arctic are in rough agreement, showing a 15-year trend in ALT increases. 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, and atmospheric behavior 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). As of 2014, the majority of representative sites shown in Figure 2 are experiencing ALTs greater than the 1991-2014 average.

Active layer trends in North America reflect the recent positive Arctic surface temperature anomalies in 2012-2014 (Figure 2). The Alaskan North Slope and Canadian regions show ALTs in 2014 10-20% thicker than their 1991-2014 average. The Alaskan interior showed a significant increase in ALT starting in 2009. In 2014, while the ALT decreased relative to 2013, it was still 61% greater than the 1991-2014 mean. Trends in thickness over Greenland have been relatively stable, but still above the mean, since the last Circumpolar Active Layer Monitoring (CALM) data update. It is likely that recent melting events as well as positive temperature anomalies over Greenland have increased active layer thickness further than 2010 levels.

ALT changes across Russia and Siberia in 2014 show less agreement. A 40% decrease in the 2014 active layer thickness occurred in Northern European Russia from near record-high levels in 2013. Western Siberia saw a small decrease in active layer thickness from previous years, but still remained within 5% of the 1991-2014 average. Year-round positive surface temperature anomalies across the Eastern Siberian and Laptev Sea may have contributed to ALTs 20% greater than normal, up substantially from previous years’ thicknesses.

Figure 2: Percent Changes in active layer thickness (ALT) relative to the 1991-2014 average for 7 differeent Arctic regions. The lower right table documents the populations of measurement sites used to create corresponding time series.

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. (1), W.L. Cable (1), A.L.Kholodov (1), S.S. Marchenko (1), S.K. Panda (1), N.I. Shiklomanov (2), and Walker, D.A. (3), 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.