Permafrost and Frozen Ground Assessments

Aaron Letterly
20 December 2018

In 2017-2018 active layer thicknesses (ALT) across the Arctic decreased slightly from their 2016 values, but did little to defray the trend of increasing active layer thickness across permafrost-covered monitoring sites. The Alaskan North Slope region, home to a high density of such sites, saw persistence or minor decreases in thaw depth across all nine sites during the last two years. Figure 1 shows that though thaw depths did not increase significantly over the last 2-4 years, and in some cases decreased, active layer thickness in general has been increasing across northern Alaska over the last decade. Though these sites are all located in the continuous permafrost zone north of the Brooks Range, continued warming of the surface and ground temperature could spell the complete melt of permafrost over much of northern Alaska by the year 2150 (Batir et al., 2017). Note: All data for Northern Hemisphere sites are from the Circumpolar Active Layer Monitoring (CALM) website.

Figure 1: Time series of thaw depths measured in Alaska’s North Slope and Brooks Range regions.

Active Layer Thickness Trends by Region

Figure 2 shows the trends of ALT changes at measurement sites in the eastern half of Siberia. Permafrost in this region is relatively well-monitored with many of the 29 sites taking measurements over 15-year (or greater) durations. The overwhelming majority of sites reported an increase in active layer depths, and the two that reported strong decreasing trends in the active layer depth have been in place for five years or less, meaning their trends are not statistically significant. Only stations that were current through 2018 were included in Figure 2. Overall, the eastern part of Siberia is experiencing a decreasing permafrost regime.

Central Siberia is a much less-monitored region in regards to permafrost. As shown in Figure 3, all sites that were current as of 2018 reported decreasing permafrost (increasing ALT) at or above 0.5 cm/year. This central region once had many more permafrost monitoring sites, but many have since been discontinued.

Figure 2: Scatterplot of active layer thickness trends versus number of years reporting over eastern Siberia. Relative circle size also relates the number of years reporting at the site. Only stations with current 2018 updates were included.

Figure 3: Scatterplot of active layer thickness trends versus number of years reporting over central Siberia. Relative circle size also relates the number of years reporting at the site. Only stations with current 2018 updates were included.

Permafrost monitoring sites near the Ural Mountains are considered western Siberian sites, and their ALT trends are shown in Figure 4. The majority of the sites show decreasing permafrost depths, but the decrease is not occurring as rapidly as in other parts of the Arctic.

Russian permafrost sites are interspersed throughout the European side of the Ural Mountains, many of them along the Arctic coast. There is no definitive trend of permafrost behavior based on the limited sites that were reporting in 2018, but more data would likely show that European Russia is experiencing a decreasing permafrost regime. Figure 5 shows the spread of ALT trends in the region.

Figure 4: Scatterplot of active layer thickness trends versus number of years reporting over western Siberia. Relative circle size also relates the number of years reporting at the site. Only stations with current 2018 updates were included.

Figure 5: Scatterplot of active layer thickness trends versus number of years reporting over European Russia. Relative circle size also relates the number of years reporting at the site. Only stations with current 2018 updates were included.

As explained in Figure 1, Alaska has a dense network of permafrost monitoring sites covering its northern and interior regions. All of the sites shown in Figure 6 have more than ten years of data from which trends are drawn. The entire Alaskan region shows consistent permafrost decrease of between 0.1-0.5 cm/year. This consensus of thawing permafrost and increasing active layer depths is supported by a large body of scientific research and field campaigns. Unfortunately, permafrost site measurements from Canada have not been updated for years. Their trends would cover a much larger land area than those in Alaska and could further support the emerging pattern of decreasing permafrost in North America.

Figure 6: Scatterplot of active layer thickness trends versus number of years reporting over the Alaska region. Relative circle size also relates the number of years reporting at the site. Only stations with current 2018 updates were included.

Combining the data from these five separate regions shows increasing ALT throughout the Northern Hemisphere. Figure 7 shows decreasing permafrost trends in nearly all of the stations with a ten year or greater data record. Among current permafrost measurement sites, typical Arctic ALT increases are between 0.45-0.65 cm/year.

Figure 7: Scatterplot of active layer thickness trends versus number of years reporting over all 2018 Northern Hemisphere sites current in 2018. Relative circle size also relates the number of years reporting at the site.

Antarctic permafrost measurement sites are extremely limited. Much of the continent is covered by an ice cap where there is no active layer. Measurements taken at the Johann Gregor Mendell site on James Ross Island (near the northern tip of the Eastern Antarctic Peninsula) show that the mean annual ground temperatures at 5 and 75cm depths have increased over the last five years (not shown) and active layer thicknesses have also increased (Figure 8). Currently, data is available for only two locations for the last 6 years as a result of the work by Hrbáček et al., 2017.

Figure 8: Scatterplot of active layer thickness trends versus number of years reporting over Antarctica. Both sites most recent measurement is from 2017.

References

Batir, J.F., Hornbach, M.J., and D.D. Blackwell, 2017. Ten years of measurements and modeling of soil temperature changes and their effects on permafrost in Northwestern Alaska. Global and Planetary Change, 149, 55-71, doi: https://doi.org/10.1016/j.gloplacha.2016.11.009

Hrbáček F., Láska, K., Engel, Z., 2016. Effect of Snow Cover on the Active-Layer Thermal Regime – A Case Study from James Ross Island, Antarctic Peninsula. Permafrost and Periglacial Processes, 27, 307–315.

Hrbáček, F., Kňažková, M., Nývlt, D., Láska, K., Mueller, C.W., Ondruch, J., 2017. Active layer monitoring at CALM-S site near J.G.Mendel Station, James Ross Island, Eastern Antarctic Peninsula. Science of Total Environment. 601–602, 987–997.